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Linear and Nonlinear Optical Properties of Few-Layer Exfoliated SnSe Nanosheets
摘要： Monochalcogenides of group IV elements have been considered as phosphorene analogs due to their similar crystal and electronic structure. Here, few-layer SnSe nanosheets are synthesized by a sonication-assisted liquid phase exfoliation process and their linear and nonlinear optical properties are examined. The as-exfoliated few-layer (FL) SnSe demonstrates layer thickness from 2 to 10 nm and lateral size of 150 nm with high crystallinity. Optical measurement confirms the layer-number dependent bandgap, which is also corroborated by first-principle calculation. In addition, the FL SnSe shows a transition from saturable absorption to reverse saturable absorption with the increase of pumping power, and the nonlinear absorption is characterized by an ultrafast response time of picosecond scale. The nonlinear optical response in FL SnSe could be explained by defect-involved recombination of photogenerated carriers as well as the Auger scattering process, which is further enhanced by structural two-dimensionality.
关键词： nonlinear optics,phosphorene analogue,SnSe,liquid-phase exfoliation,2D semiconductors
更新于2025-09-23 15:23:52
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Assessment of Different Sampling Methods for Measuring and Representing Macular Cone Density Using Flood-Illuminated Adaptive Optics
摘要： PURPOSE. To describe a standardized flood-illuminated adaptive optics (AO) imaging protocol suitable for the clinical setting and to assess sampling methods for measuring cone density. METHODS. Cone density was calculated following three measurement protocols: 50 × 50-μm sampling window values every 0.5° along the horizontal and vertical meridians (fixed-interval method), the mean density of expanding 0.5°-wide arcuate areas in the nasal, temporal, superior, and inferior quadrants (arcuate mean method), and the peak cone density of a 50 × 50-μm sampling window within expanding arcuate areas near the meridian (peak density method). Repeated imaging was performed in nine subjects to determine intersession repeatability of cone density. RESULTS. Cone density montages could be created for 67 of the 74 subjects. Image quality was determined to be adequate for automated cone counting for 35 (52%) of the 67 subjects. We found that cone density varied with different sampling methods and regions tested. In the nasal and temporal quadrants, peak density most closely resembled histological data, whereas the arcuate mean and fixed-interval methods tended to underestimate the density compared with histological data. However, in the inferior and superior quadrants, arcuate mean and fixed-interval methods most closely matched histological data, whereas the peak density method overestimated cone density compared with histological data. Intersession repeatability testing showed that repeatability was greatest when sampling by arcuate mean and lowest when sampling by fixed interval. CONCLUSIONS. We show that different methods of sampling can significantly affect cone density measurements. Therefore, care must be taken when interpreting cone density results, even in a normal population.
关键词： cone photoreceptor density,retina,adaptive optics
更新于2025-09-23 15:23:52
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Amplification of the Zeroth Order Mode in Ultra-thin Layers
摘要： The ultra-thin limit regime is characterized by a structure which at least in one dimension is signi?cantly smaller than the incoming light wavelength. The absorption in this regime is characterized either by very weak absorption or by a broad peak attributed to the zeroth order Fabry-Perot mode. We show that this mode provides a 14.5% ultimate ef?ciency in a 25 nm GaAs slab on a gold substrate. GaAs in air achieves only a 5% for a 10 nm slab. Such an ampli?cation of the resonance absorption is attributed to the high losses of the substrate. For very high losses, the zeroth order mode transits from an over-damped regime to an under-damped one.
关键词： absorption,Solar cells,optical coating,quasinormal modes,optics
更新于2025-09-23 15:23:52
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Analysis of Light Propagation on Physiological Properties of Neurons for Nanoscale Optogenetics
摘要： Miniaturization of implantable devices is an important challenge for future Brain-Computer Interface applications, and in particular for achieving precise neuron stimulation. For stimulation that utilizes light, i.e., optogenetics, the light propagation behavior and interaction at the nanoscale with elements within the neuron is an important factor that needs to be considered when designing the device. This paper analyses the effect of light behavior for a single neuron stimulation, and focuses on the impact from different cell shapes. Based on the Mie Scattering theory, the paper analyzes how the shape of the soma and the nucleus contributes to the focusing effect resulting in an intensity increase, which ensures that neurons can assist in transferring light through the tissue towards the target cells. At the same time, this intensity increase can in turn also stimulate neighboring cells leading to interference within the neural circuits. The paper also analyzes the ideal placements of the device with respect to the angle and position within the cortex that can enable axonal biophoton communications, which can contain light within the cell to avoid interference.
关键词： Optogenetics,Geometrical Optics Analysis,Nano Communications,Mie scattering
更新于2025-09-23 15:23:52
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Phase Conjugation and Mode Conversion in Stimulated Parametric Down-Conversion with Orbital Angular Momentum: a Geometrical Interpretation
摘要： We report on an experiment that investigates the spatial mode conversion in the process of parametric down-conversion seeded by a light beam in a superposition of orbital angular momentum modes. This process is interpreted in terms of a geometric representation of first-order spatial modes in a Poincaré sphere, providing an intuitive image of the phase conjugation and the topological charge conservation. We also make a comparison with the analogous phenomenon for optical parametric oscillators.
关键词： Orbital angular momentum,Parametric down-conversion,Optics,Phase conjugation
更新于2025-09-23 15:23:52
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OPD analysis and prediction in aero-optics based on dictionary learning
摘要： When aircraft ?ying at a high speed, the density and re?ective index of atmosphere around it become uneven. Thus images or videos observed from the observation window on the aircraft are usually blur or quivering, which is called the aero-optical effect. To recover the images from poor quality, it is necessary to learn about the wavefront distortion of the light, described as optical path difference (OPD). Among the existing methods, the method of computational ?uid dynamics (CFD) simulation followed by ray tracing is very time consuming, and the method of real-time OPD measurement with OPD sensor has a certain lag for OPD with high frequency. In this paper, a reconstructible dimension reduction method based on dictionary learning is employed to map the high-dimensional OPD data into a low-dimensional space, and the OPD data are calculated when rays travel across the supersonic shear layer. All the parameters of training and test datasets remain the same except the convective Mach numbers (Mc number). According to the dimension reduction results of training sets, we ?nd that OPD is obviously periodic and its distribution characteristics have a strong correlation with Mc number. By ?tting the OPD data in the low-dimensional space, every point on the ?tting curve can be reconstructed to the original high-dimensional space, which works as prediction. Compared with the truthful data, the average similarity coef?cient of the prediction for the test datasets is up to 83%, which means that the prediction result is credible.
关键词： Prediction,Aero-optics,Couple dictionary learning,Optical path difference
更新于2025-09-23 15:23:52
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Study of the scattered light with the strip beam incident to the LiNbO3:Fe crystal
摘要： In this paper, we systematically studied the scattered light when the stripe beam of ordinary polarized light and extraordinary polarized light are incident to the LiNbO3:Fe crystal, respectively. First, we observed the variation of scattered light with the different angle between the stripe beam and the c-axis. And find the scattered light is growing with time along the c-axis direction of the crystal when the e-light is incident; while the scattered light is growing along a special direction which is not the c-axis direction or the stripe direction, when o-light is incident to the crystal. Then we analyzed the causes of these phenomena. In addition, we also studied the diffraction efficiency of o-light and e-light, and the raster image in the crystal.
关键词： nonlinear optics,light scattering,photorefractive effect
更新于2025-09-23 15:23:52
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Using the multi-object adaptive optics demonstrator RAVEN to observe metal-poor stars in and towards the Galactic Centre
摘要： The chemical abundances for five metal-poor stars in and towards the Galactic bulge have been determined from the H-band infrared spectroscopy taken with the RAVEN multi-object adaptive optics science demonstrator and the Infrared Camera and Spectrograph at the Subaru 8.2-m telescope. Three of these stars are in the Galactic bulge and have metallicities between ?2.1 < [Fe/H] < ?1.5, and high [α/Fe] ～ +0.3, typical of Galactic disc and bulge stars in this metallicity range; [Al/Fe] and [N/Fe] are also high, whereas [C/Fe] < +0.3. An examination of their orbits suggests that two of these stars may be confined to the Galactic bulge and one is a halo trespasser, though proper motion values used to calculate orbits are quite uncertain. An additional two stars in the globular cluster M22 show [Fe/H] values consistent to within 1σ, although one of these two stars has [Fe/H] = ?2.01 ± 0.09, which is on the low end for this cluster. The [α/Fe] and [Ni/Fe] values differ by 2σ, with the most metal-poor star showing significantly higher values for these elements. M22 is known to show element abundance variations, consistent with a multipopulation scenario though our results cannot discriminate this clearly given our abundance uncertainties. This is the first science demonstration of multi-object adaptive optics with high-resolution infrared spectroscopy, and we also discuss the feasibility of this technique for use in the upcoming era of 30-m class telescope facilities.
关键词： stars: abundances,Galaxy: centre,instrumentation: adaptive optics,globular clusters: individual: M22,techniques: spectroscopic
更新于2025-09-23 15:23:52
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Study of the effects of phytoplankton morphology and vertical profile on lidar attenuated backscatter and depolarization ratio
摘要： Propagation of a lidar beam in a coupled atmosphere-ocean model consisting of multiple atmospheric and upper oceanic layers and a rough ocean surface is studied by using a vectorized Monte Carlo radiative transfer solver optimized specifically for lidar-based remote sensing applications. The effects of assumed phytoplankton morphology variations and its vertical distribution on the lidar attenuated backscatter and depolarization ratio are studied. In this study, a phytoplankton particle is assumed to be a sphere, a sphere with a core, or a randomly distorted hexahedron with or without a core. The single-scattering properties of the nonspherical/inhomogeneous particles are computed using appropriate state-of-the-art light-scattering computational capabilities. Vertical variation of the phytoplankton distribution is derived explicitly using a PAR (photosynthetically active radiation) limited carbon biomass balance equation that is subsequently coupled with the Monte Carlo solver.
关键词： Radiative transfer,Lidar,Ocean optics,Monte Carlo,Phytoplankton,Net primary production,Remote sensing
更新于2025-09-23 15:23:52
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Microfluidics Integrated Lithography-Free Nanophotonic Biosensor for the Detection of Small Molecules
摘要： Currently, labeling techniques are mostly used to study the binding kinetics of small molecules and their detection (molecular weights <500 Daltons) at low analyte concentrations. However, the fluorescent tags used in this method for conjugation may modify or prevent the functionality of the targeted molecules. Although label-free techniques provide more accurate quantitative and kinetic measurements by monitoring the binding of analytes in their usual forms, the monitoring of small molecule binding at lower concentrations is still impossible using conventional refractometric sensing devices. Here, a label-free and cost-effective technique is demonstrated for the detection of ultralow molecular weight biomolecules such as biotin (244 Daltons) at very low concentrations. This extreme sensitivity is realized by utilizing the phase singularity feature of lithography-free nanophotonic cavities. The proposed four-layered metal-dielectric-dielectric-metal asymmetric Fabry–Perot cavity exhibits point-of-darkness and phase singularity at the Brewster angle. Since the singular phase behavior depends on the topology of the cavity surface, it is very challenging to realize singular phase in reflection mode for a microfluidics integrated nanophotonic cavity. To overcome this issue, a specially designed polymethylmethacrylate microfluidic channel is integrated with the cavity.
关键词： flat-optics,small biomolecules,Fabry–Perot cavities,lithography-free,label-free optical biosensors,biosensors
更新于2025-09-23 15:23:52