Inhomogeneous Refractive Index Distribution Research Articles

Tomographic reconstruction of refractive index fields based on laser beam deflection measurement

Refractive index as one of the most important optical properties of materials is the key to utilize refractive index elements. However, it is very difficult to measure or estimate refractive index of media, especially for multi-dimensional inhomogeneous refractive index distribution. Therefore, an inverse scheme is presented for reconstructing refractive index field in rectangular gradient index medium based on ray deflection measurement in this work. A collection of rays from given positions along given directions transit the semitransparent medium and all exit boundary positions are recorded to construct the objective function of inverse problem. Runge–Kutta ray tracing technique is employed to simulate the curved ray path determined by Fermat principle in gradient-index medium. Conjugate gradient method as an inverse algorithm is used to reconstruct refractive index field. A series of simulation experiments are done to show the performance of the inverse strategy we developed. The results indicate that this inverse strategy for reconstructing refractive index field is accurate and effective.

Demonstration of steering acoustic waves by generalized Eaton lens

We demonstrate an acoustic generalized Eaton lens that steers an acoustic wave to a desired angle, to propose a design method for gradient-index devices using the sub-wavelength structure to shape an acoustic wave. Based on investigations on the effective parameter with several methods: the S-parameter retrieval method, Floquet-Bloch calculations, and multiple scattering theory (MST) for the cylindrical rigid rod structure, we speculate the design process to realize inhomogeneous refractive index distribution. For realization of a high effective index (∼2), the MST design inevitably fails, whereas the Floquet-Bloch calculations and S-parameter retrieval give identical results. By appropriately designing a two-dimensional array of cylindrical PMMA rods, we experimentally verify the acoustic generalized Eaton lenses for steering angles of 15° and 45°. Furthermore, the use of non-resonant metamaterials enables the proposed devices to work in the broad frequency range from 4 to 8 kHz.

Full-Field Refractive Index Variation Measurement Based on Phase Shift Interferometry and SPR Phase Detection

We propose a new approach for determining the distribution of full-field refractive index based on the angle deviation method (ADM) with a surface plasmon resonance (SPR) sensor in phase detection. The inhomogeneous distribution of refractive index causes the variant deviation angles in the test light. SPR sensor is a sensitive angular sensor especially for phase detection. For the full–field phase measurement, we should use the four-step phase shift interferometry (PSI) to measure the phase shift profile of the test light after reflected from the SPR sensor. Thus the phase shift is caused by the variation of the refractive index. The approach can plot the full-field refractive index distribution in a short time and its resolution can be better than 2.2×10-8 (RIU).

Controlling optical beam shifts upon reflection from a magneto-electric liquid-crystal-based system for applications to chemical vapor sensing

We investigate the tunability of the Goos–Hanchen shift experienced by a Gaussian light beam reflected from a multilayered heterostructure consisting of a nematic liquid-crystal cell sandwiched between electrodes and deposited on a magneto-electric/non-magnetic bilayer. Our calculations account for the optical anisotropy of the magnetic layer and of the liquid-crystal cell, as well as for the inhomogeneous refractive index distribution in the latter due to the reorientation of its molecules. We show that the Goos–Hanchen shift can be enhanced and controlled via the voltage applied to the liquid crystal cell and the magnetization direction of the magnetic film. We propose to exploit the voltage-induced tunability of the Goos–Hanchen shift in this system to design an optical sensor devoted to the detection of chemical vapors in the vicinity of the structure.

A broadband slab lens antenna formed from gradient refractive index metamaterials composed of cross‐shaped cells

AbstractA slab lens antenna has been designed to enhance the directivity of a horn antenna efficiently in a wide range of frequencies. A gradient refractive index metamaterial is utilized to compensate phases of electromagnetic waves radiated from a horn, which can transform the cylindrical wave into a plane wave. To obtain broadband property, the gradient refractive index metamaterial composed of cross‐shaped conductor, which works in a nonresonant status is designed, and the metamaterial is proved to be broadband, polarization insensitive, and lossless. The slab lens is designed by a collection of metamaterial unit cells with different geometric parameters, where the effective refractive index of the unit cells are determined by the size of cross‐shaped conductors to form the required inhomogeneous refractive index distribution. The simulation and experiment results show that the lens located in front of a horn can considerably compress the antenna's radiation patterns over a wide band range from 12 to 18 GHz. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:1124–1129, 2014

Modified Maxwell fish-eye approach for efficient coupler design by graded photonic crystals

We present a novel design of two dimensional graded index medium that provides coupling of light with high coupling efficiency between two planar dielectric waveguides of different widths (15.46 μm vs. 2.21 μm). Poor light coupling performance of butt-coupler can be mitigated by implementing tapered coupler at the expense of long coupler section. In order to reduce coupling losses, a new coupling device approach based on graded index (GRIN) concept is proposed. The refractive index distribution is in the form of modified version of the Maxwell fish-eye lens. The inhomogeneous refractive index distribution is approximated by photonic crystals (PCs) such that the positions of each PC rods are appropriately arranged. Strong electric field focusing ability of the designed GRIN PC medium provides relatively high coupling efficiency that is around 90%. Spectral region corresponding to coupling efficiency over 75% has a bandwidth of Δω = 18.56% (284 nm). Finally, we discuss the durability of the proposed coupler against the lateral displacement and angular misalignment of output waveguides.

Numerical Simulation of Radiation Heat Transfer in a One-Dimensional Graded Index Sphere

The rays will propagate along a curved path determined by the Fermat principle in medium with inhomogeneous refractive index distribution. To avoid the complicated computation of ray trajectories, a finite element method is extended to solve the radiative transfer problem in a one-dimensional absorbing-emitting semitransparent spherical graded index medium. A problem of radiative transfer inside a semitransparent spherical graded index medium is taken as an example to verify the method. The predicted temperature distributions are determined by the proposed method, and are compared with the results available in references. The results show that finite element method can predict the radiative heat transfer in one-dimensional absorbing-emitting semitransparent spherical graded index medium accurately.

Perfect focusing of scalar wave fields in three dimensions

A method to design isotropic inhomogeneous refractive index distribution is presented, in which the scalar wave field solutions propagate exactly on an eikonal function (i.e., remaining constant on the Geometrical Optics wavefronts). This method is applied to the design of "dipole lenses", which perfectly focus a scalar wave field emitted from a point source onto a point absorber, in both two and three dimensions. Also, the Maxwell fish-eye lens in two and three dimensions is analysed.

803 マッチング液を用いないレンズの不均一屈折率分布の同定(OS8.逆問題解析手法の開発と最新応用(1),オーガナイズドセッション)

803 マッチング液を用いないレンズの不均一屈折率分布の同定(OS8.逆問題解析手法の開発と最新応用(1),オーガナイズドセッション)

Spectral evaluation of local light scattering behavior in glass particle-dispersed epoxy matrix composite

Local light scattering behavior in glass particle-dispersed epoxy matrix composite was observed at different wavelengths using a wavelength-selective microscope. Observed light scattering behavior had wavelength dependence originated from the dispersion of refractive index difference between the matrix and the particle. Local fluctuation of light scattering in dispersed particle order demonstrates importance of local evaluation of light scattering in diffusive materials with inhomogeneous refractive index distribution.

247 逆解析によるレンズの不均一屈折率分布の同定(OS9.逆問題解析手法の開発と最新応用(3),オーガナイズドセッション)

247 逆解析によるレンズの不均一屈折率分布の同定(OS9.逆問題解析手法の開発と最新応用(3),オーガナイズドセッション)

Real-Time Laser Sheet Refraction To Monitor in Situ the Heterogeneity of Polymerization Process on Teflon Surface

Real-time laser sheet refraction (RT-LSR) was used to study the polymerization kinetics of the hydrophilic monomer 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) in aqueous solution and the heterogeneity of the polymerization process on the surface of a Teflon substrate. The spatial distribution of refractive index along the vertical direction in the polymer solution was obtained in real time from the deflection of refracted light during polymerization. It was found that the polymerization of AMPS in the aqueous solution follows the first-order reaction mechanism. Inhomogeneous distribution of refractive index in the interface region near the Teflon surface, which results from the suppression of polymerization in the interface region, is also observed directly.

Functional Characteristics of Optical Neural Computing Device Using Graded Index Waveguide

New optical neural computing devices consisting of a thin film with graded refractive index distribution are described. Neural signal processing is controlled by an inhomogeneous refractive index distribution in a very small region in this device. We investigate the inhomogeneous index distribution of waveguide-type neural coupling devices for the cases where the neural signals correspond to Gaussian beams. The neural coupling characteristics attained by applying the inhomogeneous index distributions are evaluated.

Stress-induced refractive index inhomogeneity in fluoride glass fibers: origin of wavelength independent scattering loss

Inhomogeneous distribution of refractive index in fluoride glass fibers was found. The inhomogeneity results from stress in the fiber and can be an origin of wavelength independent scattering which is the dominant loss factor present in fluoride glass fibers.

Optical switching in the semiconductor directional coupler with saturated gain

A detailed theoretical study of optical switching by gain saturation-induced phase shift in semiconductor directional couplers has been carried out by solving the electromagnetic equation for a spatially inhomogeneous refractive index distribution using the method of Yeh, Brown, and Szejn [Appl. Opt. 18, 489 (1979)]. The gain saturation-induced phase shift is calculated as a function of the antiguidance factor, injection current density, and input power. The results clearly show that the antiguidance factor and injection current density drastically affect the phase shift. However, the phase shift has only a weak dependence on the input power. In addition, it is also shown that the phase shift is proportional to the antiguidance factor and injection current density for a constant input power. Based on the results presented, the future of optical gain saturation-induced light switching in directional couplers should prove to be very promising.