Ray Optics Method Research Articles

Multi-scale optical simulation of crystalline silicon solar cells by combining ray and wave optics.

Optical simulations allow the evaluation of the absorption, reflection, and transmission of each functional layer of solar cells and, therefore, are of great importance for the design of high-efficiency crystalline silicon (c-Si) solar cells. Here, a multi-scale simulation method (MSM) based on ray and wave optics is proposed to investigate the optical characteristics of c-Si solar cells. The ray and wave optical methods are first independently employed on inverted pyramid glass sheets, where the latter one can describe the size-dependent interfacial scattering characteristics more accurately. Then the optical properties of a c-Si solar cell with a tunnel oxide passivated carrier-selective contact configuration are studied by employing the MSM, where scattering at the interfaces is acquired by a finite-difference time-domain method (wave optics). Since the MSM can accurately simulate optical modes such as the Rayleigh anomaly, Bloch mode, and Mie resonances, the reflection and transmission spectra of the whole device are in good agreement with the measured data. The proposed MSM has proven to be accurate for structures with functional thin films, which can be extended to hybrid tandem devices with top-level cells consisting of stacks of layers with similar dimensions.

Radiation modeling of microplasma UV lamps for design analysis and optimization

Microplasma UV lamps have recently emerged as viable excimer-based sources of UV radiation, garnering significant attention during the recent COVID-19 pandemic for their use in disinfection applications because of their ability to emit human-safe far-UVC (200–240 nm) spectrums. An accurate model to simulate the radiation profile of microplasma UV lamps is of paramount importance to develop efficient microplasma lamp-implemented systems. We developed a 3D numerical model of microplasma UV lamps using the ray optics method. The simulation results for lamp irradiance and fluence rate were experimentally validated with standard optical radiometry and actinometry measurements, respectively. To improve the optical efficiency of microplasma lamps, an in-depth analysis of radiation behavior inside the standard commercially available lamp was performed using the geometrical optics method, and several potential scenarios were explored. A 2D modeling of an individual microcavity indicated that the current common lamp design can be significantly improved by preventing radiation loss, and small modifications in optical design can greatly increase the energy performance of the system. Based on the findings of this study, several virtual design concepts were proposed, and their performances were numerically compared with that of the original design of commercial microplasma lamps. The developed model can potentially be integrated with hydrodynamic and kinetic models for the virtual prototyping of complex photoreactors operating with UV microplasma lamps.

Coexistence of transmission mechanisms for independent multi-parameter sensing in a silica capillary-based cascaded structure.

The coexistence of transmission mechanisms, including Fabry-Perot (FP), Mach-Zehnder (MZ), and anti-resonant (AR), is demonstrated via a silica capillary-based cascaded structure. The analysis for MZ shows that one pathway is formed by the beam refracted into the silica capillary cladding from the air core, rather than being transmitted into the cladding directly at the splicing interface. Using the ray optics method, the two coexistence conditions are derived for FP and MZ, and for FP, MZ and AR, respectively. The existence percentages of the three mechanisms can be obtained using the fast Fourier transform. Finally, the coexistence of multiple transmission mechanisms is applied for independent multi-parameter sensing with the FP-based temperature sensitivity of 10.0 pm/°C and AR-based strain sensitivity of 1.33 nm/N. The third mechanism MZ interference can assist in verifying changes in both the temperature and axial strain. This shows the possibility to optimize the transmission spectra for independent multi-parameter sensing by tailoring the existence percentages of different mechanisms.

3D dynamic motion of a dielectric micro-sphere within optical tweezers

Known as laser trapping, optical tweezers, with nanometer accuracy and pico-newton precision, plays a pivotal role in single bio-molecule measurements and controllable motions of micro-machines. In order to advance the flourishing applications for those achievements, it is necessary to make clear the three-dimensional dynamic process of micro-particles stepping into an optical field. In this paper, we utilize the ray optics method to calculate the optical force and optical torque of a micro-sphere in optical tweezers. With the influence of viscosity force and torque taken into account, we numerically solve and analyze the dynamic process of a dielectric micro-sphere in optical tweezers on the basis of Newton mechanical equations under various conditions of initial positions and velocity vectors of the particle. The particle trajectory over time can demonstrate whether the particle can be successfully trapped into the optical tweezers center and reveal the subtle details of this trapping process. Even in a simple pair of optical tweezers, the dielectric micro-sphere exhibits abundant phases of mechanical motions including acceleration, deceleration, and turning. These studies will be of great help to understand the particle-laser trap interaction in various situations and promote exciting possibilities for exploring novel ways to control the mechanical dynamics of microscale particles.

Ultra-Wideband Flat Metamaterial GRIN Lenses Assisted With Additive Manufacturing Technique

This article presents the designs of ultrawideband microwave flat gradient index (GRIN) lenses, which realizes over a 108% fractional bandwidth (12-40 GHz). The frequency-independent ray optics method is employed to determine the radially varying permittivity profile of the lenses. The challenge of realizing such a radially varying profile and the limitations in dielectric material choices are overcome by two additive-manufacturing-aided approaches: 1) partially infilled dielectrics with a varied periodicity, which ensures the lens performance at the higher end of the frequency range and 2) artificially engineered dielectrics (AED) with subwavelength-scale metallic inclusions, which enables-high permittivity dielectrics and leads to benefits of thickness and mass reduction for the GRIN lenses. Measured results demonstrate that the GRIN lenses improve the gain of open-ended waveguide sources by 8.7-15.6 dB over a wide frequency range from 12 to 40 GHz, with the realized gain of up to 23.6 dBi. Both the simulation and measurements of the presented design confirm the potential of implementing the proposed GRIN lens design in high directivity and beamforming antenna applications, across an ultrawideband frequency range.

Enhancement and directional control of light output of scintillators by using microlens arrays: A numerical simulation study

In this investigation, we have carried out the simulations of light output and the directionality of emission from scintillators coated with microlens arrays by using the ray-optics method. We systematically analyze the impact of an individual microlens at different positions on the light output intensity and the directional control, followed by an entire understanding of microlens arrays. It is found that microlens arrays can be used to increase the extraction efficiency and control the directionality of emission of scintillation light. The multiple reflections by the lateral and bottom surfaces play an important role. A comprehensive understanding of the impact of the ratio of width and thickness of a scintillator and the refractive index of microlens is presented and discussed in details. The maximum angle-integrated intensity could be obtained when the refractive index of microlens is slightly larger than that of scintillator. Simulations based on genetic algorithms can be used to efficiently design the parameters of microlens arrays with a specific target of a combination of emission angle and total emission intensity.

Radiation of charge moving through a dielectric spherical target: ray optics and aperture methods

Radiation of charged particles moving in the presence of dielectric targets is of significant interest for various applications in the accelerator and beam physics. The size of these targets is typically much larger than the wavelengths under consideration. This fact gives us an obvious small parameter of the problem and allows developing approximate methods for analysis. We develop two methods, which are called the “ray optics method” and the “aperture method”. In the present paper, we apply these methods to analysis of Cherenkov radiation from a charge moving through a vacuum channel in a solid dielectric sphere. We present the main analytical results and describe the physical effects. In particular, it is shown that the radiation field possesses an expressed maximum at a certain distance from the sphere at the Cherenkov angle. Additionally, we perform simulations in COMSOL Multiphysics and show a good agreement between numerical and analytical results.

Approach to fully decomposing an optical force into conservative and nonconservative components

One of the theoretical challenges in studying optical trapping is the decomposition of the optical force into the gradient force (conservative component) and scattering force (nonconservative component), which can be achieved either for Raleigh particles or for very large particles in the regime of ray optics. However, for the moderate particles in between these two limits, the scenario is still a mystery. In this paper we present a theoretical approach to bridge this gap and fully split the optical force acting on a spherical particle immersed in a generic monochromatic free-space optical field into two such essentially different components, which is efficient even for large particles with the exact consideration of light polarization, thus offering a benchmark for examining the effective range for application of ray optics. Our approach models general optical fields by a series of homogeneous plane waves. The analytical expressions for the gradient and scattering parts of the optical force exerted on a spherical particle of arbitrary size illuminated by multiple interferential plane waves are then derived. As examples of applications, we investigate the gradient and scattering forces acting on a dielectric particle immersed in the Bessel beam. Our results are in excellent agreement with those obtained based on ray optics methods when the illuminated particle is large enough, while exhibiting effects of Mie resonance that are totally missing in the ray optics for moderate particle sizes. Finally, we study the effect of particle size on the gradient force acting on a spherical particle sitting in multiple interferential plane waves. Our extensively numerical results, up to a size as large as 2000 illuminating wavelengths, suggest an overall decreasing tendency in the ratio of the magnitude of the gradient force to that of the total force as the particle size increases.

Modal Expansion Approach for Electromagnetic Propagation in Street Canyons

We present a simple and efficient model to compute electromagnetic (EM) propagation in urban street canyons, when transmitting and receiving antennas are located below the rooftop level. A parallel-plate dielectric hollow waveguide model of the street canyon is proposed, and propagation is analyzed by using the modal expansion approach. Obtained results for the case of line-of-sight (LoS) propagation are substantially equivalent to those obtained with the ray-optics method, but, with respect to the latter, our method presents important advantages: it is more efficient from a computational viewpoint, it allows deriving a very simple and reasonably accurate formulation of the average received signal strength, and it provides a fairly simple way to compute coupling at cross-junctions, exploited to obtain an analogous formulation for the non-LoS case. Obtained results are compared with simulations of a ray-tracing-based EM solver and with experimental measurements available in the literature.

Transition of Fabry-Perot and antiresonant mechanisms via a SMF-capillary-SMF structure.

In this Letter, we investigate the transition of the well-known Fabry-Perot (FP) and antiresonant (AR) mechanisms via a single-mode fiber (SMF)-capillary-SMF structure. The critical length for this transition is analytically found as a linear relation with the capillary inner diameter based on the ray optic method, which shows the agreement with both numerical simulations and experiments. Evolutions of the transmission and reflection spectra verify that FP and AR mechanisms are closely related to the critical length. An observed AR envelope modulated by the FP mechanism in the reflection strengthens gradually with the increase of the capillary length, which is expected to be a novel method for potential applications in multi-parameters sensing because of its combined mechanisms. The transition and critical lengths can be also found and explained using the same method in other types of AR fibers or waveguides with a hollow core.

Behaviors of ellipsoidal micro-particles within a two-beam optical levitator

The two-beam levitator (TBL) is a standard optical setup made of a couple of counter-propagating beams. Note worthily, TBLs allow the manipulation and trapping of particles at long working distances. While much experience has been accumulated in the trapping of single spherical particles in TBLs, the behaviors of asymmetrical particles turn out to be more complex, and even surprising. Here, we report observations with prolate ellipsoidal polystyrene particles, with varying aspect ratio and ratio of the two beam powers. Generalizing the earlier work by Mihiretie et al. in single beam geometries [JQSRT 126, 61 (2013)], we observe that particles may be either static, or permanently oscillating, and that the two-beam geometry produces new particle responses: some of them are static, but non-symmetrical, while others correspond to new types of oscillations. A two-dimensional model based on ray-optics qualitatively accounts for these configurations and for the “primary” oscillations of the particles. Furthermore, levitation powers measured in the experiments are in fair agreement with those computed from GLMT (Generalized Lorentz Mie Theory), MLFMA (Multilevel Fast Multipole Algorithm) and approximate ray-optics methods.

Spatial and temporal variability of the evaporation duct in the Gulf of Aden

An evaporation duct climatology is constructed for the Gulf of Aden using a 31-year high-resolution data set from the climate reanalysis product National Centers for Environmental Prediction Climate Forecast System Reanalysis. The climatology reveals spatio-temporal heterogeneity in the evaporation duct height (EDH) regulated by the strong interplay between the seasonal monsoon reversals and the related atmospheric and oceanic responses. It also reveals that the Gulf of Aden exhibits a special EDH distribution that is distinct from the adjacent Arabian Sea when the south-west monsoon dominates the gulf. Under these conditions, the EDH of the entire gulf is significantly higher than that of the adjoining Arabian Sea. A cliff-style EDH drop (referred to as the EDH cliff) forms at the mouth of the Gulf of Aden. Furthermore, the influence of the EDH cliff on marine radar was investigated using a ray-optics method. Based on the analysis, it was found that the local EDH significantly affects the radar detection ability beneath the evaporation duct for ships sailing in the Gulf of Aden.

Ray picture and ray-wave correspondence in triangular microlasers

We apply ray-optical methods to dielectric optical microcavities in the shape of triangles made of low refractive index material. We find ray trajectories that maximize the intensity inside the cavity to determine the far-field emission characteristics and to complement the concept of the unstable manifold applicable to chaotic microlasers. As these maximum intensity trajectories need not be periodic, we suggest that they provide a more general explanation for emission patterns of microlasers than short periodic orbits. Further, the geometrical optics description is extended by the inclusion of intensity amplification along the optical path to achieve a better description of active, lasing cavities. Far-field emission patterns of equilateral triangle cavities obtained in this way agree well with our full electromagnetic wave simulations and with previously reported experimental results.

Channel Model for the Surface Ducts: Large-Scale Path-Loss, Delay Spread, and AOA

Atmospheric ducts, which are caused by the rapid decrease in the refractive index of the lower atmosphere, can trap the propagating signals. The trapping effects of the atmospheric ducts can be utilized as a communication medium for beyond-line-of-sight (b-LoS) links. Although the wave propagation and the refractivity estimation techniques for the atmospheric ducts are well studied, there is no work that provides a channel model for the atmospheric ducts. Therefore, we develop a large-scale path-loss model for the surface ducts based on the parabolic equation (PE) methods for the first time in the literature. In addition, we develop a ray-optics (RO) method to analyze the delay spread and angle-of-arrival (AOA) of the ducting channel with the surface ducts. Using the developed RO method, we derive an analytical expression for the effective trapping beamwidth of the transmitter to predict the ranges of the beamwidth that can be trapped by the surface ducts according to the refractivity and the channel parameters.

Cross-type optical separation of elastic oblate capsules in a uniform flow

The dynamic behavior of an elastic capsule with an initially oblate spheroidal shape during cross-type optical separation was numerically investigated. The penalty immersed boundary method was adopted for the fluid-membrane interaction, and the optical force calculation was conducted by using the ray optics method including the ray-surface intersection algorithm. The oblate elastic capsule of b/a = 0.5 with different surface Young's moduli and different initial inclination angles was considered. The oblate capsule with higher surface Young's moduli was less deformed, and was more migrated for each initial inclination angle. Unlike the oblate rigid particle, the initially inclined capsules with moderate inclination angles were similarly migrated since the oblate elastic capsule was deformed during rotation near the laser beam axis. The oblate capsules can be separated according to the surface Young's modulus, except for nearly non-inclined capsules. As the fluid velocity decreased, the migration distance increased. The maximum deformation parameter was insensitive to the fluid velocity. Furthermore, a new dimensionless number (Sec) was introduced to predict the migration distance of the oblate elastic capsule.

Optical force on a large sphere illuminated by Bessel beams: comparisons between ray optics method and generalized Lorenz-Mie theory.

Optical forces are calculated for a dielectric spherical particle illuminated by a zero-order Bessel beam based on both the generalized Lorenz-Mie theory (GLMT) and the ray optics method (ROM). Particles with positive and negative refractive indices are examined. The peculiar characteristics of the Bessel beam allow for analytical expressions for the beam shape coefficients required in the GLMT as well as a decomposition of optical force into the gradient and the scattering forces irrespective of the particle size, which enable respective comparisons for the gradient and scattering forces between the results obtained from the GLMT and the ROM. Our results demonstrate that the discrepancy between the results obtained from the GLMT and the ROM depends on the particle refractive index np, the particle size, and, also, the particle location in the beam field. As the particle size increases, the difference between the results from the GLMT and the ROM shows a general tendency of decreasing, as can be expected, but the change may exhibit oscillatory rather than monotonic behavior. A phase diagram is presented that displays the regime for particle size and refractive index where a specified accuracy can be achieved for optical force by the ROM.

Microstructured Tube-Leaky Glass Waveguide for Delivery of High-Powered Er:Yag Laser

We present tube-leaky fiber with microstructured glass supports as a hollow core microstructured optical fiber. Attenuation constants of tube-leaky fibers are derived by using a ray optic method and designed the wall thickness of the fibers for Er:YAG laser which is often used for medical applications. From the loss measurements of Er:YAG laser, the tube-leaky fibers supported by microstructured glass made of borosilicate-glass can deliver laser with 0.85 dB/m despite of high material absorption around laser wavelength. The high power energy transmission results of the fabricated fibers are at the order of 80-120 mJ. These energies are enough to ablate biological tissues in surgical operations.

Side-Hole Plastic Optical Fiber for Testing Liquid’s Refractive Index

A multimode plastic optical fiber with a side-hole fabricated by precision mechanical drilling technology is presented for refractive index (RI) sensing. A higher sensitivity of 1862.1 μW/RIU, when center wavelength of optical source is 652 nm and launched power is 1 mW, is obtained in a larger hole diameter of 350 μm, indicating that the larger hole diameter can increase the sensitivity of the sensor. Ray-optics method and finite-difference time-domain method are both utilized to theoretically evaluate the characteristics of the structure for RI testing, which is well in agreement with the experimental results. Energy field distribution is analyzed to explain the sensing mechanism of the structure, which demonstrates that the divergent effect of the side-hole like a negative lens is the key to sensing. Due to the advantages of relatively easy fabrication, low cost, and capable of continuous measurement, this side-hole-based fiber sensing structure is attractive for RI monitoring in chemical, biological, and biochemical sensing region.

Artificial Tensor Impedance Surface Waveguides

A surface waveguide structure is studied using a theoretical model and simulated using Ansoft HFSS. A tensor impedance surface is surrounded by two lower impedance surfaces on a plane. Surface waves are guided losslessly within the inner region as long as it has higher impedance than the outer region. A theoretical model is proposed which predicts the dispersion relation of the waveguide using a ray optics method. Our tensor impedance surface theory can also predict the dispersion of scalar impedance surfaces. The theory and simulation show agreement for scalar and tensor impedance surfaces. Two applications of tensor impedance surface waveguides are presented.

Theoretical and experimental investigation of infrared properties of tapered silver/silver halide-coated hollow waveguides

Silver/silver halide-coated hollow-glass waveguides (HGWs) are capable of low-loss, broadband transmission at infrared wavelengths with the advantage of optical response tunability through alteration of a number of key design parameters. Generally, the design of circular HGWs has primarily involved optimization of the waveguide bore size and deposited film structure in order to obtain the desired optical response, with the waveguide bore size being held constant as a function of length. In this study, the effects of HGW structures consisting of linearly tapered inner diameters on the optical response at infrared wavelengths are theoretically and experimentally investigated. Theoretical analysis involving numerical ray optics methods accounting for the dynamic nature of bore size, and consequently light propagation, along the waveguide length is presented and compared to experimental results in order to gain a deeper understanding of these atypical HGW structures.