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  • 1
    Publication Date: 2015-04-01
    Description: The lock-in amplifier is a critical component in many different types of experiments, because of its ability to reduce spurious or environmental noise components by restricting detection to a single frequency and phase. One example application is pump-probe microscopy, a multiphoton technique that leverages excited-state dynamics for imaging contrast. With this application in mind, we present here the design and implementation of a high-speed lock-in amplifier on the field-programmable gate array (FPGA) coprocessor of a data acquisition board. The most important advantage is the inherent ability to filter signals based on more complex modulation patterns. As an example, we use the flexibility of the FPGA approach to enable a novel pump-probe detection scheme based on spread-spectrum communications techniques.
    Print ISSN: 0034-6748
    Electronic ISSN: 1089-7623
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
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  • 2
    Publication Date: 2016-03-15
    Description: Multiphoton microscopy has rapidly gained popularity in biomedical imaging and materials science because of its ability to provide three-dimensional images at high spatial and temporal resolution even in optically scattering environments. Currently the majority of commercial and home-built devices are based on two-photon fluorescence and harmonic generation contrast. These two contrast mechanisms are relatively easy to measure but can access only a limited range of endogenous targets. Recent developments in fast laser pulse generation, pulse shaping, and detection technology have made accessible a wide range of optical contrasts that utilize multiple pulses of different colors. Molecular excitation with multiple pulses offers a large number of adjustable parameters. For example, in two-pulse pump-probe microscopy, one can vary the wavelength of each excitation pulse, the detection wavelength, the timing between the excitation pulses, and the detection gating window after excitation. Such a large parameter space can provide much greater molecular specificity than existing single-color techniques and allow for structural and functional imaging without the need for exogenous dyes and labels, which might interfere with the system under study. In this review, we provide a tutorial overview, covering principles of pump-probe microscopy and experimental setup, challenges associated with signal detection and data processing, and an overview of applications.
    Print ISSN: 0034-6748
    Electronic ISSN: 1089-7623
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 68 (1990), S. 2983-2988 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: This paper presents a study of the electric field induced migration of Ga and Sb ions during anodic oxidation of Al films on GaSb. Rutherford backscattering spectrometry was used to examine the composition and thickness of the anodic oxides. Oxide thickening was found to occur at the electrolyte oxide and also at the GaSb oxide substrate interface. It was found that Ga and Sb were mobile under the growth conditions and mobile ions resulted in the formation of a surface GaSb oxide. From measurments of the thickness we are able to report on the transport number for the anion and cations in this system. The growth of the surface GaSb oxide is discussed with reference to the intrinsic oxide growth rates in the Al and GaSb.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 7108-7116 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: After UV-laser-induced desorption we observe bimodal velocity distributions independent of internal vibrational excitation [up to v=2 (4%)] applying resonance-enhanced multiphoton ionization techniques. Both contributing desorption channels are of nonthermal origin. We introduce a model where the two desorption channels are correlated with the rupture of the molecule surface bond of the librating molecule either on the way toward or away from the surface. We have performed trajectory calculations to simulate the desorption processes. The calculated momentum distributions of the desorbing molecules show either one or two maxima, depending on lifetime, in agreement with experimental results. The vibrational distribution of the desorbing molecules can be reproduced by assuming transition into a state that is characterized by an altered N–O bond length as it is found, for example, in NO−. The model calculations both for velocity distributions and vibrational excitations result in similar lifetimes of the excited state, even though the translational and the vibrational degree of freedom of the desorbing molecules are decoupled.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 65 (1989), S. 2479-2484 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Anodic oxidation of thin Al films on GaAs beyond the exhaustion point is shown to induce substrate GaAs migration into and through the Al oxide layer. Continued anodization results in a triple-layer structure composed of a surface GaAs oxide layer superimposed on top of a mixed Al oxide layer on a substrate GaAs oxide layer. The thicknesses and composition of these Al/GaAs composite oxides are measured using Rutherford backscattering spectrometry and are related to the specific final anodic voltages. The results show that, upon completion of the anodization of the Al layer, the Ga and or As ions drift under the action of the anodic field through the Al oxide. Eventually a new oxide is formed at the GaAs/oxide interface and also at the electrolyte/oxide interface in the ratio of 64:36.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 66 (1995), S. 3331-3333 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We prepared symmetric sandwich structures of the sequence gold/Langmuir–Blodgett film/gold by employing a novel evaporation technique. As molecular systems, we used an octasubstituted palladiumphthalocyanine and a perylene-3,4,9,10-tetracarboxyldiimide derivative. Electronic transport measurements show a tunneling characteristic which arises from a direct tunneling process through molecular states. The current/voltage curves are symmetric for positive and negative bias proving the identical electronic behavior of the two organic/inorganic interfaces at the top and bottom electrode. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 3318-3325 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have studied the UV-laser-induced desorption of NO adsorbed on an epitaxial film of NiO(111) grown on Ni(111). The desorbing molecules were detected state selectively via a resonance enhanced ionization technique [REMPI(1+1)] using the A 2Σ(v'=0,1,2)←X 2Π(v‘=0,1,2) transition as intermediate state. Our results are compared with our experiments on NO desorption from NiO(100). The similarities and differences of the results due to the different surface structure of the polar NiO(111) and the non polar NiO(100) are discussed. For both surfaces we observe bimodal velocity flux distributions independent of the rovibrational state. Due to a rotational temperature of about 400 K and a vibrational temperature of 1800 K thermal processes can be ruled out. The wavelength dependence of the desorption cross section strongly correlates with the electronic structure of the NiO indicating a surface mediated excitation process. The spin orientation in the NO molecules influences the life time of the excited state depending on the magnetic property of the NiO surface.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 67 (1990), S. 2608-2612 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: This paper presents the results of a study of the anodic oxidation of n-type AlGaAs and AlGaAs/GaAs heterojunctions in acid-glycol-water electrolytes. Anodic oxides with thickness up to 300 nm may easily be grown under constant-current conditions if external illumination is used. Real-time derivative spectra of the anodic voltage-time data show structure that is dependent upon the thickness of the AlGaAs epitaxial layer, its doping density, and possibly band-gap energy differences. The derivative results clearly show that epitaxial layer thickness and thickness variations are readily measured. The oxide growth constant and material consumption rate are found to be 1.52 and 1.08 nm/V, respectively. Optical dispersion in the wavelength range from the near UV to the IR range is also measured and fitted to Sellmeier's equation. Composition of the AlGaAs is measured using Rutherford backscattering, which reveals an As-rich oxide.
    Type of Medium: Electronic Resource
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  • 9
    Publication Date: 2015-08-28
    Description: We have measured the Landau-level spin-splitting of two-dimensional electrons in the composite InAs/InGaAs channels of two InAs/InGaAs/InAlAs heterostructures with different alloy compositions by magnetotransport and THz magneto-photoconductivity in magnetic fields up to 10 T. The structures differ importantly in the mobility of the channel, the electron density and the composition of the barriers. The magnitudes of the experimental g-factors for B along the quantization axis and their anisotropies are larger by at least a factor of 2 than the corresponding calculated single particle values. The angular dependence of many-body exchange contributions and the effects of broadening of Landau–level densities of states are necessary for understanding this behaviour. We find evidence for a marked decrease of the exchange contribution at low perpendicular magnetic fields in the higher mobility sample from coincidence measurements, but no indications of such behaviour in the lower mobility sample.
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
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