Gradient Refractive Index Profile Research Articles

Modified Luneburg Lens Based on Metamaterials

We present the design, fabrication, and experimental characterization of a modified two-dimensional Luneburg lens based on bulk metamaterials. The lens is composed by a number of concentric layers. By varying the geometric dimensions of unit cells in each layer, the gradient refractive index profile required for the modified Luneburg lens can be achieved. The cylindrical waves generated from a point source at the focus point of the lens could be transformed into plane waves as desired in the microwave frequency. The proposed modified Luneburg lens can realize wide-angle beam scanning when the source moves along the circumferential direction inside the lens. Numerical and experimental results validate the performance of the modified Luneberg lens.

Porous silicon Bloch surface and sub-surface wave structure for simultaneous detection of small and large molecules.

A porous silicon (PSi) Bloch surface wave (BSW) and Bloch sub-surface wave (BSSW) composite biosensor is designed and used for the size-selective detection of both small and large molecules. The BSW/BSSW structure consists of a periodic stack of high and low refractive index PSi layers and a reduced optical thickness surface layer that gives rise to a BSW with an evanescent tail that extends above the surface to enable the detection of large surface-bound molecules. Small molecules were detected in the sensor by the BSSW, which is a large electric field intensity spatially localized to a desired region of the Bragg mirror and is generated by the implementation of a step or gradient refractive index profile within the Bragg mirror. The step and gradient BSW/BSSW sensors are designed to maximize both resonance reflectance intensity and sensitivity to large molecules. Size-selective detection of large molecules including latex nanospheres and the M13KO7 bacteriophage as well as small chemical linker molecules is reported.

Adjustable internal structure for reconstructing gradient index profile of crystalline lens

Employing advanced technologies in studying the crystalline lens of the eye has improved our understanding of the refractive index gradient of the lens. Reconstructing and studying such a complex structure requires models with adaptable internal geometry that can be altered to simulate geometrical and optical changes of the lens with aging. In this Letter, we introduce an optically well-defined, geometrical structure for modeling the gradient refractive index profile of the crystalline lens with the advantage of an adjustable internal structure that is not available with existing models. The refractive index profile assigned to this rotationally symmetric geometry is calculated numerically, yet it is shown that this does not limit the model. The study provides a basis for developing lens models with sophisticated external and internal structures without the need for analytical solutions to calculate refractive index profiles.

Responsivity enhancement of mid-infrared PbSe detectors using CaF2 nano-structured antireflective coatings

The CaF2 nano-structures grown by thermal vapor deposition are presented. Significant responsivity improvement (>200%) of mid-infrared PbSe detectors incorporating a 200 nm nano-structured CaF2 coating was observed. The detector provides a detectivity of 4.2 × 1010 cm · Hz1/2/W at 3.8 μm, which outperforms all the reported un-cooled PbSe detectors. Structural investigations show that the coating is constructed by tapered-shape nanostructures, which creates a gradient refractive-index profile. Analogy to moth-eye antireflective mechanism, the gradient refractive-index nanostructures play the major roles for this antireflection effect. Some other possible mechanisms that help enhance the device performance are also discussed in the work.

Tuning the Refractive Index of Homopolymer Blends by Controlling Nanoscale Domain Size via RIR‐MAPLE Deposition

Anti‐reflection (AR) coatings designed for glass and other rigid, inorganic substrates are commercially available; however, these inorganic AR coatings tend to crack or delaminate on flexible substrates. A polymeric film with a gradient refractive index (GRIN) profile would make an ideal AR coating for flexible substrates, but such coatings are challenging to fabricate using traditional, solution‐based techniques. Emulsion‐based resonant infrared, matrix‐assisted pulsed laser evaporation (RIR‐MAPLE) offers a straightforward approach to enabling the desired GRIN profile in polymeric materials. Two homopolymers (polystyrene (PS) and poly(methyl methacrylate) (PMMA)) are deposited as a blend and one component (PMMA) is dissolved, leaving behind a porous PS film. The porosity and refractive index (RI) are controlled by the volume ratio of the two homopolymers in the film. Structural and optical characterizations, as well as comparison to modeled optical properties, confirm that porous films fabricated from polymer blends deposited by RIR‐MAPLE behave as effective media over most of the visible spectrum. While evidence for the partial collapse of the porous polymer networks is observed, the RI of the porous films is reduced from that of the bulk material. Importantly, these studies demonstrate that RIR‐MAPLE should enable broadband, omnidirectional, polymeric AR coatings appropriate for flexible substrates. image

Influence of shape and gradient refractive index in the accommodative changes of spherical aberration in nonhuman primate crystalline lenses.

To estimate changes in surface shape and gradient refractive index (GRIN) profile in primate lenses as a function of accommodation. To quantify the contribution of surface shape and GRIN to spherical aberration changes with accommodation. Crystalline lenses from 15 cynomolgus monkeys were studied in vitro under different levels of accommodation produced by a stretching system. Lens shape was obtained from optical coherence tomography (OCT) cross-sectional images. The GRIN was reconstructed with a search algorithm using the optical path measured from OCT images and the measured back focal length. The spherical aberration of the lens was estimated as a function of accommodation using the reconstructed GRIN and a homogeneous refractive index. The lens anterior and posterior radii of curvature decreased with increasing lens power. Both surfaces exhibited negative asphericities in the unaccommodated state. The anterior surface conic constant shifted toward less negative values with accommodation, while the value of the posterior remained constant. GRIN parameters remained constant with accommodation. The lens spherical aberration with GRIN distribution was negative and higher in magnitude than that with a homogeneous equivalent refractive index (by 29% and 53% in the unaccommodated and fully accommodated states, respectively). Spherical aberration with the equivalent refractive index shifted with accommodation toward negative values (-0.070 μm/diopter [D]), but the reconstructed GRIN shifted it farther (-0.124 μm/D). When compared with the lens with the homogeneous equivalent refractive index, the reconstructed GRIN lens has more negative spherical aberration and a larger shift toward more negative values with accommodation.

Polymer-stabilized liquid crystal microlens array with large dynamic range and fast response time

We report a polymer-stabilized liquid crystal (LC) microlens array with a large dynamic range and fast response time. The top substrate has a planar indium-tin oxide (ITO) electrode, while the bottom substrate has two patterned ITO electrodes for generating a fringing field and uniform longitudinal field. The fringing field is utilized to create the desired gradient refractive index profile in the LC/monomer layer, which is later stabilized by UV curing to form polymer networks. To tune the focal length, we apply a longitudinal field to change the lens shape. This microlens array offers several attractive features, such as large dynamic range, fast response time, and good mechanical stability.

Polymeric nanolayered gradient refractive index lenses: technology review and introduction of spherical gradient refractive index ball lenses

A nanolayered polymer films approach to designing and fabricating gradient refractive index (GRIN) lenses with designer refractive index distribution profiles and an independently prescribed lens surface geometry have been demonstrated to produce a new class of optics. This approach utilized nanolayered polymer materials, constructed with polymethylmethacrylate and a styrene-co-acrylonitrile copolymer with a tailorable refractive index intermediate to bulk materials, to fabricate discrete GRIN profile materials. A process to fabricate nanolayered polymer GRIN optics from these materials through thermoforming and finishing steps is reviewed. A collection of technology-demonstrating previously reported nanolayered GRIN case studies is presented that include: (1) the optical performance of a f/# 2.25 spherical GRIN plano-convex singlet with one quarter (2) the weight of a similar BK7 lens and a bio-inspired aspheric human eye GRIN lens. Original research on the fabrication and characterization of a Luneburg inspired GRIN ball lens is presented as a developing application of the nanolayered polymer technology.

Switchable focus using a polymeric lenticular microlens array and a polarization rotator

We demonstrate a flat polymeric lenticular microlens array using a mixture of rod-like diacrylate monomer and positive dielectric anisotropy nematic liquid crystal (LC). To create gradient refractive index profile in one microlens, we generate fringing fields from a planar top electrode and two striped bottom electrodes. After UV stabilization, the film is optically anisotropic and can stand alone. We then laminate this film on a 90° twisted-nematic LC cell, which works as a dynamic polarization rotator. The static polymeric lenticular lens exhibits focusing effect only to the extraordinary ray, but no optical effect to the ordinary ray. Such an integrated lens system offers several advantages, such as low voltage, fast response time, and temperature insensitivity, and can be used for switchable 2D/3D displays.

光管理在晶体硅电池中的应用

Light management is an important factor for high-efficiency crystalline silicon solar cells,which can enhance light absorption to increase a short-circuit current( J sc). In this contribution,the most common light management methods including antireflection,surface scattering,and light confinement are reviewed. Researchers have developed various surface antireflection structures to lower the surface reflection loss of the cell. For example,biological structures fabricated by moth-eye type texture create a gradient refractive index profile resulting in a reflectance below 1% over a wide wavelength range. As the wafer thickness is reduced gradually,light management becomes even more critical for cell performance. Antireflection for the wide wavelength and incident angle range has attracted much attention. And light scattering and confinement may be the most effective ways to enhance light path length and absorption for thinner substrates.

Modeling and Optimization of Sub-Wavelength Grating Nanostructures on Cu(In,Ga)Se2 Solar Cell

In this study, an optical simulation of Cu(In,Ga)Se2 (CIGS) solar cells by the rigorous coupled-wave analysis (RCWA) method is carried out to investigate the effects of surface morphology on the light absorption and power conversion efficiencies. Various sub-wavelength grating (SWG) nanostructures of periodic ZnO:Al (AZO) on CIGS solar cells were discussed in detail. SWG nanostructures were used as efficient antireflection layers. From the simulation results, AZO structures with nipple arrays effectively suppress the Fresnel reflection compared with nanorod- and cone-shaped AZO structures. The optimized reflectance decreased from 8.44 to 3.02% and the efficiency increased from 14.92 to 16.11% accordingly. The remarkable enhancement in light harvesting is attributed to the gradient refractive index profile between the AZO nanostructures and air.

Optical model of porous glasses using genetic algorithms

Porous surfaces on glasses have been proved to be effective in suppressing light reflection due to the continuous variation in the refractive index with thickness. The porous structures were fabricated on BK7 glass by neutral-solution leaching process, and broadband transmittance was measured by a spectrometer. An optical model was applied to determine gradient refractive index profiles of porous glasses using a genetic algorithm. Scanning electron microscopy (SEM) analysis of the nanostructure variants will be shown, along with spectral transmittance that is matched to theoretical models. This model has potential applications in tracking optical properties according to the depth of nanostructures measured by SEM, or obtaining gradient refractive index profiles of porous glasses by the measured transmittance. Therefore, it is useful to optimize experimental condition for special optical properties of porous glass.

Design of acoustic beam aperture modifier using gradient-index phononic crystals.

This article reports the design concept of a novel acoustic beam aperture modifier using butt-jointed gradient-index phononic crystals (GRIN PCs) consisting of steel cylinders embedded in a homogeneous epoxy background. By gradually tuning the period of a GRIN PC, the propagating direction of acoustic waves can be continuously bent to follow a sinusoidal trajectory in the structure. The aperture of an acoustic beam can therefore be shrunk or expanded through change of the gradient refractive index profiles of the butt-jointed GRIN PCs. Our computational results elucidate the effectiveness of the proposed acoustic beam aperture modifier. Such an acoustic device can be fabricated through a simple process and will be valuable in applications, such as biomedical imaging and surgery, nondestructive evaluation, communication, and acoustic absorbers.

OPTICAL COATINGS FORMED BY GRADIENT REFRACTIVE INDEX MATERIALS / METALŲ OKSIDŲ IR JŲ MIŠINIŲ PANAUDOJIMAS KINTAMO LŪŽIO RODIKLIO OPTINĖMS DANGOMS FORMUOTI

Recent advances in ion beam sputtering technology enabled efficient deposition of different oxide mixture coatings. In the present investigation, coating materials ZrO2and SiO2 were utilized for the synthesis of different mixtures. The goal of the conducted investigation was to explore the potential of ZrO2/SiO2 mixtures, especially for applications in the range of the UV spectral. Deposited dielectric mirrors having classical quarter-wave and “rised” design with ZrO2/SiO2 mixture for 266 nm and 355 nm showed good resistance to laser radiation. We also investigated the “fatigue” behavior of LIDT’s in LBO crystals with single, dual and triple wavelength anti-reflective coatings (AR@355, AR@532+1064 and AR@355+532+1064) in order to optimize design and layer materials. The influence of gradient refractive index profiles on damage resistance is of a special interest. We selected a few designs of antireflective coatings which demonstrated the best resistance to laser radiation. An experimental study on rugate filter coatings showed the need for a more accurate characterization of optical properties of metal oxides and their corresponding mixtures. Santrauka Šiame darbe tiriamas jonapluoščio dulkinimo technologija dengtų ZrO2/SiO2 ir jų mišinių optinių savybių pritaikymas kintamo lūžio rodiklio (Rugate tipo) optinėms interferencinėms dangoms formuoti. Mišiniuose naudojant mažą ZrO2 frakciją, buvo išplėstas šios medžiagos pritaikymas UV srityje. Remiantis gautais rezultatais, suformuotos didelio atspindžio (HR) ir skaidrinančios (AR) dangos esant λ = 355 nm bangos ilgio spinduliuotei, taip pat teoriškai sumodeliuotos ir suformuotos daugiabangio skaidrinimo (AR@ 355+532+ 064) Rugate tipo dangos ant LBO (LiB3O5) netiesinių kristalų. Palygintos šių dangų optinės savybės su tomis dangomis, kuriose panaudoti gryni metalo oksidai (fiksuoto lūžio rodiklio). Atliktas Rugate tipo dangų, pasižyminčių siaura didelio (R > 99,9 %) atspindžio zona ties 532 nm ir pralaidumu likusioje spektro dalyje, teorinis modeliavimas ir eksperimentinis garinimas.

Analysis of the robustness of the lens GRIN profile in a schematic model eye

In this work, an improvement of a previously-published human eye model with aging is presented. The previous eye model showed the overestimated performance of an averaged MTF at low spatial frequencies at all ages. Since that model had rotationally symmetrical corneal surfaces, these have been modeled to resemble an astigmatic element according to the recent experimental published data and have been included to produce a more accurate eye model. The gradient refractive index (GRIN) profile proposed for the crystalline lens has not been modified to test its robustness. Further, a tilt and decentration of the lens, and only a decentration for the iris, have been permitted in order to fit the average performance of the new eye model with aging. The results demonstrate that the GRIN profile for the crystalline lens fits the model well, since the decentration and/or tilt of the lens and the iris are sufficient free parameters to simulate the performance of the retinal image quality of an emmetropic human eye with aging, having an average astigmatic cornea.

Electromagnetic Modeling of Multiwalled Carbon Nanotubes as Nanorod Electrodes for Optimizing Device Geometry in a Nanophotonic Device

We present electric-field modeling of carbon nanotubes (CNTs) as nanorods to optimize electrode geometry in a light-modulating nanophotonic device based on CNTs and liquid crystals. The electric fields spawned by the nanotube electrodes are used to align the liquid crystal molecules to generate a gradient refractive-index profile. We considered an array of CNTs on a 2-D conducting substrate. Different geometries were realized by choosing one, two, three, and four CNTs at each point. The static electric fields produced by these different geometries were simulated. Our results show that the “four nanotube” groups generated a wide and symmetrical electric field as compared to other geometries. We have verified the simulation results by experimentally fabricating the nanophotonic device and found that the “four nanotube” groups formed a negative lens array in the liquid crystal cell with enhanced performance.

Advanced key technologies for magnetron sputtering and PECVD of inorganic and hybrid transparent coatings

Besides classical multilayer systems with alternating low and high refractive indices, reactive pulse magnetron sputtering processes offer various possibilities of depositing gradient films with continuously varying refractive index. Using nanoscale film growth control it is possible to achieve optical filter systems with a defined dependency of refractive index on film thickness, e.g. by sputtering a silicon target in a time variant mixture of oxygen and nitrogen. Also reactive co-sputtering of different target materials such as silicon and tantalum in oxygen is suitable as well. Rugate filters made from SiO x N y or Si x Ta y O z gradient refractive index profiles find their application in spectroscopy, laser optics and solar concentrator systems. Furthermore polymer substrates are increasingly relevant for the application of optical coatings due to their mechanical and economical advantages. Magnetron PECVD (magPECVD) using HMDSO as precursor allows to deposit carbon containing films with polymer-like properties. Results show the suitability of these coatings as hard coatings or matching layers. Multifunctional coatings with antireflective and scratch-resistant properties were deposited on polymer substrates using a combined magPECVD and sputter deposition process.

Fabrication and Characterization of Copper-Exchanged BK7 Waveguides

In this paper, we report fabrication of a copper-exchanged optical planar waveguide in BK7 glass by using ion exchange with ZnCl2+Cu2Cl2 as ion source. The measured loss of the fabricated waveguides was as low as about 0.36 db/cm. The waveguides can hold 2 – 4 modes with 5to 30-minute exchange and 1 mode with 2or 3-minute exchanges. The effective refractive index of each mode was determined by using the m-line test system at a wavelength of 0.65 μm. In addition, the cubic spline function interpolation method was employed to recover the gradient refractive index profiles of the copper-containing waveguides. The study on the annealing process for copper-exchanged waveguide is also discussed.

Measurement of gradient refractive index profile of crystalline lens of fisheye in vivo using optical coherence tomography

We report the use of optical coherence tomography (OCT) for noninvasive measurement of the refractive index profile of a crystalline lens of a fisheye. The approach exploits the fact that OCT provides a direct measurement of the optical path of the light traveled through the medium. The gradient refractive index profile for the crystalline lens was retrieved by iterative fitting of the optical path calculated by the ray tracing method with that experimentally measured using OCT. The estimated error in refractive index is approximately 1%. The measured gradient refractive index and Matthiessen’s ratio (ratio of focal length to lens radius) are in good agreement with the previously reported values.

Electrically tunable polymer stabilized liquid-crystal lens

A tunable focal lens using flat electro-optical liquid-crystal cell with uniform pixel-free electrodes is developed. The lenslike gradient refractive index profile is created in the cell via the spatially distributed polymer network obtained by photopolymerization using a spatially nonuniform laser beam. The conditions of the polymer network generation are optimized to improve the optical quality of the lens and its focusing properties. Low optical loss (scattering) is achieved for a focal length smoothly tunable from infinity to 0.8m. Obtained results can be applied to develop lenses that have no moving parts and allow the electro-optical zooming.