- 标题
- 摘要
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- 实验方案
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Super-Oscillatory Metalens at Terahertz for Enhanced Focusing with Reduced Side Lobes
摘要： In this paper, we design and numerically demonstrate an ultra-thin super-oscillatory metalens with a resolution below the diffraction limit. The zones of the lens are implemented using metasurface concepts with hexagonal unit cells. This way, the transparency and, hence, ef?ciency is optimized, compared to the conventional transparent–opaque zoning approach that introduces, inevitably, a high re?ection in the opaque regions. Furthermore, a novel two-step optimization technique, based on evolutionary algorithms, is developed to reduce the side lobes and boost the intensity at the focus. After the design process, we demonstrate that the metalens is able to generate a focal spot of 0.46λ0 (1.4 times below the resolution limit) at the design focal length of 10λ0 with reduced side lobes (the side lobe level being approximately ?11 dB). The metalens is optimized at 0.327 THz, and has been validated with numerical simulations.
关键词： metamaterials,evolutionary algorithm,metasurfaces,terahertz,metalens,terahertz focusing,subwavelength focusing,super-oscillatory lens
更新于2025-09-09 09:28:46
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[IEEE 2018 20th International Conference on Transparent Optical Networks (ICTON) - Bucharest (2018.7.1-2018.7.5)] 2018 20th International Conference on Transparent Optical Networks (ICTON) - Localized Photonic Jets Generated by Step-Like Dielectric Microstructures
摘要： In this contribution we reveal how the step-like topology of a microstructure can contribute to the formation of a single high-intensity nanojet (NJ) beam located on its axis of symmetry. The proposed method for generating condensed optical NJ beams relies on the complex electromagnetic phenomenon associated with the light diffraction on the edges of step-like dielectric microstructures embedded in a host medium with lower refractive index. The possibility of NJ beam intensification in the near zone of such microstructure illuminated by a plane wave is demonstrated and explained by the recombination of multiple NJ beams associated with different edges or edge segments of the step-like microlens. We demonstrate that by changing materials of the layers we can intensify the NJ beam. We examine the dependence of the generated beam on the step size, shape and material.
关键词： near field focusing,microlens,photonic nanojet,edge diffraction
更新于2025-09-09 09:28:46
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Deep-subwavelength control of acoustic waves in an ultra-compact metasurface lens
摘要： Space-coiling acoustic metasurfaces have been largely exploited and shown their outstanding wave manipulation capacity. However, they are complex in realization and cannot directly manipulate acoustic near-fields by controlling the effective path length. Here, we propose a comprehensive paradigm for acoustic metasurfaces to extend the wave manipulations to both far- and near-fields and markedly reduce the implementation complexity with a simple structure, which consists of an array of deep-subwavelength-spaced slits perforated in a thin plate. A semi-analytical approach for such a design is established using a microscopic coupled-wave model, which reveals that the acoustic diffractive pattern at every slit exit is the sum of the initial transmission and the secondary scatterings of the coupled fields from other slits. For proof-of-concept, we examine two metasurface lenses for sound focusing within and beyond the diffraction limit. This work provides a feasible strategy for creating ultra-compact acoustic components with versatile potentials.
关键词： acoustic metasurfaces,sound focusing,microscopic coupled-wave model,wave manipulation,deep-subwavelength
更新于2025-09-09 09:28:46
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Riemann problems and dispersive shocks in self-focusing media
摘要： The dynamical behavior resulting from an initial discontinuity in focusing media is investigated using a combination of numerical simulations and Whitham modulation theory for the focusing nonlinear Schr?dinger equation. Initial conditions with a jump in either or both the amplitude and the local wave number are considered. It is shown analytically and numerically that the space-time plane divides into expanding domains in which the solution is described by a slow modulation of genus-0, genus-1 or genus-2 solutions, their precise arrangement depending on the specifics of the initial datum.
关键词： dispersive shocks,self-focusing media,Riemann problems,nonlinear Schr?dinger equation,Whitham modulation theory
更新于2025-09-09 09:28:46
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Highly c-axis oriented crystal growth induced giant unusual saturable absorption behavior in wet chemically grown ZnO thin films
摘要： Wet-chemical sol-gel spin coating technique was considered to fabricate ZnO thin films on bare glass substrates with different thickness values (30 nm, 90 nm, 110 nm, 150 nm). The XRD reflections showed a transition from semi-polar crystal growth orientation to polar crystal growth orientation when the thickness was reduced to 30 nm. The nonlinear characteristics of prepared nano-films were quantized by performing open aperture and closed aperture Z-scan measurements under a continuous wavelength regime. The open aperture Z-scan transmittance curves showed a remarkable transition from normalized valley to normalized peak in 30 nm thick ZnO film. The presence of normalized valley and peak were attributed to reverse saturable absorption (RSA) and saturable absorption (SA) behavior respectively. The RSA behavior was attributed to well known two-photon absorption (TPA) mechanisms and the SA behavior was due to the reduction in electron-hole wave function overlapping along c-axis. The closed aperture Z-scan measurement of 30 nm thick ZnO film revealed the violation of two parabolic band (TPB) and Kramer-Kronig relations by exhibiting self-focusing mechanism. The self-focusing mechanism was attributed to thermal effects due to the smaller thickness of ZnO. However, self-defocusing behavior was observed in 90 nm, 110 nm, and 150 nm thick ZnO films.
关键词： electron-hole wavefunction,saturable absorption,polar ZnO,thermo-optic effect,self-focusing
更新于2025-09-09 09:28:46
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Focus Assessment Method of Gaze Tracking Camera Based on ε-Support Vector Regression
摘要： In order to capture an eye image of high quality in a gaze-tracking camera, an auto-focusing mechanism is used, which requires accurate focus assessment. Although there has been previous research on focus assessment in the spatial or wavelet domains, there are few previous studies that combine all of the methods of spatial and wavelet domains. Since all of the previous focus assessments in the spatial or wavelet domain methods have disadvantages, such as being affected by illumination variation, etc., we propose a new focus assessment method by combining the spatial and wavelet domain methods for the gaze-tracking camera. This research is novel in the following three ways, in comparison with the previous methods. First, the proposed focus assessment method combines the advantages of spatial and wavelet domain methods by using ε-support vector regression (SVR) with a symmetrical Gaussian radial basis function (RBF) kernel. In order to prevent the focus score from being affected by a change in image brightness, both linear and nonlinear normalizations are adopted in the focus score calculation. Second, based on the camera optics, we mathematically prove the reason for the increase in the focus score in the case of daytime images or a brighter illuminator compared to nighttime images or a darker illuminator. Third, we propose a new criterion to compare the accuracies of the focus measurement methods. This criterion is based on the ratio of relative overlapping amount (standard deviation of focus score) between two adjacent positions along the Z-axis to the entire range of focus score variety between these two points. Experimental results showed that the proposed method outperforms other methods.
关键词： ε-support vector regression with a symmetrical Gaussian radial basis function kernel,auto-focusing,focus assessment,camera optics,gaze-tracking camera
更新于2025-09-09 09:28:46
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Sample Volume Reduction Using the Schwarzschild Objective for a Circular Dichroism Spectrophotometer and an Application to the Structural Analysis of Lysine-36 Trimethylated Histone H3 Protein
摘要： With the increasing interest in scarce proteins, reducing the sample volume for circular dichroism (CD) spectroscopy has become desirable. Demagni?cation of the incident beam size is required to reduce the sample volume for CD spectroscopy detecting transmitted light passed through the sample. In this study, the beam size was demagni?ed using a focal mirror, and small-capacity sample cells were developed in an attempt to reduce the sample volume. The original beam size was 6 × 6 mm2; we successfully converged it to a size of 25 × 25 μm2 using the Schwarzschild objective (SO). The new sample cell and SO allowed the required sample volume to be reduced to 1/10 (15 → 1.5 μL), when using a 15 μm path length cell. By adopting a smaller sample cell, further sample reduction could be achieved. By using the SO system, the secondary structural contents of the lysine-36 trimethylated histone H3 protein were analyzed. The trimethylation induced the increment of helix structures and decrement of unordered structures. These structural alterations may play a role in regulating cellular function(s), such as DNA damage repair processes.
关键词： post-translational modi?cation,Schwarzschild objective,histone,beam focusing,structural alteration
更新于2025-09-09 09:28:46
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Composite spiral zone plate
摘要： A new kind of optical element, i.e., composite spiral zone plate (CSZP), is proposed to generate a twin-vortex beam along the optic axis in this paper. The axial diffraction property of the CSZP is analyzed theoretically and experimentally. The results demonstrate that the CSZP can possess two focused vortices with the same intensity peak, different topological charges, and customized axial positions. The proposed optical device exhibits significant potential for various applications including three-dimensional optical manipulation.
关键词： Composite spiral zone plate (CSZP),Focusing property,Spiral zone plate (SZP)
更新于2025-09-09 09:28:46
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Brightening of a supersonic beam of neutral atoms
摘要： Well-collimated, high-intensity beams of neutral atoms have many applications ranging from atom microscopy to atom interferometry to ultracold atomic physics. We experimentally demonstrate a method for brightening a supersonic atomic beam and observe an increase in the phase space density by a factor of at least 20. Our scheme relies upon a permanent magnetic hexapole lens to focus a divergent beam of neutral atoms emitted from a supersonic nozzle and transverse laser cooling as the beam converges downstream from the lens so as to create a dense, narrow, Doppler-collimated atomic beam. In principle, this method can be repeated multiple times in series for further beam brightening.
关键词： atomic focusing,beam brightening,supersonic beam,laser cooling,magnetic hexapole
更新于2025-09-09 09:28:46
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[IEEE ICASSP 2018 - 2018 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP) - Calgary, AB (2018.4.15-2018.4.20)] 2018 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP) - Real-Time Total Focusing Method for Ultrasonic Imaging of Multilayered Object
摘要： In imaging of multilayered object, total focusing method (TFM) combined with ray-tracing appeals to numerous researchers because of its superior image quality. But lots of iterative computation to calculate the incident points greatly increases its processing time. Based on line scan-conversion algorithm and Snell’s principle, this paper proposes an efficient method to calculate the incident point. By dividing the image area into multiple regions, the time-consuming iterative calculation of point of incidence (POI) is eliminated. After introducing this method into TFM, the time complexity of Region-Division (RD) TFM is reduced by one order of magnitude. The simulation experiments validate that the imaging speed of RD-TFM is improved dozens of times compared to traditional TFM with ray-tracing. Besides, RD-TFM applies to multilayered objects with both planar and non-planar interface.
关键词： Ultrasonic phased array,Multi-layered object,Full matrix capture,Total focusing method
更新于2025-09-09 09:28:46