ALBERT

Beschreibung: In this article we propose a new type of optical vortex, the X-type vortex. This vortex inherits and develops the conventional noncanonical vortex, i.e., it no longer has a constant phase gradient around the center, while the intensity keeps invariant azimuthally. The strongly focusing properties of the Xtype vortex and its effect on the beam shaping in three-dimensional (3D) fields are analyzed. The interesting phenomena, which cannot be seen in canonical vortices, are observed, for instance the `switch effect' which shows that the intensity pattern can switch from one transverse axis to another in the focal plane by controlling the phase gradient parameter. It is shown that by adjusting the phase gradient of this vortex, the focal field can have marvelous patterns, from the doughnut shape to the shapes with different lobes, and the beam along propagation direction will form a twisting shape in 3D space with controllable rotation direction and location. The physical mechanisms underlying the rule of the beam shaping are also discussed, which generally say that the phase gradient of the X-type vortex, the orbital angular momentum, the polarization and the `nongeneric' characteristic contribute differently in shaping fields. This new type of vortex may supply a new freedom for tailoring 3D optical fields, and our work will pave a way for exploration of new vortices and their applications.

Beschreibung: In this study, a femtosecond laser with a repetition frequency of 0–400 kHz was used to join soda lime glass and 304 stainless steel. The effects of single-pulse power, repetition frequency, welding speed, and defocusing on the weld quality were investigated. The joining mechanism and fracture surface morphologies were studied using scanning electron microscopy and x-ray diffraction analysis. The results show that no new phases were formed between the glass and stainless steel, and that the joining mechanism consisted mainly of mechanical mixing between the two materials. Using a suitable combination of process parameters, a good weld with a strength of 8.79 MPa was obtained. The weld strength was influenced mainly by the amount of glass that adhered to the stainless steel, the bonding strength between the glass base material and the remelted glass, and the wetting of the stainless steel by the molten glass.

Beschreibung: The aim of the present work is to introduce a lens whose faces are a conical surface and a spherical surface. We illuminate this lens by a plane wavefront and its associated refracted wavefronts, light rays and caustic are computed. We find that the caustic region has two branches and can be virtual, real or one part virtual and the other real, depending on the values of the parameters characterizing the lens. Furthermore, we present a particular example where one of the branches of the caustic region is constituted by two segments of a line, one part is real and the other one virtual. The second branch is a two-dimensional surface with a singularity of the cusp ridge type such that its Gaussian curvature is different from zero. It is important to remark that for this example, the two branches of the caustic are disconnected. Because of this property and the result obtained by Berry and Balazs on the relationship between the acceleration of an Airy beam and the curvature of its corresponding caustic, we believe that using this optical element one could generate a scalar optical accelerating beam in the region where the caustic is a two-dimensional surface of revolution, and at the same time a scalar optical beam with similar properties to the Bessel beam of zero order in the region were the real caustic is a segment of a line along the optical axis.

Beschreibung: In this study, n-dodecane used as the coupling layer for reusable acoustofluidic microchips was investigated. n-dodecane has the similar viscosity to that of water. However, it possesses much lower surface tension and higher boiling point compared to water. When dispensing a droplet of n-dodecane on lithium niobate (LiNO3) substrate with interdigital transducers and placing the polydimethylsiloxane microchannel on top of it, n-dodecane can easily wick through and completely occupy the interstitial space. Moreover, it can be readily removed from the substrate without leaving residue. The experimental results showed that the reusable acoustofluidic microchips can be operated at higher input voltages and longer duration when applying n-dodecane as the coupling layer. Attenuation of the acoustic radiant force was observed through decrease of the particle velocity, which is in agreement with the literature results. Decreasing the thickness of the coupling layer can alleviate the attenuation effect and a linear relationship between particle velocity and thickness on a semi-log plot was obtained.

Beschreibung: Accurate prediction of the East Asian summer monsoon (EASM) is beneficial to billions of people’s production and lives. Here convolutional neural networks (CNN) and transfer learning are used for predicting the EASM. The results of the constructed CNN regression model show that the prediction of the CNN regression model is highly consistent with the reanalysis dataset, with correlation coefficient of 0.78, which is higher than that of each of the current state-of-the-art dynamic models. The heat map method indicates that the robust precursor signals in the CNN regression model agree well with previous theoretical studies, and can provide the quantitative contribution of different signals for EASM prediction. The CNN regression model can predict the EASM one year ahead with a confidence level above 95%. The above method can not only improve the prediction of the EASM but also help to identify the involved physical predictors.

Beschreibung: Objective. The role of the crypt microarchitecture and surrounding tissue curvature on intestinal stem/proliferative cell physiology is unknown. The utility of liquid lithography in creating polydimethylsiloxane (PDMS) micropillar stamps with controlled tip curvature was assessed. Using these stamps, the impact of microcurvature at the crypt base on intestinal cell and cytoskeletal behavior was studied. Approach. An SU-8 master mold as a support, polyols of varying surface energies as sacrificial liquids, and liquid PDMS as the solidifiable material were combined using liquid lithography to form PDMS micropillar arrays. Vapor phase deposition of organosilane onto the master mold was used to modify the surface energy of the master mold to shape the micropillar tips. Collagen was molded using the micropillar arrays forming a scaffold for culture of human primary colonic epithelial cells. Cell proliferation and cytoskeletal properties were assessed using fluorescent stains. Main results. Liquid lithography using low surface energy polyols (55 dynes cm−1) yielded concave-tipped PDMS micropillars. Gradients of octyltrichlorosilane deposition across a master mold with an array of microwells yielded a PDMS micropillar array with a range of tip curvatures. Human primary colonic epithelial cells cultured on micropillar-molded collagen scaffolds demonstrated a stem/proliferative cell compartment at the crypt base. Crypts with a convex base demonstrated significantly lower cell proliferation at the crypt base than that of cells in crypts with either flat or concave bases. Crypts with a convex base also displayed higher levels of G-actin activity compared to that of crypts with flat or concave bases. Significance. Liquid lithography enabled creation of arrays of in vitro colonic crypts with programmable curvature. Primary cells at the crypt base sensed and responded to surface curvature by altering their proliferation and cytoskeletal properties.

Beschreibung: The generation of a large cold plasma jet while maintaining the reproducibility and homogeneity of the discharge is one of the major challenges encountered by the plasma community to efficiently apply this technology in the industry. Here, we report on the discharge in a recently developed device called the plasma candle (PC), wherein a stable plasma jet with a diameter of 20 mm can be generated at atmospheric pressure and temperature. Unlike the discharge morphology previously reported for conventional plasma jet devices, the unique configuration of PC device resulted in distinctive discharge patterns. Homogenous discharge was generated in the electrode gap and followed by a swirling discharge toward the tube nozzle. Fast photography and electrical measurements revealed that filament propagation and its morphology form the visually observable swirl discharge. Detailed analysis indicated that residual helium metastable species (Hem) and their penning ionization play an essential role in the discharge mode and its transition, which was verified by changing the feeding gas and the frequency of the applied voltage. For instance, it is found that only filamentary discharge was observed along the entire tube at frequencies less than 3 kHz, at which the time between consecutive discharges was long enough for Hem decay. Consequently, the homogenous discharge pattern was recovered by increasing the pre-ionization levels by adding a trace of impurities (N2, O2 or H2O) to the feeding gas. However, the level of these impurities must be carefully adjusted to achieve a homogenous discharge without negatively affecting the jet properties. A trivial change in the gas impurity, in the range of adsorption and desorption of water from the gas tubing, is sufficient to cause a noticeable change and instability in the discharge mode. This finding is critical to predicting the production of reactive species and plasma-surface interaction for different applications.

Beschreibung: MnBi 2 n Te 3 n + 1 (MBT) is the first intrinsic magnetic topological insulator and is promising to host emergent phenomena such as quantum anomalous Hall effect. They can be made ferromagnetic by having n   ⩾   4 or with Sb doping. We studied the magnetic dynamics in a few selected ferromagnetic (FM) MBT compounds, including MnBi8Te13 and Sb doped MnBi 2 n Te 3 n + 1 ( n = 2 , 3 ) using AC susceptibility and magneto-optical imaging. Slow relaxation behavior is observed in all three compounds, suggesting its universality among FM MBT. We attribute the origin of the relaxation behavior to the irreversible domain movements since they only appear below the saturation fields when ferromagnetic domains form. The very soft ferromagnetic domain nature is revealed by the low-field fine-structured domains and high-field sea-urchin-shaped remanent-state domains imaged via our magneto-optical measurements. Finally, we ascribe the rare ‘double-peak’ behavior observed in the AC susceptibility under small DC bias fields to the very soft ferromagnetic domain formations.