ALBERT

Description: Systematic numerical study of near-infrared radiation formed during filamentation in air revealed the formation of robust light bullet first registered in the experiment (Chen et al. in Appl Phys B 91:219, 2008 ). The near-infrared light bullet propagates along the filament axis with the divergence 〈1 mrad and the quasi-constant duration of ~30 fs. The central wavelength of the bullet gradually increases from 860 to 900 nm during the propagation. The results of our numerical simulation are in agreement with the experiments (Chen et al. in Appl Phys B 91:219,  2008 ; Uryupina et al. in Appl Phys B 110:123, 2013 ).

Description: Flame flatness is one of the most critical factors in evaluating the performance of a flat-flame burner. In this paper, the flame flatness of a flat-flame burner is validated using a resolution-doubled one-dimensional wavelength modulation spectroscopy tomography (1D-WMST) technique that only uses one view of multiple parallel laser beams. When the interval of two neighboring parallel laser beams is Δ r , a designed novel geometry of the parallel laser beams realizes a doubled tomographic resolution of Δ r /2. Using the proposed technique, the distributions of temperature and H 2 O mole fraction in an axisymmetric premixed flame are simultaneously reconstructed and hence the flame flatness of a flat-flame burner can be validated. The flatness factor is quantitatively described by the similarity between the reconstructed and expected distributions of H 2 O mole fraction. For flat and non-flat flames, the experimental results agree well with the CFD simulation results, denoting that the resolution-doubled 1D-WMST technique provides a noninvasive, reliable and low cost way to validate the flame flatness of the flat-flame burner.

Description: We report on the optical performances of laccaic acid dye in solution at different concentrations and dye–poly(methyl methacrylate) composite thin films. The linear spectral characteristics including optical constants, i.e. refractive index ( n ) and extinction coefficient ( k ), were carried out in a comprehensive way through absorbance, fluorescence and ellipsometric studies. The nonlinear optical parameters such as nonlinear absorption coefficient β eff (or β 2 ), the imaginary third-order susceptibility (Im[ χ (3) ]) and the imaginary part of second-order hyperpolarizability ( γ ) of the samples were evaluated using the open-aperture Z-scan technique with a laser pulse duration of 10 ns at 532 nm wavelength. The corresponding numerical values of these parameters were of 10 −10 , 10 −11 and 10 −32 order, respectively. Two-photon absorption was revealed to be the main driving physical mechanism in the nonlinear response. This suggests that laccaic acid dye can be a potential candidate for NLO materials application.

Description: We demonstrate a sensor based on tunable diode laser absorption spectroscopy for the detection of hydrogen fluoride (HF) gas at ambient pressure. Absorption from the HF R(1) ro-vibrational peak at ν̃  = 4038.962 cm −1 (2.476 µm) in the fundamental (Δ ν  = 1) band is measured. A quantitative spectral fit based on HITRAN data is used to account for overlapping spectral peaks of HF and water vapor, with an rms residual noise of 5 × 10 −4 absorbance units. The sensor is optimized for the detection of transient variations in HF concentration. We measure noise-equivalent concentrations for HF of 38 parts-per-trillion by volume (ppt) for 1-s integration times and 2.3 ppt for 10-min integration times.

Description: We report on the absorption profile of the monoclinic holmium-doped \(\hbox {KY}(\hbox {WO}_4)_2\) crystal near the optic axis for the maximal absorption wavelength at 1960 nm. The full angular distribution of the absorption coefficient at the vicinity of such optical singularity has been experimentally and numerically investigated. Furthermore, laser experiments along the optic axis have been carried out. So-called conical refraction laser and classical Gaussian laser operation are compared near the optic axis, taking into account the complex absorption profile.

Description: We demonstrate a novel and compact fiber-probe pressure sensor based on a micro-Fabry–Perot interferometer (FPI). The device is fabricated by splicing both ends of a short-section simplified hollow-core photonic crystal fiber (SHC-PCF) with single-mode fibers. Then, a microchannel is drilled by a femtosecond laser micromachining in the SHC-PCF to allow air to diffuse in. The pressure sensing mechanism is based on the dependence of the air refractive index on pressure. We use both theory and experiment to investigate the sensing characteristics. A micro-FPI with a length of 272 μm demonstrates a pressure sensitivity of 4.071 nm/MPa at 1580 nm and a low-temperature sensitivity of 1.1 pm/°C at atmospheric pressure. We further study the temperature cross sensitivity of the sensor under different pressures. The sensor also shows strong stability and good reversibility, and may be potentially used in pressure sensing applications.

Description: We study the thermo-optic Goos–Hänchen (TOGH) effect in a prism–waveguide coupling structure with silicon-on-insulator waveguide. Stationary-phase method is utilized to calculate the TOGH shift. When the waveguide is regarded as a two-dimensional planar waveguide, a nonlinear relation between GH shift and temperature is obtained. Based on the noticeable TOGH effect, a sensitive temperature modulator or sensor can be realized. As the waveguide width is limited, the proposed structure can be regarded as a three-dimensional rectangular waveguide. We explore the GH shift and TOGH effect for different modes propagating in rectangular waveguide which show different linear relations between GH shift and temperature, which can be used to design mode-selective device based on TO effect.

Description: We report on an experimental study of vacuum-induced suppression and enhancement of four-wave mixing (FWM) signal in a composite atom–cavity system. By scanning the additional dressing field, the suppression ratio of the FWM signal can reach 90 % compared with 40 % without cavity. We attribute the enhanced suppression and enhancement to the atom–cavity coupling arising from a vacuum-induced Raman process, which amplifies the dressing effect from the additional field. Also, the dressing asymmetry of the atom–cavity coupling is discussed and used to estimate the nonlinearity of atomic medium in the cavity. The suppression and enhancement can be interpreted by a dressed-state picture and agree with theoretical calculations. The investigation may find applications in optical switch and quantum memory controlled by cavity.

Description: By simultaneously using a multi-walled carbon nanotube (MWCNT) and a monolayer graphene saturable absorber (SA) in the cavity, a laser-diode-pumped dual-loss-modulated passively Q-switched Tm:LuAG laser at 2 μm is demonstrated for the first time. In comparison with the singly passively Q-switched laser with MWCNT or monolayer graphene SA, the doubly passively Q-switched laser with both MWCNT and monolayer graphene SA can generate shorter pulse width and higher peak power. A maximum pulse width compression ratio of 2.8 and a highest peak power enhancement factor of 4 were obtained at the incident pump power of 5.8 W, respectively. The experimental results show that the dual-loss modulation is an efficient method to compress the pulse widths and improve the peak powers of the Q-switched lasers at 2 μm.

Description: Photoexcitation dynamics in a three-step photoionization of atomic uranium has been investigated using time-resolved two-color three-photon and delayed three-color three-photon photoionization signals. Investigations are carried out in an atomic beam of uranium coupled to a high-resolution time-of-flight mass spectrometer using three tunable pulsed dye lasers. Dependence of both the signals on the second-step laser photon fluence is studied. Excited-level-to-excited-level photoexcitation cross section and photoionization cross section from the second excited level are simultaneously determined by analyzing the two-color three-photon and three-color three-photon photoionization signals using population rate equation model. Using this methodology, photoexcitation and photoionization cross sections at seven values of the second-step laser wavelength have been measured. From the measured values of the photoexcitation cross sections, we have obtained excited-level-to-excited-level transition probabilities and compared these with the values reported in the literature.

Description: A theoretical analysis of the photorefractive sensitivity of Ti:PPLN ridge waveguides in comparison with conventional Ti:PPLN channel waveguides is presented. In particular, intensity-dependent photorefraction, effective indices, waveguide modes and power-dependent SHG in Ti:PPLN ridge and channel waveguides are modeled for a wide range of parameters. Results predict a much better damage resistance of Ti:PPLN waveguides with ridge geometry in comparison with conventional indiffused channels. This superiority of ridge waveguides is attributed to their higher effective refractive index contrast and more tightly confined guided modes. The theoretical predictions are supported by experimental results for second harmonic generation (SHG) at room temperature and for light-induced detuning characteristics of the phase-matching wavelength.

Description: Two-color laser-induced incandescence (LII) measurements are carried out in diffusion flames and at the exhaust of a homemade soot generator, both fueled with ethylene and methane. Two-color prompt LII signals, their ratio and the corresponding temperature have been analyzed as a function of laser fluence. In particular, the effect of fuel, soot load and gas/particle initial temperature on LII measurements have been investigated. LII spectral measurements have also been performed in all conditions for validation. The results suggest that the incandescence is sensitive to both optical and non-optical physical properties of the particles. Moreover, soot volume fraction measurements are dependent on the laser fluence used, indicating that the soot temperature influences the refractive index absorption function. Such issues can be overcome by working at high laser fluences, where the saturation curves are independent from the experimental conditions if the soot absorption function near soot sublimation threshold is known.

Description: In this study, In 2 O 3 /Ag/MoO 3 (IAM) nano-multilayer films are designed, and optimum thickness of each layer is calculated. These films were deposited by thermal evaporation technique and then annealed in air atmosphere at different temperatures for 1 h. The effects of annealing temperature on electrical, optical, and structural properties of the IAM system were investigated. The UV–visible–near-IR transmittance and reflectance spectra confirmed that the annealing temperature has significant influence on the electro‐optical characteristics of IAM films. High-quality IAM films with a low sheet resistance of 8.2 (Ω/□) and the maximum optical transmittance of 85 % at 120 °C annealing temperature were obtained. The effect of heat treatment on surface roughness of the layers was also investigated. Figure-of-merit quantity showed that the IAM films annealed at 120 °C have the best performance. X-ray diffraction patterns showed that the crystallinity of the structures enhanced with increase in annealing temperature. Organic light-emitting diodes (OLEDs) were fabricated on IAM anodes. The current density–voltage–luminance (J–V–L) characteristic measurements show that the electroluminescence performances of OLED with IAM anode are improved compared with the conventional ITO-based device. The results indicate that the designed system is suitable for use as transparent conductive anode in optoelectronic devices.

Description: We demonstrate the effectiveness of silicon phase masks to implement spatially resolved, multispectral imaging capabilities in the range of terahertz frequencies, using a standard setup of basic interest for time-domain spectrometry with a single-cell source and a single-cell detector. Our principle primarily aims at the development of robust and inexpensive systems. It consists of appropriate space-to-time encoding, in order to ensure single-scan triggering and then take advantage of rapid and self-consistent measurements in the two-dimensional space. The process enables very efficient discrimination giving access to a relevant spatial resolution in the analysis of small size, planar assemblies made of inhomogeneous materials. Benchmark results are provided to validate the concept, thanks to prototyping phase masks with 2 × 2 pixels, prior evidencing actual performance limitations in the case of 3 × 3 pixels. Due to the frequency bandwidth of 0.1–1.5 THz in our setup and to the available operating conditions, currently acceptable pixel resolutions lie in the range of 3–4 mm. Numerical modeling by means of finite elements helps to discuss these numbers and to investigate the relevant theoretical issues, figuring the main propagation issues in connection with a sub-picosecond seed pulse throughout various masks. This involves diffraction and trailing edge effects when crossing the mask together with residual, parasitic reflections. Finally, we give a consistent prospective for improved performance, via realistic updates regarding the architecture of the setup and complementary post-processing. Further values for the attainable spatial resolution then range from 5 × 5 to 6 × 6 pixels.

Description: A combination of optical emission spectroscopy (OES) and cavity ring-down spectroscopy (CRDS) has enabled to determinate the number densities of CH(A 2 Δ) and CH(X 2 Π) radicals simultaneously in a cascaded arc plasma reactor operating with a CH 4 /Ar mixture. It is found that the number density of CH(A 2 Δ) radical increases with discharge current at first and then decreases. However, the number density of CH(X 2 Π) radical decreases with discharge current when the rate of CH 4 flow to total flow is lower than 1 %, while it increases slightly with discharge current when the rate is 1.5 %. The results reveal that CH radicals are deviation from excitation equilibrium. Although OES is the simplest and most straightforward means to investigate the CH radical behavior, it is not enough to provide the information of the CH(X 2 Π) number density, and additional methods, such as CRDS, are needed in the cascaded arc plasma jet.

Description: In this paper, we present a numerical study based on 3D FDTD (finite-difference time-domain) simulations that demonstrate the high sensitivity of the optical response of a bowtie nano-aperture antenna (BNA), engraved at the apex of a metal-coated tip, to the distance variation between it and a given substrate. This study mainly discussed the case of the collection mode regime of the BNA and considered the case of two different substrates (high and low refractive index). The coupling between the substrate and the BNA may greatly affect the properties of the optical resonance of the nano-antenna. A blueshift of the resonance wavelength, as large as \(\frac{\varDelta \lambda }{\lambda }=0.3\) , is obtained when the tip moves away over only 10 nm from the substrate (InP) interface. These results open the way to the design of stand-alone optical near-field probes that allow faithful interaction control with a given sample as a function of the distance between them.

Description: The present paper describes the research of the optical device for two-dimensional straightness measurement of technological machines. Mathematical study of an optical device, operating on the phase principle and measuring transversal displacements of machine parts in two directions ( X and Y ) during their linear longitudinal motion in a machine (alongside the Z axis), is presented. How to estimate the range of travel along the Z axis is analytically shown. At this range, the measurer gives correct measurements of transverse displacement. The necessary distance from the objective focus to the image plane was defined mathematically. The sample results of measuring the displacement of the table of a technological machine by using the optical device are presented in the paper. This optical device for non-contact straightness measurement can be used for measurement straightness in turning, milling, drilling, grinding machines and other technological machines, also in geodesy and cartography, and for moving accuracy testing of mechatronic devices, robotics and others.

Description: Non-resonant laser-induced thermal acoustics (LITA), a four-wave mixing technique, was applied to post-shock flows within a shock tube. Simultaneous single-shot determination of temperature, speed of sound and flow velocity behind incident and reflected shock waves at different pressure and temperature levels are presented. Measurements were performed non-intrusively and without any seeding. The paper describes the technique and outlines its advantages compared to more established laser-based methods with respect to the challenges of shock tube experiments. The experiments include argon and nitrogen as test gas at temperatures of up to 1000 K and pressures of up to 43 bar. The experimental data are compared to calculated values based on inviscid one-dimensional shock wave theory. The single-shot uncertainty of the technique is investigated for worst-case test conditions resulting in relative standard deviations of 1, 1.7 and 3.4 % for Mach number, speed of sound and temperature, respectively. For all further experimental conditions, calculated values stay well within the 95 % confidence intervals of the LITA measurement.

Description: Airflow induced by femtosecond laser (800 nm/1 kHz/25 fs) filamentation with different lengths was investigated in a laboratory cloud chamber. Various filament lengths were generated by adjusting laser energy and lens focal length. It was found that airflow patterns are closely related to filament intensity and length. Intense and long filaments are beneficial in updraft generation with large vortices above the filament, while intense and short filaments tend to promote the formation of well-contacted vortices below the filament. Differently patterned airflows induced elliptical snow piles with different masses. We simulated airflow in a cloud chamber numerically taking laser filaments as heat sources. The mechanisms of differently patterned airflow and snow formation induced by filaments were discussed.

Description: Tissue simulators, the so-called tissue phantoms, have been used to mimic human tissue for spectroscopic applications. Phantoms’ design depends on patterning the optical properties, namely absorption and scattering coefficients which characterize light propagation mechanisms inside the tissues. In this work, two calibration models based on measurements adopting integrating sphere systems have been used to determine the optical properties of the studied solid phantoms. Integrating sphere measurement results were fed into the calibration models using the multiple polynomial regression method and Newton–Raphson algorithm. The third-order polynomials have been used for optical properties predictions. Good agreement between the two models has been obtained. Role of solid phantoms’ components, namely titanium dioxide as a scatterer and black carbon as an absorber, has been discussed. Both of the two components showed observable effects on the absorption and scattering of light inside the solid tissue phantoms.

Description: We report a novel time-resolved photoacoustic-based technique for studying the thermal decomposition mechanisms of some secondary explosives such as RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), picric acid, 4,6-dinitro-5-(4-nitro-1 H -imidazol-1-yl)-1 H -benzo[ d ] [ 1 – 3 ] triazole, and 5-chloro-1-(4-nitrophenyl)-1 H -tetrazole. A comparison of the thermal decomposition mechanisms of these secondary explosives was made by detecting NO 2 molecules released under controlled pyrolysis between 25 and 350 °C. The results show excellent agreement with the thermogravimetric and differential thermal analysis (TGA–DTA) results. A specially designed PA cell made of stainless steel was filled with explosive vapor and pumped using second harmonic, i.e., λ  = 532 nm, pulses of duration 7 ns at a 10 Hz repetition rate, obtained using a Q-switched Nd:YAG laser. The use of a combination of PA and TGA–DTA techniques enables the study of NO 2 generation, and this method can be used to scale the performance of these explosives as rocket fuels. The minimum detection limits of the four explosives were 38 ppmv to 69 ppbv, depending on their respective vapor pressures.

Description: For the first time, spatial distribution of major and trace elements has been studied in cholesterol gallstones using time-of-flight secondary mass ion mass spectrometry (TOF-SIMS). The TOF-SIMS has been used to study the elemental constituents of the center and surface parts of the gallstone sample. We have classified the gallstone sample using Fourier transform spectroscopy. The detected elements in cholesterol gallstone sample were carbon (C), hydrogen (H), calcium (Ca), sodium (Na), potassium (K), strontium (Sr), copper (Cu), iron (Fe), chromium (Cr), mercury (Hg) and lead (Pb). The detected molecules in the cholesterol gallstone were CH 3 + , CO 3 + , CaCO 3 + and C 3 H + . Our results revealed that the contents of these elements in cholesterol gallstone were higher in the center part than that in the surface part. In the present paper, we have also presented the UV–Vis spectroscopic studies of the center and surface parts of the gallstone sample which indicated the presence of a higher content of cholesterol in the surface part and bilirubin in the center part.

Description: Numerical experiments are carried out to calculate continuum emissivity and opacity of plasmas produced from laser-irradiated Au and Pb targets as hohlraum wall materials. Targets are considered to be solid or porous with different initial densities. Simulation results show a good agreement compared with the measured data. The results show that under identical conditions, X-ray emission is higher for Au plasma; however, by decreasing initial densities, X-ray yield enhancement is greater for Pb plasma. By using a Pb target with initial density of about 1.14 g cm −3 instead of solid Au target, the same X-ray yield even more can be obtained. Calculations also show that in the conditions of solid density targets, Pb plasma offers a little lower opacity in soft X-ray region. Decreasing initial density of Pb causes its opacity to increase and get closer to the opacity of solid Au which in turn reduces energy losses in hohlraum wall.

Description: We report the two-photon interference properties of a photon pair generated in a type-II collinear periodically poled KTiOPO 4 (PPKTP) crystal pumped by a 406-nm diode laser capable of producing a single or dual longitudinal mode (LM). When the Hong–Ou–Mandel (HOM) interference signals in the PPKTP crystal pumped by a dual-mode diode laser were investigated at various crystal temperatures, it was found that the maximum visibility of the HOM interference signal depends on the relative strength of the dual LMs of the pump laser. The HOM interference pattern was numerically calculated considering the dual LM components of the pump laser diode and the crystal temperature, and was found to be in good agreement with the experimental results.

Description: One- and two-ring aromatics such as toluene and naphthalene are frequently used molecular tracer species in laser-induced fluorescence (LIF) imaging diagnostics. Quantifying LIF signal intensities requires knowledge of the photo-physical processes that determine the fluorescence quantum yield. Collision-induced and intramolecular energy transfer processes in the excited electronic state closely interact under practical conditions. They can be separated through experiments at variable low pressures. Effective fluorescence lifetimes of gaseous toluene, 1,2,4-trimethylbenzene, anisole, naphthalene, and 1-methylnaphthalene diluted in CO 2 were measured after picosecond laser excitation at 266 nm and time-resolved detection of fluorescence intensities. Measurements in an optically accessible externally heated cell between 296 and 475 K and 0.010–1 bar showed that effective fluorescence lifetimes generally decrease with temperature, while the influence of the bath-gas pressure depends on the respective target species and temperature. The results provide non-radiative and fluorescence rate constants and experimentally validate the effect of photo-induced cooling.

Description: We present a mid-infrared pulsed cavity ring-down spectroscopy (CRDS) setup based on difference frequency mixing of a tunable dye laser pumped by a Nd:YAG laser and the residual 1064 nm pulse in a temperature-stabilized lithium niobate (LiNbO 3 ) crystal. The performances of such a spectrometer have been investigated by studying the absorption spectrum of methane around 3 µm at room temperature. Our results show a minimum detectable absorption coefficient of 5 × 10 −8  cm −1 . Our approach combines the excellent sensitivity of CRDS with the wide tunable range of difference frequency generation (DFG) with a dye laser. However, the relatively low resolution of this spectrometer prevents the quantification of methane in a wide pressure range. To our knowledge, we report here the first realization and application of a pulsed DFG-based CRDS experiment used for sensitive trace gas detection.

Description: A telescope-grating-deformable scheme is proposed to compensate the angular dispersion of ultrabroadband spectrum. A simple design consideration is formulated based on the large angular dispersion of the idler from noncollinear optical parametric amplification. A proof of principle experiment is demonstrated. A 3-μJ ultrabroadband near-infrared pulse with spectrum range from 700 to 1400 nm has been generated. The technique has great potential to provide an ultrabroadband seed with negligible angular dispersion for high-power amplification of few-cycle pulses.

Description: Up-conversion can be the main mechanism of energy losses in laser glasses with high concentration of erbium ions. This investigation is devoted to the evaluation of up-conversion parameters in several phosphate and silicate Er-doped glasses. Analysis of the luminescent lifetime shortening at high excitation level has shown that the up-conversion parameters in different glasses can differ by an order of magnitude. The smallest up-conversion was observed in Ba crown silicate glass and Li–Ln-phosphate glass.

Description: Wavefront aberrations are one of the largest uncertainty factors in present atom interferometers. We present a detailed numerical and experimental analysis of this effect based on measured aberrations from optical windows. By placing windows into the Raman beam path of our atomic gravimeter, we verify for the first time the induced bias in very good agreement with theory. Our method can be used to reduce the uncertainty in atomic gravimeters by one order of magnitude, resulting in an error of 〈3 × 10 −10 g, and it is suitable in a wide variety of atom interferometers with thermal or ultracold atoms. We discuss the limitations of our method, potential improvements, and its role in future generation experiments.

Description: In this review, we have discussed the potential application of the emerging imaging modality, i.e., optical coherence tomography (OCT) for glucose monitoring in biological tissues. OCT provides monitoring of glucose diffusion in different fibrous tissues like in sclera by determining the permeability rate with acceptable accuracy both in type 1 and in type 2 diabetes. The maximum precision of glucose measurement in Intralipid suspensions, for example, with the OCT technique yields the accuracy up to 4.4 mM for 10 % Intralipid and 2.2 mM for 3 % Intralipid.

Description: Optical-feedback cavity-enhanced absorption spectroscopy is a highly sensitive trace gas sensing technique that relies on feedback from a resonant intracavity field to successively lock the laser to the cavity as the wavelength is scanned across a molecular absorption with a comb of resonant frequencies. V-shaped optical cavities have been favoured in the past in order to avoid additional feedback fields from non-resonant reflections that potentially suppress the locking to the resonant cavity frequency. A model of the laser–cavity coupling demonstrates, however, that the laser can stably lock to a resonant linear cavity, within certain constraints on the relative intensity of the two feedback sources. By mode mismatching the field into the linear cavity, we have shown that it is theoretically and practically possible to spatially filter out the unwanted non-resonant component in order for the resonant field to dominate the feedback competition at the laser. A 5.3  \(\upmu \hbox {m}\)  cw quantum cascade laser scanning across a \(\hbox {CO}_2\) absorption feature demonstrated stable locking to achieve a minimum detectable absorption coefficient of \(2.7\,\times \,10^{-9}\,\hbox {cm}^{-1}\) for 1-s averaging. Detailed investigations of feedback effects on the laser output verified the validity of our theoretical models.

Description: A new method of pump-coupling in diode-pumped alkali vapor amplifier is reported, which uses ring-LD to tightly surround the alkali vapor cell for directly coupled side-pumping. The kinetic and fluid dynamic modeling, numerical approaches of the ring-LD side-pumped configuration are proposed and applied to the static and the flowing-gas Cs vapor amplifiers. Pump intensity and temperature distribution in the cell are simulated. Influences of some important factors on laser power are calculated and analyzed. Comparisons of different pumped configurations are made, demonstrating the highest utilizing efficiency of pump power of the ring-LD side-pumped configuration. Thus the model is very helpful for designing high-power side-pumped alkali vapor amplifiers.

Description: Within the frame of the one-electron approximation, we calculate the electron binding energies of the \(\hbox {Na}_{55}^-\) cluster which allows for the identification of the icosahedral structure of the cluster through comparison with experimental photoelectron spectroscopy data. The surface of the icosahedral cluster is represented as a slightly deformed spherical surface, and the corresponding splitting of the energy levels caused by this symmetry reduction is calculated. Subsequently, we demonstrate that the calculated energies of photoelectrons agree very well with the experimental values. This gives an unambiguous demonstration of the role of the cluster structure in photoelectron spectra, whereas electronic shell filling effects are less important.

Description: Turbulent mixing is highly important in flows that involve heat and mass transfer. Information on turbulent heat flux is needed to validate the mixing models implemented in numerical simulations. The calculation of turbulent heat fluxes requires instantaneous information on temperature and velocity. Even using minimally intrusive laser optical methods, simultaneous measurement of temperature and velocity is still a challenge. In this study, thermographic phosphor particles are used for simultaneous thermometry and velocimetry: conventional particle image velocimetry is combined with temperature-dependent spectral shifts of BAM:Eu 2+ phosphor particles upon UV excitation. The novelty of this approach is the analysis of systematic errors and verification using the well-known properties of a heated turbulent jet issuing into a low velocity, cold coflow. The analysis showed that systematic errors caused by laser fluence, multiple scattering, or preferential signal absorption can be reduced such that reliable measurement of scalar fluxes becomes feasible, which is a prerequisite for applying the method to more complex heat transfer problems.

Description: Multicolor deep-ultraviolet femtosecond pulses are generated in a spectral range of 220–300 nm with pulse energies exceeding 1 μJ. Pulses with shorter wavelengths are also generated with the shortest wavelength of 185 nm. These pulses are generated through cascaded four-wave mixing induced in hydrogen gas by use of three-color femtosecond pump pulses at 1200, 800, and 267 nm. The third pulse dramatically enhances the intensities of the multicolor deep-ultraviolet pulses. The cross-phase modulation induced by the two intense near-infrared pulses broadens the spectral width of each multicolor emission, resulting in a spectral width supporting a transform-limited duration shorter than 10 fs.

Description: In this work, we present a modification to conventional X-rays fluorescence using electrons as excitation source and compare it with the traditional X-ray excitation for the study of pigments. For this purpose, we have constructed a laser-based source capable to produce X-rays as well as electrons. Because of the large penetration depth of X-rays, the collected fluorescence signal is a combination of several material layers of the artwork under study. However, electrons are stopped in the first layers, allowing a more superficial analysis. We show that the combination of both excitation sources can provide extremely valuable information about the structure of the artwork.

Description: A correction for the undesirable effects of direct and indirect cross-interference from water vapour on ammonia (NH 3 ) measurements was developed using an optical laser sensor based on cavity ring-down spectroscopy. This correction relied on new measurements of the collisional broadening due to water vapour of two NH 3 spectral lines in the near infra-red (6548.6 and 6548.8 cm −1 ), and on the development of novel stable primary standard gas mixtures (PSMs) of ammonia prepared by gravimetry in passivated gas cylinders at 100 μmol mol −1 . The PSMs were diluted dynamically to provide calibration mixtures of dry and humidified ammonia atmospheres of known composition in the nmol mol −1 range and were employed as part of establishing a metrological traceability chain to improve the reliability and accuracy of ambient ammonia measurements. The successful implementation of this correction will allow the extension of this rapid on-line spectroscopic technique to exposure chamber validation tests under controlled conditions and ambient monitoring in the field.

Description: In this paper, we analyze the influence of large-scale segmentation errors in the morphology of high-performance optical gratings. It is thus assumed that the optical grating under consideration (typical lateral extends S are 10–1000 mm) can be spatially decomposed into a great many but unique sub-segments ( \(\ll S\) ; typical extends are 10–100  \(\upmu \mathrm{m}\) ). Any violation of the perfect periodicity will result in the generation of stray light, especially Rowland ghosts, which radiate into a small angular region around the grating’s diffraction orders. In this paper, we focus on three different kinds of segmentation errors. On the one hand, there are statistic as well as deterministic alignment errors between otherwise perfect sub-segments. On the other hand, we analyze the effect of chirping of geometrical parameters, i.e., the groove width, within every sub-segment. Most importantly, we find that the particular type of imperfection results in a unique characteristic of the according stray light spectrum which thus acts as a fingerprint. We come to this conclusion on three different ways. First, we rely on a simple theoretical model that is based on scalar diffraction theory. Second, we have performed rigorous numerical simulations for a high aspect ratio purely dielectric spectrometer grating ( \(\hbox {period} = {667}\,\mathrm{nm}\) ). Third, the very same grating was then fabricated by e-beam lithography and its stray light spectrum was measured with a purposely designed optical setup. Eventually, all different routes to analyze the problem turn out to be in very good agreement, and we are confident that stray light measurements can be used as an important tool in the detection of fabrication imperfections.

Description: We report diffusion coefficients of optically pumped lithium atoms in helium buffer gas. The free-induction decay and the spin-echo signals of ground-state atoms were optically detected in an external magnetic field with the addition of field gradient. Lithium hot vapor was produced in a borosilicate-glass cell at a temperature between 290 and \(360\,^{\circ }\hbox {C}\) . The simple setup using the glass cells enabled lithium atomic spectroscopy in a similar way to other alkali-metal atoms and study of the collisional properties of lithium atoms in a hot-vapor phase.

Description: We propose and demonstrate a laser frequency stabilization scheme which generates a dispersion-like tunable Doppler-free dichroic lock (TDFDL) signal. This signal offers a wide tuning range for lock point (i.e. zero-crossing) without compromising on the slope of the locking signal. The method involves measurement of magnetically induced dichroism in an atomic vapour for a weak probe laser beam in the presence of a counter-propagating strong pump laser beam. A simple model is presented to explain the basic principles of this method to generate the TDFDL signal. The spectral shift in the locking signal is achieved by tuning the frequency of the pump beam. The TDFDL signal is shown to be useful for locking the frequency of a cooling laser used for magneto-optical trap (MOT) for 87 Rb atoms.

Description: Detection of multiple transitions in NO and H 2 O using multi-mode absorption spectroscopy, MUMAS, with a quantum cascade laser, QCL, operating at 5.3 μm at scan rates up to 10 kHz is reported. The linewidth of longitudinal modes of the QCL is derived from pressure-dependent fits to experimental MUMAS data. Variations in the spectral structure of the broadband, multi-mode, output of the commercially available QCL employed are analysed to provide accurate fits of modelled MUMAS signatures to the experimental data.

Description: We propose a novel approach to site-resolved detection of a 2D gas of ultracold atoms in an optical lattice. A near-resonant laser beam is coherently scattered by the atomic array, and after passing a lens its interference pattern is holographically recorded by superimposing it with a reference laser beam on a CCD chip. Fourier transformation of the recorded intensity pattern reconstructs the atomic distribution in the lattice with single-site resolution. The holographic detection method requires only about two hundred scattered photons per atom in order to achieve a high reconstruction fidelity of 99.9 %. Therefore, additional cooling during detection might not be necessary even for light atomic elements such as lithium. Furthermore, first investigations suggest that small aberrations of the lens can be post-corrected in imaging processing.

Description: We experimentally demonstrate that the spectral resolution of Fourier transform interferometer could be greatly enhanced by using the dispersive property of semiconductor GaAs in the near infrared region and obtain the frequency distribution of the input light from interference pattern by defining a new frequency transform factor. The results show the effectiveness of this method in the slow light interference system.

Description: Characteristics of a ZnSe:Fe 2+ laser operating at room temperature of active polycrystalline elements with large transversal dimensions are investigated. The active elements had a shape of plates with diameters D  = 25–63 mm and width of ~4 mm, doped from two sides with iron ions by the diffusion method. The plates were doped in the process of hot isostatic pressing at an argon pressure of 100 MPa and temperature of 1250 °C for 75–151 h. The laser was pumped by a non-chain electrodischarge HF laser operated in a single-pulse mode. The employment of active elements with greater transversal dimensions resulted in a suppressed transversal parasitic oscillation at large diameters of the pumping spot. The generation energy of 1.43 J with the slope efficiency η slope  = 53 % and the total efficiency with respect to the energy absorbed in an active element η abs  ≈ 48 % was obtained on the sample of diameter D  = 63 mm.

Description: The hole transport buffer layer (HTL) known as PEDOT:PSS is found to be sensitive to polar solvents often used in the preparation of solution-processed perovskite-based solar cell. We employed \(\hbox {CH}_{3}\,\hbox {NH}_{3}\,\hbox {PbI}_{3}\) perovskite absorber sandwiched between two charge transport layers to analyze the effect of precursor solvent. By introducing skin-depth interfacial defect layer (IDL) on PEDOT:PSS film we studied the overall performance of the devices using one-dimensional device simulator. Both enhanced conductivity and variations in valence band offset (VBO) of IDL were considered to analyze device performance. A power conversion efficiency (PCE) of the devices was found to grow by 35 % due to increased conductivity of IDL by a factor of 1000. Furthermore, we noted a drastic reduction in PCE of the device by reducing the work function of IDL by more than 0.3eV . The thickness of interfacial defect layer was also analyzed and found to decrease the PCE of the devices by 18 % for fourfold increase in IDL thickness. The analysis was remarkably reproduced the experimentally generated device parameters and will help to understand the underlying physical process in perovskite-based solar cell.

Description: We present a diode laser system optimized for laser cooling and atom interferometry with ultra-cold rubidium atoms aboard sounding rockets as an important milestone toward space-borne quantum sensors. Design, assembly and qualification of the system, combing micro-integrated distributed feedback (DFB) diode laser modules and free space optical bench technology, is presented in the context of the MAIUS (Matter-wave Interferometry in Microgravity) mission. This laser system, with a volume of 21 l and total mass of 27 kg, passed all qualification tests for operation on sounding rockets and is currently used in the integrated MAIUS flight system producing Bose–Einstein condensates and performing atom interferometry based on Bragg diffraction. The MAIUS payload is being prepared for launch in fall 2016. We further report on a reference laser system, comprising a rubidium stabilized DFB laser, which was operated successfully on the TEXUS 51 mission in April 2015. The system demonstrated a high level of technological maturity by remaining frequency stabilized throughout the mission including the rocket’s boost phase.

Description: We demonstrate fourfold coherent combining in a gas-filled hollow-fiber compressor with 92 % efficiency. Our passive approach relies on the use of carefully oriented birefringent plates for temporal pulse dividing and combining. We perform a detailed theoretical and experimental analysis of the effects degrading the combining process, as polarization change or nonlinear interactions between pulse replicas. We show how to overcome these limitations to generate 10-fs output pulses with high temporal quality.

Description: Characteristics of annular beams propagating through atmospheric turbulence along a downlink path and an uplink path are studied in detail by using numerical simulation method. It is found that in downlink the influence of atmospheric turbulence on the characteristics is quite different from that in uplink because of the altitude-dependent index structure constant. It is shown that, when the zenith angle θ is not large enough, it is always \(\sigma_{{I\,{\text{up}}}}^{2} 〉 \sigma_{{I\,{\text{down}}}}^{2}\) on propagation whatever the value of the obscure ratio ε is, where \(\sigma_{{I\,{\text{up}}}}^{2}\) and \(\sigma_{{I\,{\text{down}}}}^{2}\) are the on-axis scintillation index in uplink and downlink, respectively. However, when θ is large enough, \(\sigma_{{I\,{\text{down}}}}^{2}\) is close to \(\sigma_{{I\,{\text{up}}}}^{2}\) as the propagation distance z increases, and \(\sigma_{{I\,{\text{up}}}}^{2}\) and \(\sigma_{{I\,{\text{down}}}}^{2}\) overlap each other as ε increases. Furthermore, as z increases, \(\sigma_{{I\,{\text{up}}}}^{2}\) approaches an asymptotical value when θ is not large enough, and the saturation phenomenon of \(\sigma_{{I\,{\text{up}}}}^{2}\) appears when θ is large enough. But the asymptotical value and the saturation phenomenon of \(\sigma_{{I\,{\text{down}}}}^{2}\) never appear. On the other hand, the energy focusability in downlink is better than that in uplink, and the difference of energy focusability between a downlink and an uplink increases with increasing θ or decreasing ε . In addition, in downlink there may exist sidelobes of intensity distributions when θ is not large enough, but the sidelobes never appear in uplink.

Description: We study the dispersion of the extraordinary dielectric function real and imaginary parts in the wide terahertz-frequency range of the lowest polariton branch for bulk LiNbO 3 and Mg:LiNbO 3 crystals. At frequencies 0.1–2.5 THz, both dispersion parts are measured by means of standard time-domain terahertz spectroscopy, and at higher frequencies up to 5.5 THz, the dielectric function real part is determined using a common scheme of spontaneous parametric down-conversion under near-forward Raman scattering by phonon polaritons. A special approach is applied for measuring of the dielectric function imaginary part at frequencies 1–3 THz, based on the analysis of visibility of three-wave second-order interference under spontaneous parametric down-conversion. The generalized approximate expressions are obtained for complex dielectric function dispersion within the lower polariton branches of LiNbO 3 and Mg:LiNbO 3 . It is shown that the well-known decrease in terahertz-wave absorption of lithium niobate crystals under Mg-doping is caused by changes in the defect structure and reduction of coupling of the terahertz-frequency polaritons with Debye relaxational mode.

Description: We report improved fluorescence contrast between dyes by two-photon excitation with polarization-shaped laser pulses after transmission through a kagome fiber utilizing the anisotropy of the dye molecules. Particularly phase- and polarization-tailored pulse shapes are employed for two-photon excited fluorescence of dyes in a liquid environment at the distal end of the kagome fiber. The distortions due to the optical fiber properties are precompensated in order to receive predefined polarization-shaped laser pulses after the kagome fiber. This enables to optimally excite one dye in one polarization direction and simultaneously the other dye in the other polarization direction. The presented method has a high potential for endoscopic applications due to the unique properties of kagome fibers for guiding ultrashort laser pulses.

Description: A new nonlinear optical material, 4-[(2 E )-3-(3-fluorophenyl) prop-2-enoyl] benzonitrile (3FPB), belonging to chalcone family was synthesized and characterized by FTIR and linear absorption spectroscopy. Single-crystal X-ray diffraction reveals that the new material crystallizes in monoclinic system with P 2 1 /c space group and lattice parameters a  = 6.4841(2) Å, b  = 13.6038(5) Å, c  = 14.6418(6) Å, α  = 90.00°, β  = 94.552(2)° and γ  = 90°. The crystallographic perfection of the synthesized material has been analyzed by X-ray powder diffraction. The X-ray powder diffraction peaks of the sample were indexed with hkl values. The UV–visible spectrum for 3FPB crystals showed the optical transmittance window and a lower cutoff wavelength of absorption at 343 nm. The direct transition band gap energy and indirect transition energy gap were determined using Tauc’s plots. The thermal stability and melting point of the material have been investigated by thermogravimetric analysis/differential thermal analysis (TGA/DTA). The Thermogravimetric curve showed the absence of any phase transition before melting point. Third-order nonlinear absorption and optical limiting experiment were carried out using open-aperture Z -scan experiment with Nd:YAG laser nanosecond pulses at a wavelength of 532 nm.

Description: We analyzed the surfaces of vitreous silica (quartz) and borosilicate glass (Pyrex) substrates exposed to rubidium (Rb) vapor by X-ray photoelectron spectroscopy (XPS) to understand the surface conditions of alkali metal vapor cells. XPS spectra indicated that Rb atoms adopted different bonding states in quartz and Pyrex. Furthermore, Rb atoms in quartz remained in the near-surface region, while they diffused into the bulk in Pyrex. For these characterized surfaces, we measured light-induced atom desorption (LIAD) of Rb atoms. Clear differences in time evolution, photon energy dependence, and substrate temperature dependence were found; the decay of LIAD by continuous ultraviolet irradiation for quartz was faster than that for Pyrex, a monotonic increase in LIAD with increasing photon energy from 1.8 to 4.3 eV was more prominent for quartz, and LIAD from quartz was more efficient at higher temperatures in the range from 300 to 580 K, while that from Pyrex was almost independent of temperature.

Description: Mid-infrared cavity ring-down spectroscopy (CRDS) using an external cavity, widely tunable pulsed quantum cascade laser operating at 3.8 μm, was employed for simultaneous detections of ethanol, ether and acetone in this paper. The experiments were performed with a maximum cavity mirror reflectivity of 99.915 % between the wave number 2614 and 2634 cm −1 , leading to an effective optical path length of 588 m. The absorption spectra of ethanol, ether and acetone were measured with high spectral resolution in the range of 2614–2634 cm −1 , and the spectroscopic analysis of the mixture of ethanol, ether and acetone with overlapping absorption spectra was demonstrated. The experimentally achieved detection limits ( \(3\sigma\) , or three times of standard deviation) for ethanol, ether and acetone were 157, 60 and 280 ppb, respectively. The simultaneously measured concentration results were in good agreement with the results with the standard gravimetric method, indicated that the mid-infrared CRDS has the potential for multi-component trace gas detection as well as for spectroscopic measurements of multi-broadband absorbers.

Description: Infrared to visible upconversion fluorescent nanoparticles of Gd 2 O 3 codoped with Ho 3+ /Yb 3+ ions are synthesized via thermal decomposition process. The X-ray diffraction analysis of as-synthesized nanoparticles and annealed sample at 1000 °C has shown body-centered cubic phase of Gd 2 O 3 . The synthesized phosphor has shown intense green emission upon 980-nm excitation. High-contrast latent fingermarks on some difficult semi-porous and non-porous surfaces under 980-nm diode laser excitation were developed through powder dusting and colloidal solution spraying techniques and the results are compared with the commercial green luminescent fingermark powder. The latent fingermarks were developed on transparent (biological glass slides), single-color (aluminum foil) and multicolor (plywood, plastic bottle and book cover page) background surfaces. The present study depicts that the upconversion-based latent fingermarks detection using Gd 2 O 3 :Ho 3+ /Yb 3+ phosphor material is suitable over the other conventional powders and has potential for practical applications in forensic science.

Description: We have found the new ways to investigate the linear/non-linear optical properties of nanostructure pentacene thin film deposited by thermal evaporation technique. Pentacene is the key material in organic semiconductor technology. The existence of nano-structured thin film was confirmed by atomic force microscopy and X-ray diffraction. The wavelength-dependent transmittance and reflectance were calculated to observe the optical behavior of the pentacene thin film. It has been observed the anomalous dispersion at wavelength λ  〈 800 nm, whereas the normal dispersion was found at wavelength λ  〉 800. The non-linear refractive index of the deposited films was investigated. The linear optical susceptibility of pentacene thin film was calculated, and we observed the non-linear optical susceptibility of pentacene thin film at about 6 × 10 −13  esu. The advantage of this work is to use of spectroscopic method to calculate the liner and non-liner optical response of pentacene thin films rather than expensive  Z-scan. The calculated optical behavior of the pentacene thin films could be used in the organic thin films base advanced optoelectronic devices such as telecommunications devices.

Description: We report a laboratory demonstration of the dissemination of an ultrastable optical frequency signal to two distant users simultaneously using a branching network. The ultrastable signal is extracted along a main fibre link; it is optically tracked by a narrow linewidth laser diode, which light is injected in a secondary link. The propagation noise of both links is actively compensated. We implement this scheme with two links of 50-km fibre spools, the extraction being set up at the mid-point of the main link. We show that the extracted signal at the end of the secondary link exhibits a fractional frequency instability of 1.4 × 10 −15 at 1-s measurement time, almost equal to the 1.3 × 10 −15 instability of the main link output end. The long-term instabilities are also very similar, at a level of 3–5 × 10 −20 at 3 × 10 4 -s integration time. We also show that the setting up of this extraction device, or of a simpler one, at the main link input, can test the proper functioning of the noise rejection on this main link. This work is a significant step towards a robust and flexible ultrastable network for multi-users dissemination.

Description: A hologram can be used for high-power laser processing applications such as cutting, drilling, patterning, or welding. However, not much progress has been made in cutting application compared to the others, because it requires optical reconstruction of static and uniform line images using holograms which have a high damage threshold. These static and uniform line images are difficult to be reconstructed with a single hologram, since they usually suffer from speckle between neighboring spots. We propose a method to reconstruct reduced-speckle static line images using two interlaced holograms which reconstruct odd and even pixel line images, corresponding to two orthogonal polarizations. Then, the two orthogonally polarized line images are superposed for interlacing in the image plane. The proposed method was studied by numerical simulations and demonstrated experimentally. The experimental results show that speckle contrast decreased by about one-third, compared to that of a non-interlaced hologram. This method can be applied also for complex-shaped images which include curved lines as well as straight lines, and we have a plan for laser cutting with this method in the near future.

Description: Two commercial femtosecond laser sources have been used to implement a dual-comb spectrometer tuneable across a spectral range from 1.5 to 2.2 μm. The optical linewidth of the comb modes was characterized for different time scales in order to estimate the achievable spectral resolution for an optimal acquisition time. The transmission spectra of three different gas samples were recorded, demonstrating good agreement with reference data. Frequency axis calibration was provided via the parallel monitoring of a reference sample. This technique allows an accurate calibration of the frequency axis of the spectrometer, with no need for stabilization or optical referencing of the frequency combs. Our set-up represents a good compromise for a compact and versatile dual-comb spectrometer based on commercially available parts with possible applications in trace-gas monitoring, remote sensing and spectroscopy of short-lived processes.

Description: A ppb-level mid-infrared ethane (C 2 H 6 ) sensor was developed using a continuous-wave, thermoelectrically cooled, distributed feedback interband cascade laser emitting at 3.34 μm and a miniature dense patterned multipass gas cell with a 54.6-m optical path length. The performance of the sensor was investigated using two different techniques based on the tunable interband cascade laser: direct absorption spectroscopy (DAS) and second-harmonic wavelength modulation spectroscopy (2 f -WMS). Three measurement schemes, DAS, WMS and quasi-simultaneous DAS and WMS, were realized based on the same optical sensor core. A detection limit of ~7.92 ppbv with a precision of ±30 ppbv for the separate DAS scheme with an averaging time of 1 s and a detection limit of ~1.19 ppbv with a precision of about ±4 ppbv for the separate WMS scheme with a 4-s averaging time were achieved. An Allan–Werle variance analysis indicated that the precisions can be further improved to 777 pptv @ 166 s for the separate DAS scheme and 269 pptv @ 108 s for the WMS scheme, respectively. For the quasi-simultaneous DAS and WMS scheme, both the 2 f signal and the direct absorption signal were simultaneously extracted using a LabVIEW platform, and four C 2 H 6 samples (0, 30, 60 and 90 ppbv with nitrogen as the balance gas) were used as the target gases to assess the sensor performance. A detailed comparison of the three measurement schemes is reported. Atmospheric C 2 H 6 measurements on the Rice University campus and a field test at a compressed natural gas station in Houston, TX, were conducted to evaluate the performance of the sensor system as a robust and reliable field-deployable sensor system.

Description: We report on broadband light generation in the impulsive regime in an un-doped lithium niobate (LiNbO 3 ) crystal by two femtosecond laser pulses (36 fs) from a Ti-sapphire laser amplifier. We systematically investigate the role of incident intensity on spectral broadening. At relatively low incident intensity (0.7 TW cm −2 ), spectral broadening in the transmitted beam occurs due to the combined effect of self-phase modulation and cross-phase modulation. At higher incident intensity (10.2 TW cm −2 ), we observe generation of as many as 21 anti-Stokes orders due to coherent anti-Stokes Raman scattering in self-diffraction geometry. Moreover, we observe order-dependent spectral broadening of anti-Stokes lines that may be attributed to the competition with other nonlinear optical effects like cross-phase modulation.

Description: We describe a tunable Fabry–Perot filter and grating hybrid modulator to achieve a higher spectral resolution compared with that produced by a single grating with the same period. In the hybrid modulator, a tunable Fabry–Perot filter is designed with a long cavity to accommodate a multi-order narrowband pre-filter. A grating is then utilized to separate these multi-orders spatially. Scanning the air gap of the tunable Fabry–Perot filter within 1/2 wavelength, the entire spectrogram can be achieved by compositing each group of transmitted multi-orders. Light passes first through the Fabry–Perot cavity and then into the grating. Thus, all of the light is incident on the Fabry–Perot cavity at a given angle, which can reduce the requirement for incident beam alignment and simplify the operation of the hybrid modulator. The structural matching conditions of the tunable Fabry–Perot filter and grating were presented based on the operating law of the hybrid modulator. In terms of the Rayleigh criterion, the practical spectral resolution of the hybrid modulator can be increased by at least twice that of the single grating. Experiments with a neon lamp revealed that the spectral resolution of the hybrid modulator was nearly double that of a single grating.

Description: We reported the development of an evanescent-wave quartz-enhanced photoacoustic sensor (EW-QEPAS) using a single-mode optical fiber tip for sensitive gas detection in the extended near-infrared region. It is a spectroscopic technique based on the combination of quartz-enhanced photoacoustic spectroscopy with fiber-optic evanescent-wave absorption to achieve low optical noise, easy optical alignment, and high compactness. Carbon monoxide (CO) detection at 2.3 μm using a fiber-coupled, continuous-wave, distributed-feedback laser was selected for the sensor demonstration. By tapering the optical fiber down to 2.5 μm diameter using the flame-brushing technique, an evanescent field of ~0.6 mW around the fiber tip was absorbed by CO molecules. Besides an excellent linear response ( R 2  = 0.9996) to CO concentrations, the EW-QEPAS sensor achieved a normalized noise-equivalent absorption (NNEA) coefficient of 8.6 × 10 −8  cm −1 W/√Hz for an incident optical power of 1.8 mW and integration time of 1 s. The sensor detection sensitivity can be further improved by enhancing the evanescent-wave power on the fiber tip.

Description: We describe a collisional radiative model (CRE) of homogeneously expanded nickel plasmas in vacuum. The CRE model is coupled with two separate electron and ion temperature magneto-hydrodynamic equations. On the output, the model provides the temporal variation of the electron temperature, ion temperature, and average charge state. We demonstrate the effect of three-body recombination \(({\propto}N_{\rm e} T^{-9/2}_{\rm e})\) on plasma parameters, as it changes the time dependence of electron temperature from \(t^{-2}\) to \(t^{-1}\) and exhibits a pronounced effect leading to a freezing feature in the average charge state. In addition, the effect of the three-body recombination on the warm up of ions and delaying the equilibration is addressed.

Description: An experimental and numerical investigation of the applicability of the temperature-controlled focal length of a thermally induced lens is reported. The thermal lens is formed as a result of absorption of a heating laser beam. Numerical simulations and experimental results show that changing the bulk temperature of the material of the lensing element allows for the selection of its focal length. The range of focal length changes for an example lens is given.

Description: The technique of measuring O 2 rotational temperature by coherent microwave Rayleigh scattering from resonance-enhanced multiphoton ionization (Radar REMPI) has been studied to determine temperature sensitivity and range. The molecular oxygen Rydberg state of \(\left( {3s\sigma } \right)C{{}^{3}}\Pi_{g} \left( {v^{\prime } = 2} \right)\) has been selected as the intermediate state in the 2 + 1 REMPI process, which is known to provide a relatively strong REMPI signal. Rotational-resolved spectra representing the two-photon \(C{{}^{3}}\Pi_{g} \left( {v^{\prime } = 2} \right) \leftarrow \leftarrow X{{}^{3}}\varSigma _{g}^{ - } \left( {v^{\prime \prime } = 0} \right)\) transition have been obtained under several gas conditions including pure oxygen, air-like syngas, ambient air, and flame environments from room temperature (~300 K) to flame temperature (~1700 K). An O 2 REMPI spectral model has been developed to simulate the experimental spectral line intensity distribution which is dependent on the O 2 ground-state temperature. The model has been verified at a low-temperature condition (~5 K) and then applied to various oxygen environments over an extended temperature range with an overall error of less than ±10 %. The current O 2 REMPI spectral model is an improvement over a previously reported version in both accuracy and the quantity of lines fit to provide rotational temperature measurements. This work details an optimized model that fits simulated spectra to full experimental spectral bands over various conditions with a wide temperature range, including both low temperature (〈300 K) and high temperature ranges (〉1300 K).

Description: We demonstrate a 980-nm Q-switch Yb-doped photonic crystal fiber laser by a multilayer molybdenum sulfide polymer composite as the broadband saturable absorber which is prepared by the chemical vapor deposition method. We achieve passively Q-switching operations at 978 nm with the pulse width of 2.7 and 0.63 μs, corresponding to the repetition rate of 212 and 221 kHz, respectively. The maximum output power is 127 mW. It is the first time that MoS 2 Q-switched Yb-doped photonic crystal fiber laser at 980 nm is demonstrated. The experimental results show that few-layer MoS 2 is a promising broadband saturable absorber material.

Description: First continuous laser oscillation on many lines in the range of 533–635 nm on different transitions of \(\hbox {Na}_{2}\) and \(\hbox {Te}_{2}\) molecules has been obtained, optically pumped with common cw blue emitting InGaN diode lasers operating around 445 and 460 nm. Spectral narrowing of the diode laser is achieved with a beamsplitter and grating setup, allowing use of more than 50 % of the diode power. Operation conditions and properties of the laser systems are presented, and perspectives for the realization of compact low cost molecular lasers are discussed.

Description: The field enhancement of individual cross-shaped nanoantennas for normal incident light has been measured by the relative photoemission yield using a photoemission electron microscope. We not only measured the electron yield in dependence on the intensity of infrared light (800 nm, 100 fs), but also the polarization dependence. In the normal incidence geometry, the electrical field vector of the illuminating light lies in the surface plane of the sample, independent of the polarization state. Strong yield variations due to an out-of-plane field component as well as changes in the polarization state described by the Fresnel laws are avoided. The electron yield is related to the near-field enhancement as a function of the polarization state of the incident light. The polarization dependence is well explained by numerical simulations.

Description: Germania/ormosil hybrid matrix with large third-order nonlinearity is prepared by a low-temperature sol–gel process. Z-scan measurements indicate that the film fabricated from the pure Germania/ormosil hybrid solution shows an excellent third-order nonlinearity at all measured wavelengths. In order to explore its potential to be a functional matrix, a well-investigated organic dopant disperse red 1 (DR1) azoaromatic chromophore is introduced into the Germania/ormosil system. As a comparison, the poly(methyl methacrylate) (PMMA) polymer is employed and doped with the same content of DR1 molecule. Results indicate that by employing Germania/ormosil matrix system, the figure of merit of DR1-doped material at 532 nm can be greatly improved as compared to that of the PMMA/DR1 polymer film and also other published reports. This improvement helps broaden the limited applications of DR1-doped material and make it acceptable for devices fabrication at 532 nm. Results demonstrate that the as-prepared hybrid matrix might be a promising candidate for all-optical applications.

Description: We have proposed and demonstrated a novel technique to measure distance with high range precision. To meet the stringent requirements of a variety of applications, range precision is an important specification for laser radar systems. Range precision in conventional laser radar systems is limited by several factors, namely laser pulse width, the bandwidth of a detector, the timing resolution of the time to digital converter, shot noise and timing jitters generated by electronics. The proposed laser radar system adopts a Pockels cell and a quadrant photodiode and only measures the energy of a laser pulse to obtain range so that the effect of those factors is reduced in comparison to conventional systems. In the proposed system, the measured range precision was 5.7 mm with 100 laser pulses. The proposed method is expected to be an alternative method for laser radar system requiring high range precision in many applications.

Description: Spatiotemporal nanolocalization of ultrashort pulses in a random scattering nanostructure via time reversal and adaptive optimization employing a genetic algorithm and a suitably defined fitness function is studied for two embedded nanoparticles that are separated by only a tenth of the free space wavelength. The nanostructure is composed of resonant core–shell nanoparticles (TiO 2 core and Ag shell) placed randomly surrounding these two nanoparticles acting as targets. The time reversal scheme achieves selective nanolocalization only by chance if the incident radiation can couple efficiently to dipolar local modes interacting with the target/emitter particle. Even embedding the structure in a reverberation chamber fails improving the nanolocalization. In contrast, the adaptive optimization strategy reliably yields nanolocalization of the radiation and allows a highly selective excitation of either target position. This demonstrates that random scattering structures are interesting multi-purpose optical nanoantennas to realize highly flexible spatiotemporal optical near-field control.

Description: The thermal lens technique is applied to vibrational overtone spectroscopy of solutions of benzene. The pump and probe thermal lens technique has been found to be very sensitive for detecting samples of low concentration in transparent solvents. The C–H fifth vibrational (Δ υ  = 6) overtone spectrum of benzene is detected at room temperature for compositions per volume in the range (1 to 1 × 10 −4 ) using CCl 4 and n -C 6 H 14 as solvents. By detecting the absorption band in a 100-ppm solution, the peak absorption of the signal is approximately (2.2 ± 0.3) × 10 −7 cm −1 . The parameters that determine the magnitude of the thermal lens signal such as the pump laser power and the thermodynamic properties of the solvent and solute are discussed. A plot of normalized integrated intensity as a function of composition of benzene in solution reveals a nonlinear behavior. The nonlinearity cannot be explained assuming solvent enhancement at low concentrations. A two-color absorption model that includes the simultaneous absorption of the pump and probe lasers explains the enhanced magnitude and the nonlinear behavior of the thermal lens signal for solutions of composition below 0.01.

Description: In this paper, we investigate propagation effects and interference switching of surface plasmon-polaritons (SPPs) in a junction of multiple crossed waveguides. These waveguides are produced on a thin gold layer by a simple photolithographic procedure. The waveguide dimensions are optimized for SPP excitation and propagation along two crossed input waveguides. At the waveguide intersection, different possibilities for SPP propagation into multiple output waveguides are offered. Using leakage radiation microscopy, we find that the SPPs preferably propagate into only one specific direction different from the direction of the input waveguides with avoidance of signal backscattering into the input direction. Furthermore, it is demonstrated that the SPP intensity at the output waveguide can be tuned by interference effects induced by a phase shift of the excitation laser beams. Additionally, we study the influence of different angles between the two input and the one specific output waveguides of the junction structure on the propagation properties of SPP modes in order to demonstrate a highest possible energy flux into the output waveguide. The experimental investigations are supported by finite-difference time-domain simulations. Good agreement between experimental results and numerical simulations is obtained. Applications of this effect are discussed for realization of ultrafast optical/plasmonic switches and optical logic gate structures with potential for integration and cascading.

Description: Simultaneous dual-wavelength laser oscillation with orthogonal polarizations has been observed and analyzed in a continuous wave N g -cut Yb:KGW oscillator. Without inserting any optical elements for polarization control, the N m - and N p -polarized modes, each of which possessed a distinct wavelength, coexisted and switched twice in two power regimes as the pump power was varied. The two wavelengths and their separation slightly depended on output coupling level. The wavelength switching and coexistence was studied and explained by considering the thermal and spectral anisotropy of the Yb:KGW crystals, which led to polarization-dependent reabsorption loss in the unpumped regions of the crystal. The maximum average output power obtained in the dual-wavelength regime was 4.6 W.

Description: This paper investigates Dy 3+ -doped and Dy 3+ , Er 3+ -co-doped yttrium aluminum garnets (YAG) with the admixture of boron nitride with the aim of using them as efficient thermographic phosphors at high temperatures. The phosphors were synthesized using a conventional high-temperature solid-state method. The influence of two fluxes, B 2 O 3 and LiF/NH 4 F, and the effect of activator and coactivator concentrations were investigated. Additionally, the effect of B 3+ and N 3− substituting for Al 3+ and O 2− ions, respectively, in the YAG:Dy 3+ co-doped with Er 3+ was studied for the first time. The changes in the host lattice led to a much stronger photoluminescence compared with the samples without B 3+ and N 3− substitution. The admixture of BN also improves the thermal sensitivity of the YAG:Dy and YAG:Dy, Er thermographic phosphors.

Description: Identification of agricultural pest insects is an important aspect in insect research and agricultural monitoring. We have performed a methodological study of how spectroscopic techniques and wing-beat frequency analysis might provide relevant information. An optical system based on the combination of close-range remote sensing and reflectance spectroscopy was developed to study the optical characteristics of different flying insects, collected in Southern China. The results demonstrate that the combination of wing-beat frequency assessment and reflectance spectral analysis has the potential to successfully differentiate between insect species. Further, studies of spectroscopic characteristics of fixed specimen of insects, also from Central China, showed the possibility of refined agricultural pest identification. Here, in addition to reflectance recordings also laser-induced fluorescence spectra were investigated for all the species of insects under study and found to provide complementary information to optically distinguish insects. In order to prove the practicality of the techniques explored, clearly fieldwork aiming at elucidating the variability of parameters, even within species, must be performed.

Description: We designed and fabricated the transmission quarter-wave plate phase retarder at 1064 nm using optical nanometric thin films of silicon oxide and titanium oxide. Final design consists of 32 layers. Transmissions of polarizations are equal and ≥99 % and their phase difference is 90°. System consists of two 16 layers systems that coated with the same condition on BK7 glass substrates then attached together with optical glue. Electron beam evaporation method was used for depositing materials. Photo spectrometer was used for measuring transmission spectrum of system. Transmission of polarizations was ≥95 % and equal. A polarimeter was used for testing systems. Polarization of beam was circular.

Description: In this work, we present suitable phase accuracy indicators, which are obtained from the first three obtained eigenvalues of the principal component analysis (PCA) demodulation algorithm. These indicators can be used in the measuring process to determine a blind phase goodness assessment, without the need of using any ground truth phase information. Therefore, it is possible to perform further actions if required, as obtaining more interferograms or repeat the measure. Additionally, we present simulated and experimental results that support our mathematical analysis and conclusions. A complete MATLAB software package reproducing any result and figure shown in this work is provided in ( http://goo.gl/fy5EC ).

Description: We propose the use of graded-index few-mode fibers for mode conversion by long-period gratings (LPG) transiently written by ultrashort laser pulses using the optical Kerr effect. The mode interaction is studied by numerically solving the multi-mode coupled nonlinear Schrödinger equations. We present highly efficient conversion of the LP 01 - into the LP 11 -mode preserving the pulse shape in contrast to previous results in step-index fibers. Furthermore, mode conversion using different wavelengths for inducing and probing the LPG is shown. Due to the flat phase-matching curve of the examined modes in the graded-index fiber, mode conversion can be observed for probe center wavelengths of 1,100 nm up to 1,800 nm with a write beam centered around 1,030 nm. Therefore, a complete separation of the probe from the write beam should be possible as well as the application of optically induced guided-mode conversion for all-optical modulation across a broad wavelength range.

Description: Laser-induced fluorescence of anisole as tracer of isooctane at an excitation wavelength of 266 nm was investigated for conditions relevant to rapid compression machine studies and for more general application of internal combustion engines regarding temperature, pressure, and ambient gas composition. An optically accessible high pressure and high temperature chamber was operated by using different ambient gases (Ar, N 2 , CO 2 , air, and gas mixtures). Fluorescence experiments were investigated at a large range of pressure and temperature (0.2–4 MPa and 473–823 K). Anisole fluorescence quantum yield decreases strongly with temperature for every considered ambient gas, due to efficient radiative mechanisms of intersystem crossing. Concerning the pressure effect, the fluorescence signal decreases with increasing pressure, because increasing the collisional rate leads to more important non-radiative collisional relaxation. The quenching effect is strongly efficient in oxygen, with a fluorescence evolution described by Stern–Volmer relation. The dependence of anisole fluorescence versus thermodynamic parameters suggests the use of this tracer for temperature imaging in specific conditions detailed in this paper. The calibration procedure for temperature measurements is established for the single-excitation wavelength and two-color detection technique.

Description: A core-mode Fabry–Perot (FP) interferometer is constructed by using a dual-core photonic crystal fiber (DCPCF). The FP cavity is formed by a single piece of DCPCF, which can also serve as a direct sensing probe without any additional components. We theoretically and experimentally studied its temperature responses in the range of 40–480 °C. The temperature sensitivity is 13 pm/°C which matches the theoretical results. Since the temperature sensitivity of the proposed sensor is independent on cavity length, precise control of the length of FP cavity or photonic crystal fiber is not required. The sensor size can be as short as 100–200 μm, and its fabrication only involves splicing and cleaving, which make the sensor production very cost-effective. The proposed FP interferometric sensor based on a DCPCF can find applications in high-temperature measurement especially those that need accurate point measurement with high sensitivity.

Description: We present measurements of the absorption and emission cross-sections for Yb:YAG , Yb:LuAG and Yb:CaF 2 as a function of temperature between 80 and 340 K. The cross-sections are determined by the combination of the McCumber relation and the Fuchtbauer–Ladenburg (FL) equation to achieve reliable results in spectral regions of high and low absorption. The experimental setup used for the fluorescence measurements minimizes re-absorption effects due to the measurement from small sample volume, providing nearly undisturbed raw data for the FL approach. The retrieved cross-sections together with the spectral characteristics of the tested materials provide important information for the design of energy efficient, high-power laser amplifiers.

Description: An asymmetric chiral metamaterial (CMM) circular polarizer based on bilayer twisted split-ring resonator structure was proposed and investigated. Both numerical simulations and experiments reveal that when a y -polarized wave is incident on this CMM propagating along backward (− z ) direction, the two linear components of the transmitted wave have nearly equal amplitudes and 90°(−90°) phase difference at the resonant frequencies. This means that the right-hand circular polarization and left-hand circular polarization are realized in transmission at 6.4 and 8.1 GHz, respectively. The surface current distributions are studied to illustrate the transformation behavior for both circular polarizations. Further, the influences of the structural parameters of the circular polarizer to the transformation transmissions spectra have been investigated numerically.

Description: A tunable multiwavelength Brillouin-erbium fiber laser (MW-BEFL) using a twin-core fiber (TCF) coupler is proposed and demonstrated. The TCF coupler is formed by splicing a section of TCF between two single-mode fibers. By simply applying bending curvature on the TCF coupler, the peak net gain is shifted close to the Brillouin pump (BP), which has advantage for suppressing self-lasing cavity modes with low-BP-power injection. In this work, the dependency of the Stokes signals tuning range on the free spectral range (FSR) of TCF coupler is studied. It is also found that the tuning range of MW-BEFL can exceed the FSR of TCF coupler by adopting proper BP power and 980-nm pump power. Up to 40 nm tuning range of MW-BEFL in the absence of self-lasing cavity modes is achieved.

Description: The high-precise star sensor calibration method requires high-accurate turntable, collimator, star point plate or other high-precision devices that are very expensive. We present a simple and available method to calibrate the principal point, focal length, radial distortion, tangential distortion and installation error of star sensor in laboratory, and without having high accurate or expensive devices. The calibration model takes the ordinary camera calibration methods and installation error into account. The installation error is modeled by combination of three typical effects: the installation of pan-tilt-zoom (PTZ) initial status, PTZ and charge-coupled device, which result in six parameters. The proposed procedure consists of a closed-form solution, followed by a nonlinear refining based on maximum likelihood criterion. Our calibration method is validated through simulation and real data that shows the superiority with respect to the traditional methods and has the same level as the state-of-the-art methods. The accuracy of our calibration method is 0.015° in the root of mean square distances between testing points and projected ones.

Description: We report on the development of semiconductor double-chirped mirrors with the group delay dispersion of −3,800 ± 100 fs 2 in the wavelength range between 1,058 ÷ 1,064 nm and reflectivity of 99.1 %. The simplified plane-wave reflection transfer method was used to design the mirror multilayer stack. The mirror contains an epitaxial AlAs/GaAs structure topped with a SiNx antireflective layer.

Description: There is significant need for optical diagnostic techniques to measure instantaneous volumetric vector and scalar distributions in fluid flows and combustion processes. This is especially true for investigations where only limited optical access is available, such as in internal combustion engines, furnaces, flow reactors, etc. While techniques such as tomographic PIV for velocity measurement have emerged and reached a good level of maturity, instantaneous 3D measurements of scalar quantities are not available at the same level. Recently, developments in light field technology have progressed to a degree where implementation into scientific 3D imaging becomes feasible. Others have already demonstrated the utility of light field technology toward imaging high-contrast particles for PIV and for imaging flames when treated as single-surface objects. Here, the applicability and shortcomings of current commercially available light field technology toward volumetric imaging of translucent scalar distributions and flames are investigated. Results are presented from imaging canonical chemiluminescent and laser-induced fluorescent systems. While the current light field technology is able to qualitatively determine the position of surfaces by locating high-contrast features, the correlation-based reconstruction algorithm is unable to fully reconstruct the imaged objects for quantitative diagnostics. Current analysis algorithms are based on high-contrast correlation schemes, and new tools, possibly based on tomographic concepts, will have to be implemented to reconstruct the full 3D structure of translucent objects for quantitative analysis.

Description: We describe laser systems for photoionization, Doppler cooling, and quantum state manipulation of beryllium ions. For photoionization of neutral beryllium, we have developed a continuous-wave 235 nm source obtained by two stages of frequency doubling from a diode laser at 940 nm. The system delivers up to 400 mW at 470 nm and 28 mW at 235 nm. For control of the beryllium ion, three laser wavelengths at 313 nm are produced by sum-frequency generation and second-harmonic generation from four infrared fiber lasers. Up to 7.2 W at 626 nm and 1.9 W at 313 nm are obtained using two pump beams at 1051 and 1551 nm. Intensity drifts of around 0.5 % per hour have been measured over 8 h at a 313 nm power of 1 W. These systems have been used to load beryllium ions into a segmented ion trap.

Description: A novel approach to mass measurements at the 10 −9 level for short-lived nuclides with half-lives well below one second is presented. It is based on the projection of the radial ion motion in a Penning trap onto a position-sensitive detector. Compared with the presently employed time-of-flight ion-cyclotron-resonance technique, the novel approach is 25-times faster and provides a 40-fold gain in resolving power. Moreover, it offers a substantially higher sensitivity since just two ions are sufficient to determine the ion’s cyclotron frequency. Systematic effects specific to the technique that can change the measured cyclotron frequency are considered in detail. It is shown that the main factors that limit the maximal accuracy and resolving power of the technique are collisions of the stored ions with residual gas in the trap, the temporal instability of the trapping voltage, the anharmonicities of the trapping potential and the uncertainty introduced by the conversion of the cyclotron to magnetron motion.

Description: The kinetics of signal formation in collinear photofragmentation and atomic absorption spectroscopy (CPFAAS) are discussed, and theoretical equations describing the relation between the concentration of the target molecule and the detected atomic absorption in case of pure and impure samples are derived. The validity of the equation for pure samples is studied experimentally by comparing measured target molecule concentrations to concentrations determined using two other independent techniques. Our study shows that CPFAAS is capable of measuring target molecule concentrations from parts per billion (ppb) to hundreds of parts per million (ppm) in microsecond timescale. Moreover, the possibility to extend the dynamic range to cover eight orders of magnitude with a proper selection of fragmentation light source is discussed. The maximum deviation between the CPFAAS technique and a reference measurement technique is found to be less than 5 %. In this study, potassium chloride vapor and atomic potassium are used as a target molecule and a probed atom, respectively.

Description: The photorefractive (PR) properties of semi-insulating GaAs/AlGaAs multiple quantum wells (MQWs) operating in the Franz–Keldysh geometry are modelled by solving the material equations including the nonlinear transport of hot electrons. This work studies the PR response of MQWs in a two-wave mixing geometry under a moving grating. Calculations were made under the small intensity modulation approximation, and the simulation results are compared with experimental data available in the literature. A reasonable qualitative agreement regarding most experimental characteristics was found. The results can be treated as a test of the correctness of the commonly used band transport model of PR behaviour in MQWs. Analytic solutions for the stationary and transient regimes under negligible diffusion are given. In addition, the conditions for the occurrence of a strong resonance predicted by the model are noted.