Large Refractive Index Research Articles

VO2 nanophotonics

The intriguing physics of vanadium dioxide (VO2) makes it not only a fascinating object of study for fundamental research on solid-state physics but also an attractive means to actively modify the properties of integrated devices. In particular, the exceptionally large complex refractive index variation produced by the insulator-to-metal transition of this material opens up interesting opportunities to dynamically tune optical systems. This Perspective reviews some of the exciting work done on VO2 for nanophotonics in the last decade and suggests promising directions to explore for this burgeoning field.

Strain measurement range enhanced chirped pulse φ-OTDR for distributed static and dynamic strain measurement based on random fiber grating array

A high-performance distributed sensing system based on a random fiber grating array (RFGA) and multi-frequency database demodulation (MFDD) method for strain induced delay time measurement is demonstrated. It enables a wide measurement range for both static and dynamic strain sensing. The proposed MFDD method can enlarge the strain measurement range, since the large strain variation induced time domain trace distortion could be compensated for by laser initial frequency changes. Furthermore, a random fiber grating made with embedded large random refractive index changes along the single-mode fiber could provide a stable reflection with a wide reflection spectrum range. Such a structure successfully improves the time delay measurement precision and achieves a large tuning range, as demonstrated by the database in which a set of pre-recorded undisturbed reflected Rayleigh traces form RFGA at various laser frequencies. Ultimately, a dynamic strain with a peak-to-peak value of 12.5µε at a vibration frequency of 50 Hz is accurately reconstructed when the pulse repetition rate is 1 kHz, which was not detected using a conventional chirped pulse phase-sensitive optical frequency domain reflectometers. The maximum measurable strain variation of about 12.5µε represents a factor of 3 improvement. This number is limited by a pre-recorded frequency scanning range of RFGA response in the database.

The Potential of Combining Thermal Scanning Probes and Phase‐Change Materials for Tunable Metasurfaces

AbstractMetasurfaces allow for the spatiotemporal variation of amplitude, phase, and polarization of optical wavefronts. Implementation of active tunability of metasurfaces promises compact flat optics capable of reconfigurable wavefront shaping. Phase‐change materials (PCMs) are a prominent material class enabling reconfigurable metasurfaces due to their large refractive index change upon structural transition. However, commonly employed laser‐induced switching of PCMs limits the achievable feature sizes and restricts device miniaturization. Thermal scanning‐probe‐induced local switching of the PCM germanium telluride is proposed to realize near‐infrared metasurfaces with feature sizes far below what is achievable with diffraction‐limited optical switching. The design is based on a planar multilayer and does not require fabrication of protruding resonators as commonly applied in the literature. Instead, it is numerically demonstrated that a broad‐band tuning of perfect absorption can be realized by the localized tip‐induced crystallization of the PCM. The spectral response of the metasurface is explained using resonance mode analysis and numerical simulations. To facilitate experimental realization, a theoretical description of the tip‐induced crystallization employing multiphysics simulations is provided to demonstrate the great potential for fabricating compact reconfigurable metasurfaces. The concept can be applied not only for plasmonic sensing and spatial frequency filtering, but also be transferred to all‐dielectric metasurfaces.

Fabrication of nanodiamond-dispersed composite holographic gratings and their light and slow-neutron diffraction properties

We demonstrate the use of nanodiamond in constructing holographic nanoparticle-polymer composite transmission gratings with large saturated refractive index modulation amplitudes at both optical and slow-neutron wavelengths, resulting in efficient control of light and slow-neutron beams. Nanodiamond possesses a high refractive index at optical wavelengths and large coherent and small incoherent scattering cross sections with low absorption at slow-neutron wavelengths. We describe the synthesis of nanodiamond, the preparation of photopolymerizable nanodiamond-polymer composite films, the construction of transmission gratings in nanodiamond-polymer composite films and light optical diffraction experiments. Results of slow-neutron diffraction from such gratings are also presented.

Hybrid State Engineering of Phase‐Change Metasurface for All‐Optical Cryptography

AbstractChalcogenide material Ge2Sb2Te5 (GST) has bistable phases, the so‐called amorphous and crystalline phases that exhibit large refractive index contrast. It can be reversibly switched within a nanosecond time scale through applying thermal bias, especially optical or electrical pulse signals. Recently, GST has been exploited as an ingredient of all‐optical dynamic metasurfaces, thanks to its ultrafast and efficient switching functionality. However, most of these devices provide only two‐level switching functionality and this limitation hinders their application to diverse all‐optical systems. In this paper, the method to expand switching functionality of GST metasurfaces to three level through engineering thermo‐optically creatable hybrid state that is co‐existing state of amorphous and crystalline GST‐based meta‐atoms is proposed. Furthermore, the novel hologram technique is introduced for providing the visual information that is only recognizable in the hybrid state GST metasurface. Thanks to thermo‐optical complexity to make the hybrid state, the metasurface allows the realization of highly secured visual cryptography architecture without the complex optical setup. The phase‐change metasurface based on multi‐physical design has significant potential for applications such as all‐optical image encryption, security, and anti‐counterfeiting.

Demonstration of Spin-Multiplexed and Direction-Multiplexed All-Dielectric Visible Metaholograms.

The optical holography technique realized by metasurfaces has emerged as a novel approach to projective volumetric display and information encryption display in the form of ultrathin and almost flat optical devices. Compared to the conventional holographic technique with spatial light modulators, the metahologram has numerous advantages such as miniaturization of optical setup, higher image resolution and larger field of visibility for holographic images. Here, a protocol is reported for the fabrication and optical characterization of optical metaholograms that are sensitive to the spin and direction of incident light. The metasurfaces are composed of hydrogenated amorphous silicon (a-Si:H), which has large refractive index and small extinction coefficient in the entire visible range resulting in high transmittance and diffraction efficiency. The device produces different holographic images when the spin or direction of incident light are switched. Therefore, they can encode multiple types of visual information simultaneously. The fabrication protocol consists of film deposition, electron beam writing and subsequent etching. The fabricated device can be characterized using a customized optical setup that consists of a laser, a linear polarizer, a quarter waveplate, a lens and a charge-coupled device (CCD).

Hybrid Adaptive Wavelet-Based Optical Flow Algorithm for Background Oriented Schlieren (BOS) Experiments

Quantitative analysis of the flow field is an effective method to study hydrodynamics. As a flow field measurement technology, the Background Oriented Schlieren (BOS) is widely used. However, it is difficult to measure the complex transparent flow field (flow field with large refractive index gradient) using the BOS experiment. In order to overcome this difficulty and improve the accuracy of the BOS experiment, this paper presents a hybrid adaptive wavelet-based optical flow algorithm for the BOS. The current algorithm is a combination of the traditional optical flow algorithm and the wavelet-based optical flow algorithm. By adding the initial value constraints, the adaptive scale constraints, and the adaptive regularization constraints, the algorithm can effectively overcome the above-mentioned difficulty and also improve its accuracy. To further illustrate the feasibility of the proposed method, this paper uses the simulation data, the data of the DNS datasets, and the data of the BOS experiment to verify the performance of the algorithm. The experiment of comparing the proposed algorithm with the existing ones is carried out on the DNS datasets and the data of the BOS experiment. Finally, the proposed method is verified by a practical BOS experiment. The results show that the proposed algorithm can effectively improve the measurement accuracy of displacements.

Broadband dispersive free, large, and ultrafast nonlinear material platforms for photonics

Abstract Broadband dispersion free, large and ultrafast nonlinear material platforms comprise the essential foundation for the study of nonlinear optics, integrated optics, intense field optical physics, and quantum optics. Despite substantial research efforts, such material platforms have not been established up to now because of intrinsic contradictions between large nonlinear optical coefficient, broad operating bandwidth, and ultrafast response time. In this work, a broadband dispersion free, large and ultrafast nonlinear material platform based on broadband epsilon-near-zero (ENZ) material is experimentally demonstrated, which is designed through a novel physical mechanism of combining structural dispersion and material dispersion. The broadband ENZ material is constructed of periodically nanostructured indium tin oxide (ITO) films, and the structure is designed with the help of theoretical predictions combined with algorithm optimization. Within the whole broad ENZ wavelength range (from 1300 to 1500 nm), a wavelength-independent and large average nonlinear refractive index of −4.85 × 10−11 cm2/W, which is enlarged by around 20 times than that of an unstructured ITO film at its single ENZ wavelength, and an ultrafast response speed at the scale of Tbit/s are experimentally reached simultaneously. This work not only provides a new approach for constructing nonlinear optical materials but also lays the material foundation for the application of nanophotonics.

Very high contrast volume holographic gratings recorded in photopolymerizable nanocomposite materials.

Volume holographic phase gratings possessing the saturated refractive index modulation amplitudes as large as 4.5×10-2 were recorded at a wavelength of 532 nm in a photopolymerizable nanoparticle-polymer composite (NPC) film dispersed with ultrahigh refractive index hyperbranched-polymer (HBP) organic nanoparticles. This prominent result was achieved by a combination of the HBP nanoparticles with triazine and aromatic ring units and an electron donor/acceptor photo-initiator system doped in an acrylate monomer blend with low viscosity. As a result, efficient mutual diffusion of HBP nanoparticles and monomer having their very large refractive index difference took place. Obtained results suggest a potentiality of our newly developed HBP-dispersed NPC gratings as efficient volume holographic optical elements for various photonic applications including wearable headsets for augmented and mixed reality.

An analysis of the factors influencing the modeling accuracy of the invariant imbedding T-Matrix method and the optimal design of the parameter settings for particles with different geometrical and optical properties

The Invariant Imbedding (IIM) T-matrix method has been recognized as one of the most promising method to derive exact solution of the light scattering properties of the nonspherical atmospheric particles after the significant improvement by Bi L. and Yang P. Compared with DDA (Discrete Dipole Approximation) and the time domain methods, it can calculate the scattering properties of the random-oriented particles analytically once the T-matrix of the particle is obtained. In order to better understand the modeling capability of this model and optimize the parameter settings for the particles with different geometrical and optical properties, the factors influencing the modeling accuracy are systematically analyzed, which includes the discretization schemes of the radial integral, the discrete point numbers of the radial, zenith and azimuthal integrals. The results show that, the calculation accuracy of Gaussian quadrature technique is higher than that of the trapezoidal rule in the radial discretization. For particles with large size parameter or refractive index, the density of the discrete points (both for the radial discretization and the mesh generation of the zenith and azimuth angles) should be increased correspondingly to ensure the calculation accuracy. Finer discrete grids are also required for particles with extreme shapes (e.g., particles with large aspect ratios). Based on the analysis above, the method to determine the parameters of scattering simulation is further proposed for particles with different geometrical and optical properties, by which the discretization of the computational domain can be optimized once the refractive index and geometry of the particle are known. The method is also validated for cylinders and hexagonal prisms, the results shows that the optimization scheme can be applicable to particles with different shapes.

Optical properties of charged nonspherical particles determined using the discrete dipole approximation

The optical properties of non-spherical particles have been studied for decades and there are a number of solution techniques to model distinct geometries. Of these methods, the Discrete Dipole Approximation (DDA) is known to compute electromagnetic scattering from irregularly shaped, heterogeneous particles. DDSCAT, which is the numerical implementation of DDA, is identified as a potential solver for electrically charged particles; however, there is a set of limitations and shortcomings to be addressed. The main concern is the conductivity, which introduces an infinitesimally thin charged monolayer on the particle surface. The side effect of this concept is a steep increase of refractive index and impedance. The DDSCAT can have trouble converging to a solution when the thin shell at the particle surface produces large losses due to having electromagnetic properties significantly different from the media on either side of the particle interface.In this paper we introduce a method to calculate the optical response of charged particles at wavelengths where charge-induced resonances typically occur. The method takes advantage of the interfaces to DDSCAT, while the conductive shell is simulated by a multi-level material distribution. The discretization involves both the 3D model of the particle and the charge distribution. The latter is determined numerically by solving the Laplace equation. The surface- and volume-averaging parameter is used to avoid large refractive indices, but the results are still accurate within a few percent. The method is validated for sensitivity and specificity in modeling optical resonances that are analytically retrievable for ideal spheres. The applicability to an irregularly shaped particle is demonstrated consequently. Implementation algorithms and numerical solvers are made publicly available, which allows light-scattering modelers to study particles in different media under different conditions.

Role of the Exciton–Polariton in a Continuous-Wave Optically Pumped CsPbBr3 Perovskite Laser

Lead halide perovskites have emerged as excellent optical gain materials for solution-processable and flexible lasers. Recently, continuous-wave (CW) optically driven lasing was established in perovskite crystals; however, the mechanism of low-threshold operation is still disputed. In this study, CW-pumped lasing from one-dimensional CsPbBr3 nanoribbons (NBs) with a threshold of ∼130 W cm-2 is demonstrated, which can be ascribed to the large refractive index induced by the exciton-polariton (EP) effect. Increasing the temperature reduces the exciton fraction of EPs, which decreases the group and phase refractive indices and inhibits lasing above 100 K. Thermal management, including reducing the NB height to ∼120 ± 60 nm and adopting a high-thermal-conductivity sink, e.g., sapphire, is critical for CW-driven lasing, even at cryogenic temperatures. These results reveal the nature of ultralow-threshold lasing with CsPbBr3 and provide insights into the construction of room-temperature CW and electrically driven perovskite macro/microlasers.

Fabrication of Periodic Nanostructures on Silicon Suboxide Films with Plasmonic Near-Field Ablation Induced by Low-Fluence Femtosecond Laser Pulses.

Silicon suboxide (SiOx, x ≈ 1) is a substoichiometric silicon oxide with a large refractive index and optical absorption coefficient that oxidizes to silica (SiO2) by annealing in air at ~1000 °C. We demonstrate that nanostructures with a groove period of 200–330 nm can be formed in air on a silicon suboxide film with 800 nm, 100 fs, and 10 Hz laser pulses at a fluence an order of magnitude lower than that needed for glass materials such as fused silica and borosilicate glass. Experimental results show that high-density electrons can be produced with low-fluence femtosecond laser pulses, and plasmonic near-fields are subsequently excited to create nanostructures on the surface because silicon suboxide has a larger optical absorption coefficient than glass. Calculations using a model target reproduce the observed groove periods well and explain the mechanism of the nanostructure formation.

Design of InGaAsP phase modulator with asymmetrically coupled quantum wells for efficient polarization modulation

Monolithically integrated high-speed polarization modulators are finding emerging applications in the fields of optical communication, quantum key distribution, and radio-frequency photonics. Here, we design an efficient polarization modulator with InGaAsP asymmetrically coupled quantum wells (ACQWs) on InP. By using two quantum wells separated by a thin barrier layer, the behaviors of the heavy holes and the light holes can be controlled independently under an external bias. As a result, we can induce an anomalous quantum-confined Stark effect, having a strong polarization dependence. With an optimized ACQW design, we numerically demonstrate a large polarization-dependent refractive index modulation of 7.1 × 10−3 at an applied field of 1.8 V μm−1. Using a 1 mm long device, we estimate that 2π rotation of the Stokes vector can be achieved with negligible absorption at a driving voltage of only 0.75 V.

Thermally Controllable Infrared Absorption in Cylindrical Groove Array Covered by Phase Change Material

A perfect infrared spectrum absorber in a cylindrical groove array perforated in a metallic layer and followed by vanadium dioxide (VO2), a phase-change material, is introduced and analytically investigated. The phase-change material can change from insulator to metallic phase if proper stimuli, such as heat, voltage, and current, are applied, herein the phase change initiated by heating beyond the phase-transition temperature of VO2. The large refractive index variations between two phases enable the manipulation of optical properties. Simulation results reveal that in the metallic phase, the absorption is suppressed to one third, while VO2 works as a transparent medium for impinging light, and perfect absorption is observed in the insulation phase. The process is reversible in a picosecond time scale. Given the structure symmetry, the absorption is irreverent to impinging light polarization. The thermally controlled infrared absorber may have applications in various optical systems, like imaging and sensing, due to its simple and easy fabrication process.

Numerical modeling of integrated electro-optic modulators based on mode-gap shifting in photonic crystal slab waveguides containing a phase change material

This paper presents novel designs and simulation results of electro-optic modulators in integrated silicon photonics platforms. The electro-optic modulators described in this paper are based on the modulation of the photonic bandgap in a silicon photonic crystal slab waveguide, consisting of a phase change material (germanium selenide) sandwiched in between silicon layers. Three-dimensional finite difference time domain (FDTD) modeling is employed to simulate two configurations of electro-optic modulators based on mode-gap shifting in photonic crystal slab waveguides—one that is designed for operation with the transverse electric (TE) polarization of light and the other for the transverse magnetic (TM) polarization. When the electric field is applied across the germanium selenide layer surrounded by doped silicon layers, there is a change of phase of the germanium selenide layer. A large refractive index contrast between the two phases of the germanium selenide layer—i.e., between the amorphous phase and the crystalline phase—on application of the electric field is responsible for shifting of the mode gap in the dispersion characteristics of the PhC slab waveguide. Our results show excellent performance of the electro-optic modulators in terms of the high On/Off extinction ratio ( > --> 37 d B in the TE configuration and > --> 28 d B in the TM configuration), broadband switching capability ( ∼ 100 n m ), low insertion loss in the On state ( < 2 d B ), high range of tunability (by changing the lattice constant and radius of air holes), and low operating voltage ( < 4 V ). The proposed devices can be used as electro-optic modulators in future nanoscale photonic integrated circuits.

The mechanism of microwave-induced discharge between submillimeter active coke

Microwave-induced active coke discharge has been used in much research related to the environment and energy due to its kinetics and thermal effect. However, the mechanisms of discharge for submillimeter particles are still not clear. This paper presents evidence that the particle shape and gap distance between particles are the possible mechanisms of the microwave discharge. This paper also gives the scope of application of several mechanisms for different particulate materials. Submillimeter particles with about 40–110° tip angle and micron gap distance have the best effect on microwave electric field enhancement. The gap effect is only applicable to materials with a large refractive index, in detail, n > 3 or k > 2 for a 300 μm particle. When the gap distance is too narrow, the electron loss from gap to particle surfaces is large enough, leading to the failure of discharge.

Diffraction ring patterns and nonlinear measurements of the Tris(2′,2-bipyridyl)iron(II) tetrafluoroborate

The nonlinear optical properties of the Tris(2,2′-bipyridyl)iron(II) tetrafluoroborate in aqueous solution was investigated by the spatial self-phase modulation (SSPM) technique using CW laser at λ = 532 nm. The experimental results show large nonlinear refractive index (n2) and the thermo-optic coefficient (dn/dT) equal to −3.17 × 10−6 cm2/W and the 3.17 × 10−3 k−1, respectively. The effect of input power of excitation laser and the temporal evolution on the shape of the diffraction patterns were studied. Consequently, the collapse effect of the diffraction ring patterns was explained.

Asymmetric-waveguide, short cavity designs with a bulk active layer for high pulsed power eye-safe spectral range laser diodes

It is shown, by calculations calibrated against the authors’ recent experimental data, that an eye-safe wavelength range InGaAsP/InP high pulsed power laser design using a bulk active layer, which has a large refractive index step with respect to the optical confinement layer and is located close to the p-cladding, can provide substantial performance improvement compared to the best results achieved so far for this operating regime and wavelength. The dependence of the laser performance on the design parameters such as the thicknesses of the active layer and the waveguide, as well as the cavity length, are analysed. It is shown that the relatively thick bulk active layer in such InGaAsP/InP lasers allows the use of short cavity lengths (∼1 mm or even shorter), for achieving high pulsed power while maintaining a low p-cladding series resistance (making for high efficiency) and a narrow far field (making for high brightness). A single-asymmetry structure with the asymmetric active layer location but symmetric optical confinement layer/cladding refractive index steps gives performance only marginally inferior to that of a double-asymmetric one including asymmetric refractive index steps.

All-dielectric metasurfaces for polarization manipulation: principles and emerging applications

Abstract Metasurfaces, composed of specifically designed subwavelength units in a two-dimensional plane, offer a new paradigm to design ultracompact optical elements that show great potentials for miniaturizing optical systems. In the past few decades, metasurfaces have drawn broad interests in multidisciplinary communities owing to their capability of manipulating various parameters of the light wave with plentiful functionalities. Among them, pixelated polarization manipulation in the subwavelength scale is a distinguished ability of metasurfaces compared to traditional optical components. However, the inherent ohmic loss of plasmonic-type metasurfaces severely hinders their broad applications due to the low efficiency. Therefore, metasurfaces composed of high-refractive-index all-dielectric antennas have been proposed to achieve high-efficiency devices. Moreover, anisotropic dielectric nanostructures have been shown to support large refractive index contrast between orthogonal polarizations of light and thus provide an ideal platform for polarization manipulation. Herein, we present a review of recent progress on all-dielectric metasurfaces for polarization manipulation, including principles and emerging applications. We believe that high efficient all-dielectric metasurfaces with the unprecedented capability of the polarization control can be widely applied in areas of polarization detection and imaging, data encryption, display, optical communication and quantum optics to realize ultracompact and miniaturized optical systems.