Incident Light Beam Research Articles

GaAs gradient metasurface for vortex beam emission with high deflection angles.

This study experimentally demonstrates metasurfaces capable of producing output emissions with deflection angles greater than 75 degrees. These metasurfaces are composed of high-aspect-ratio gallium arsenide (GaAs) nano-resonators on double-sided polished GaAs substrates, operating reflectively at a wavelength of 650 nm. Beyond deflecting the incident light beam, the metasurfaces are successfully shown to generate high-deflection-angle, doughnut-shaped emissions by incorporating vortex beam (VB) structured light with a topological charge of up to 8. The broadband capabilities of these metasurfaces are also explored in this work. This work significantly advances the manipulation of structured light. Achieving high deflection angles opens new pathways for developing compact optical systems that leverage the unique properties of vortex beams in a range of applications.

Roll angle measurement method based on a double-sided optical wedge

The roll angle is an important albeit difficult-to-measure geometric parameter. This study proposes a roll angle measurement method for an entire circle (360°). An elaborately designed double-sided optical wedge (DOW) is employed as a roll angle probe to divide the incident light beam into two beams. Both beams are imaged using a photodetector, and the roll angle of the DOW is obtained through the detection of the motion trajectories of the two images. The measurement principle is comprehensively analyzed, and the experimental results indicate that the accuracy of the constructed measurement system is better than ±7.72arcsec in the range of 360°. The proposed method is confirmed to be effective for large-range roll angle measurements.

Observation of the Goos-Hänchen shift in monolayer WSe2 for an arbitrary linearly polarized incident light beam using weak measurement

We report, to our knowledge, the first experimental investigation of the spatial Goos-Hänchen (GH) shift at an absorbing material interface comprised of monolayer (ML) tungsten di-selenide (WSe2) on a SiO2/Si substrate under a total internal reflection (TIR) condition. The critical angle for this design is drastically shifted to 23.31°, compared to the glass-air interface, which was at 41.3°. Utilizing the weak value amplification (WVA) approach, the behavior of spatial GH shifts at this interface with various regulating parameters such as angle of incidence, polarization angle, and post-selection angle has systematically been studied. At critical incidence, the greatest shift of approximately 116 µm exceeds the maximum limit of beam shift w0/2, where w0 is the beam waist (180 µm). A generic theoretical model compatible with polarization-dependent studies is also established that has demonstrated excellent agreement with experimental results. Moreover, this work established three distinct features that allow us to readily tweak the value of spatial GH shifts. The observation of a controllable spatial GH shift at the ML WSe2-SiO2/Si configuration has potential implications for optical sensors, optical differential operation, and other photonic manipulations.

Single-shot polarization detection with a highly scattering system

Polarization detection plays a significant role in optics. However, the current detection methods usually involve mechanically rotating components, multiple measurement steps, complicated optical design, and precise microfabrication process. To address this issue, we propose a single-shot method to detect the polarization state of light based on a highly scattering system, which is constituted by a spatial light modulator and a highly scattering medium. When the incident light beam shaped by a superimposed wavefront is incident on a highly scattering medium, the foci represented the six components at horizontal, vertical, diagonal, antidiagonal, right circularly polarized, and left circularly polarized directions will appear behind the highly scattering medium simultaneously. By measuring the intensities of these six foci, all the Stokes parameters can be extracted. Taking advantage of the measured Stokes parameters, the orientation angle of major axis, the ellipticity, and the handedness of the polarization ellipse of incident light beam can be determined. Various light beams with different polarization states are detected to demonstrate the viability of the method. The experimental results and theoretical values are in a good agreement. Compared to the existing methods, this approach is fast, free of complicated fabrication, and independent of mechanical movement. The proposed method is expected to promote the development of real-time and broadband polarimetry.

Influence of launch light beam conditions on the bandwidth in multimode graded-index microstructured POFs

The time-dependent power flow equation (TD PFE) is used to theoretically examine the wavelength dependency of the bandwidth in a multimode graded-index microstructured polymer optical fiber (GI mPOF) with a solid core. The TD PFE is numerically solved using the explicit finite difference method (EFDM) and physics-informed neural networks (PINNs). Our numerical results show that the bandwidth decreases with an increasing wavelength from 476 to 522 and finally to 568 nm. With further increasing of the wavelength from 568 to 633 nm, the bandwidth increases. In this way, we have demonstrated that the maximum bandwidth of the analyzed GI mPOF in the analyzed wavelength region is obtained at 633 nm. Such bandwidth behavior is a consequence of the different influences of the analyzed wavelengths on the parameters that characterize the refractive index distribution and the maximum propagating principal mode number of the analyzed GI mPOF. We also showed that, when the radial offset of the incident light beam increases, the bandwidth decreases. This results from a greater modal dispersion when higher guided modes are excited. As a result, based on the observations reported in this work, it is simpler to modify the GI mPOF for a particular application at various wavelengths.

Achromatic CMOS-Integrated Four-Bit Orbital Angular Momentum Mode Detector at Three Wavelengths.

The simultaneous detection of the orbital angular momentum (OAM) and wavelength offers new opportunities for optical multiplexing. However, because of the dispersion of lens functions for Fourier transformation, the mode conversions at distinct wavelengths cannot be achieved in the same plane. Here we propose an ultracompact achromatic complementary metal oxide semiconductor (CMOS)-integrated OAM mode detector. Specifically, a spatial multiplexed scheme, randomly interleaving the phase distributions for distributing the superposed OAM modes into preset positions at distinct wavelengths, is presented. In addition, such a nanoprinted achromatic OAM detector featuring a microscale size and a short focal length can be integrated onto a CMOS chip. Consequently, the four-bit incident light beams at three discrete wavelengths (633, 532, and 488 nm) can be distinguished with a high degree of accuracy evaluated by the average standardized Euclidean distance of ∼0.75 between the analytical and target results. Our results showcase a miniaturized platform for achieving high-capacity information processing.

The Impacts of Deformed Fabry–Perot Interferometer Transmission Spectrum on Wind Lidar Measurements

The Fabry–Perot interferometer (FPI) plays a crucial role as the frequency discriminator in the incoherent Doppler wind lidar. However, in the practical receiver system, reflections occurring between optical elements introduce non-normal incident components in the light beams passing through the FPI. This phenomenon results in the deformation of the FPI transmission spectral lines. Based on that, a theoretical model has been developed to describe the transmission spectrum of the FPI when subjected to obliquely incident light beams with a divergence angle. By appropriately adjusting the model parameters, the simulated transmission spectrum of the FPI edge channels can coincide with the experimentally measured FPI spectral line. Subsequently, the impact of deformations in the transmission spectrum of the two edge channels on wind measurements is evaluated. The first implication is a systematic shift of 30.7 m/s in line-of-sight (LOS) wind velocities. This shift is based on the assumption that the lidar echo is solely backscattered from atmospheric molecules. The second consequence is the inconsistency in the response sensitivities of Doppler frequency shift between Rayleigh signals and Mie signals. As a result, the lidar system fails to fully achieve its initial design objectives, particularly in effectively suppressing interference from Mie signals. The presence of aerosols can introduce a significant error of several meters per second in the measurement of LOS wind velocity.

Optimizing Polarization Selective Unidirectional Photoluminescence from Phased‐Array Metasurfaces

AbstractMetasurface‐based optical elements offer a wide design space for miniature and lightweight optical applications. Typically, metasurface optical elements transform an incident light beam into a desired output waveform. Recent demonstrations of light‐emitting metasurfaces highlight the potential for directly producing desired output waveforms via metasurface‐mediated spontaneous emission. In this work, reciprocal finite‐difference time‐domain (FDTD) simulations and machine learning are used to enable the inverse design of highly unidirectional photoluminescent III‐Nitride quantum well metasurfaces capable of directive p‐, s‐, or combined p‐ and s‐ polarized emission at arbitrary angles. In comparison with previous intuition‐guided designs using the same quantum well architectures, the inverse design approach enables new polarization capabilities and experimentally demonstrated improvements in directivity of 54%. An analysis of ways in which the inverse design both validates and contradicts previous intuition‐guided design heuristics is presented. Ultimately, the combination of reciprocal simulations and efficient global optimization (EGO) grants remarkable improvements in emission directivity and results in full control over the polarization and momentum of emitted light, including simultaneous directional emission of s‐ and p‐polarized light.

Tackling the inverse problem in ellipsometry: analytic expressions for supported coatings with nonuniform refractive index profiles in the thin film and weak contrast limits

Ellipsometry is a powerful tool for the evaluation of the refractive index profile of a film or coating supported on a solid substrate. A well-acknowledged problem, however, is the inverse problem: Given a set of data, under what conditions can a refractive index profile be determined unambiguously? To this end, a series expansion of the Abeles matrix method has been applied to an arbitrary refractive index profile to determine analytic expressions of the ellipsometric ratio ρ. Two types of expressions are found: The thin film limit in which the film thickness L is much less than the wavelength of the incident light beam ( L≪λ ) and the weak contrast limit in which the refractive index of the coating is very near the refractive index of the supporting substrate. In the thin film limit, the first two terms in the series expansion are relatively straight-forward, and they depend on two types of integrals involving the difference between the dielectric profile of the coating and the dielectric constant of the substrate. While higher order terms are possible, they are quite convoluted and do not assist in the inverse problem. In the weak contrast limit, however, the series expansion of ρ depends on the moments of the difference between the dielectric profile of the coating and the dielectric constant of the substrate, allowing an analytic expression that applies to coatings that are even much larger than the incident light beam. The expressions associated with both limits are verified through comparison to the numerical evaluation of ρ with the Abeles matrix method. The results demonstrate that through judicious selection of the substrate refractive index and incident wavelength, conditions can be created that permit critical insights into the inverse problem for either thin coatings or for coatings that are very near the refractive index of the substrate.

Plasmonic slanted slit gratings for efficient through-substrate light-plasmon coupling and sensing

We present an experimental study of plasmonic slanted slit gratings (PSSGs) designed to achieve directional coupling between an incident light beam and surface plasmon polaritons (SPPs) propagating along the surface of the structure. We also investigate mirrored PSSG pairs interconnected by a plasmonic slab waveguide. The structures are fabricated using direct milling by a gallium focused ion beam (FIB). In a mirrored pair arrangement, the first PSSG couples a perpendicularly-incident light beam to SPPs propagating in one direction along the waveguide, while the second PSSG decouples SPPs to perpendicularly-emerging light. This configuration shows promise for sensing applications due to the high sensitivity of the excited SPPs to changes in the refractive index of the bounding medium, and the separation of the optics from the fluidics by the substrate. The design also exhibits robustness to fabrication tolerances. The optical characteristics and sensing potential are investigated theoretically and experimentally, highlighting its potential for a wide range of applications.

Enhanced opposite Imbert–Fedorov shifts of vortex beams for precise sensing of temperature and thickness

The Imbert–Fedorov (IF) shift, which refers to a tiny transverse splitting induced by spin–orbit interaction at a reflection/refraction interface, is sensitive to the refractive index of a medium and momentum state of incident light. Most studies have focused on the shift for an incident light beam with a spin angular momentum (SAM), i.e., circular polarization. We demonstrate experimentally that the relative IF shifts of vortex beams with large opposite orbital angular momentums (OAMs) are highly enhanced in resonant structures when light refracts through a double-prism structure (DPS), in which the thickness and temperature of the air gap are precisely sensed via the observed relative IF shifts. The thickness and temperature sensitivities increase as the absolute value of opposite OAMs increases. Our results offer a technological and practical platform for applications in sensing of thickness and temperature, ingredients of environment gas, spatial displacement, chemical substances and deformation structure.

High-precision two-dimensional displacement metrology based on matrix metasurface.

A long-range, high-precision, and compact transverse displacement metrology is of crucial importance in both industries and scientific researches. However, it is a great challenge to measure arbitrary two-dimensional (2D) displacement with angstrom-level precision and hundred-micrometer range. Here, we demonstrated a prototype of high-precision 2D-displacement metrology with matrix metasurface. Light passing through the metasurface is diffracted into three beams in horizontal (H), vertical (V), and diagonal (D) linear polarization. 2D transverse displacement of the metasurface relative to the incident light beam is retrieved from the interferential optical powers arisen from coherent superposition between H-polarized and D-polarized beams or V-polarized and D-polarized beams. We experimentally demonstrate that arbitrary displacement in 2D plane can be determined with high precision down to 0.3 nm in a large range of 200 micrometers. Our work broadens the application scope of metasurface and paves the way for development of ultrasensitive optical 2D displacement metrology.

A liquid-crystal microlens with dual-layers arrayed pattern electrode for the effective modulation of incident light beams

In this paper, an electrically controlled liquid crystal (LC) microlens array based on dual-layer arrayed planar pattern electrode is reported for effective light wave modulation. By adjusting the voltages applied to two planar arrayed electrodes within one substrate of the LC microlens, an adjustable focused circular ring or point-shaped light spot can be formed. Dual-layers arrayed pattern electrode with a minimum interval spacing of approximately ~10μm and a maximum of 130μm are achieved on one substrate by using standard microelectronic lithography, etching, coating film, and other processes. Experimental results are presented and discussed to illustrate outstanding performance of electrically focusing and modulation capability of the proposed LC device. This approach will serve as inspiration for the continued development and design of LC optical elements, enabling applications such as light field imaging, wavefront detection/correction, and the advancement of imaging augmented systems.

Development of a miniaturized PZT-based MEMS Fabry-Perot interferometer with eutectic wafer bonding and its interface electronics

A miniaturized lead zirconate titanate (PZT)-actuated Fabry-Perot interferometer (FPI) was developed for an 850 nm light beam selection by modulating the air gap between two refractive mirrors. The developed FPI consists of mirrors, a PZT actuator for air gap modulation, metal electrodes, and interface electronics capable of applying a square voltage at a frequency of 0.1 Hz. Eutectic wafer bonding method was employed to bond two mirrors through rapid thermal annealing (RTA) and applying mechanical pressure for a short time to minimize the interdiffusion between layers. The PZT actuator and metal electrodes serve as a thickness spacer to create an air gap between the two mirror plates. Alternating high and low dielectric layers (TiO2 and SiO2) with precise thickness and high crystallization quality were utilized for the formation of the refractive mirror because these layers offer high reflectivity and low absorption loss, enabling effective transmission of a specific wavelength in the incident light beam. By applying 0.1 Hz square pulses to the PZT actuator, the air gap was modulated, resulting in significant shifts in the center wavelength of the filtered beam at the desired wavelength. The study demonstrates the successful modulation and optimization of transmission peaks, ensuring a sharp full width at half maximum (FWHM) and high transmission rate. It also provides comprehensive experimental process factors for optimizing heat treatment conditions for PZT and refractive mirrors, minimizing metal interdiffusion between the dielectric layers, and ensuring mirror plate flatness. The device parameter effects such as cavity length orders, sequence of refractive indices for dielectric layers, and the extent of elongation of PZT with applied voltage were systematically studied to achieve optimal performance of FPI filter. The developed interface electronics allow voltage modulation up to 21.9 V at a frequency of 0.1 Hz, while preserving the elasticity of the PZT actuator.

Wax appearance temperature in crude oils measured by surface plasmon resonance

To predict and quantify the wax appearance temperature in crude oils, we proposed and implemented a well-explored sensing method known as surface plasmon resonance. This phenomenon occurs when an incident light beam couples into a metallic surface. As sensing method, this phenomenon has been widely used in biosensing applications. In this research, we showed the effectiveness of the surface plasmon resonance sensing technique in hydrocarbon sensing in general and in crude oil wax appearance prediction in particular. To the authors knowledge, this is the first exploration of monitoring wax deposition events from crude oil using surface plasmon resonance measurements. A compact optical sensor was developed from an ultra-thin layer of gold deposited on a sapphire prism and supported by a new data analyzing metric to analyze the acquired reflected beam intensity profile and detect the precipitation of crude oil wax or other hydrocarbon particles under varying-temperature and low-pressure conditions. The obtained wax appearance temperatures using the introduced on-site method were found to be comparable with the literature measurements using the common in-lab Cross-Polarization Microscopy method.

Suppression of laser speckle contrast using negative dielectric anisotropy nematic liquid crystal doped with an ionic medium

ABSTRACT A liquid crystal (LC) with negative dielectric anisotropy called N-(4-Methoxybenzylidene)-4-Butylaniline (MBBA) doped with the ionic agent cetyltrimethylammonium bromide (CTAB) has been investigated as a speckle suppression device to reduce the laser’s speckle contrast. The electrohydrodynamic instabilities (EHDI) effect in LC is exploited to reduce speckle contrast. The EHDI effect causes chaotic turbulence in LC, resulting in dynamic scattering of the incident light beam. In order to investigate the electro-optic properties and frequency region for optimal performance, optical measurements and dielectric spectroscopy of the pristine and doped LC are performed. The dielectric characteristics of the LC demonstrate that the frequency range to produce the EHDI effect in the LC cell is broadened as the ionic agent is doped. The inclusion of ions also enhances the ion density, mobility, and conductivity in LC. The increase in ion density and mobility enhances the turbulence in the LC at a faster rate; as a result, the dynamic scattering becomes more rapid. When measured with a charge-coupled device (CCD) camera, the faster dynamic scattering provides higher suppression in the laser’s speckle contrast. This method can be used to reduce the speckle contrast in laser source-based imaging and projection displays.

Polarization-independent and all-optically modulated multiband metamaterial coherent perfect absorber

A polarization-independent, all-optically modulated multiband metamaterial coherent perfect absorber (CPA) is proposed. Numerical simulation results reveal that the absorber can be controlled over a wide range of amplitudes by changing the phase difference of two incident light beams. The all-optical modulation can be extended to telecom wavelengths, allowing for simultaneous modulation of absorption at 1310 nm and 1550 nm. In addition, switchable logical gate states can be achieved at the dip between absorption peaks. The proposed absorber utilizes bound states in the continuum (BIC) mode to realize another absorption band, expanding the application range and enhancing its flexibility. The results suggest that the proposed absorber has a promising modulated optical response, with potential applications in developing optical switches, smart multi-band absorbers, telecom devices and so on.

Two-dimensional spatial coherence measurement of X-ray sources using aperture array mask.

Fourth-generation synchrotron radiation delivers x-ray sources with unprecedented coherence and brilliance, which enables the development of many advanced coherent techniques taking advantage of the inherent high coherence of the x-ray beams. Simple and accurate measurement of two-dimensional (2D) coherence is of utmost importance for the applications of these coherent experimental techniques. Here, we propose a novel approach based on diffraction of aperture array mask (AAM) to obtain accurate 2D spatial coherence with a single-shot measurement. We utilize a coherent mode decomposition algorithm to simulate the diffraction of AAM illuminated by Gaussian-Schell model beam and demonstrate that spatial coherence function of the incident light beam can be accurately and robustly retrieved. We expect that this new approach will be applied into transverse coherence measurements for the new-generation synchrotron radiation source and relevant coherent experimental techniques.

Light scattering by a periodically time-modulated object of arbitrary shape: the extended boundary condition method

A proper generalization of the extended boundary condition method to calculate the transition matrix, T, for electromagnetic scattering from a homogeneous and isotropic body of arbitrary shape, characterized by a periodically time-varying electric permittivity, is presented. The application of the method on a specific example of a spheroidal dielectric particle confirms that time modulation induces strong inelastic scattering, accompanied by energy transfer between the scatterer and the light field, when the difference of the incident wave frequency to a particle optical resonance matches an integer multiple of the modulation frequency. Moreover, it is shown that, for nonspherical scatterers, these effects can be selectively tuned by external means such as the polarization and the propagation direction of the incident light beam. The method is readily implementable in available dynamic multiple-scattering computer codes, and, because of its versatility and computational efficiency, it can offer new opportunities for studying more complex time-varying photonic structures.

Photonic eigenmodes and transmittance of finite-length 1D cholesteric liquid crystal resonators

Cholesteric liquid crystals exhibit a periodic helical structure that partially reflects light with wavelengths comparable to the period of the structure, thus performing as a one-dimensional photonic crystal. Here, we demonstrate a combined experimental and numerical study of light transmittance spectra of finite-length helical structure of cholesteric liquid crystals, as affected by the main system and material parameters, as well as the corresponding eigenmodes and frequency eigenspectra with their Q-factors. Specifically, we have measured and simulated transmittance spectra of samples with different thicknesses, birefringences and for various incident light polarisation configurations as well as quantified the role of refractive index dispersion and the divergence of the incident light beam on transmittance spectra. We identify the relation between transmittance spectra and the eigenfrequencies of the photonic eigenmodes. Furthermore, we present and visualize the geometry of these eigenmodes and corresponding Q-factors. More generally, this work systematically studies the properties of light propagation in a one-dimensional helical cholesteric liquid crystal birefringent profile, which is known to be of interest for the design of micro-lasers and other soft matter photonic devices.