Anisotropic Refractive Index Research Articles

CVD‐Synthesized CsAg2I3 Single Crystals toward Polarization UV Photodetection and Anti‐Counterfeiting Identification

AbstractLow‐dimensional ternary silver halide (CsAg2I3), an emerging inorganic lead‐free perovskite, has garnered significant scientific attention due to its strong quantum confinement effects, wide bandgaps, non‐toxicity, and others. However, the photophysical properties and optoelectronic performance of CsAg2I3 materials and devices are significantly compromised by crystal quality and defects arising from liquid‐phase preparation. Herein, 1D CsAg2I3 single crystals newly grown by the chemical vapor deposition method are reported. These crystals display exceptional quality and stability over a 3‐month period despite exposure to air and moisture. The high in‐plane anisotropic structure, phonon vibrations, and refractive index of CsAg2I3 crystals are also experimentally elucidated. Polarization‐sensitive ultraviolet photodetector based on the CsAg2I3 monocrystal features a responsivity of 139 mA W−1, a specific detectivity of 1.05 × 1011 cm Hz1/2 W−1, an ultra‐fast photoresponse speed of 48.2/69.1 µs, and a dichroic ratio of 2.66 upon 360 nm irradiation at −5.0 V bias. The photodetector excels amongst its competitors, moving toward the potential application of rapid optical anti‐counterfeiting identification. This work, which leverages grown high‐crystalline and highly stable CsAg2I3 monocrystals, holds promises for advancing this functional material into a robust next‐generation optoelectronic devices capable of probing sensitive optical sensing and specific azimuth recognition.

Accurate Determination of the Uniaxial Complex Refractive Index and the Optical Band Gap of Polymer Thin Films to Correlate Their Absorption Strength and Onset of Absorption.

The advanced development of optoelectronic devices requires a methodical knowledge of the fundamental material properties of the key active components. Systematic investigations and correlations of such basic optical properties can lead to new insights for the design of more potent materials. In this perspective, we provide a systematic overview of the uniaxial anisotropic complex refractive indices and the absorption coefficients obtained by ellipsometry as well as the optical band gap energies derived from Tauc plots of six selected solution-processed polymer thin films. While the optical band gap energies are intentionally distributed over the visible spectral range, we found that the absorption strength of all polymer samples are grouped in a random distribution within a rather uniform range of values.

Birefringence modulation via intense coherent phonons engineering with asymmetric VO2/TiO2 heterostructures

The ultrafast modulation of optical crystal birefringence, involving rapid deformations of crystalline lattices, holds significant scientific and technological importance. High-frequency coherent phonons, through transient perturbation of lattice order, have emerged as a powerful tool for modifying the properties of materials. Here, we systematically investigate coherent phonons in the asymmetric crystal directions [011], [110], and [100] of VO2/TiO2 heterostructures. Notably, in the (011)-VO2/TiO2 system, a remarkable shear mode coherent phonon signal was excited, exhibiting a marginally higher intensity compared to the longitudinal mode. By changing the probe light polarization, we observed a fascinating reversal in the birefringence sign induced by the giant coherent phonons. Density functional theory (DFT) calculations indicate that TiO2 possesses excellent photoelastic properties, with strong coherent phonons efficiently modulating refractive index anisotropy, accounting for this phenomenon. This finding provides novel insights into the development of ultrafast acousto-optic devices.

Building nanoplatelet α-MoO3 films: A high quality crystal anisotropic 2D material for integration

The successful development of nanoplatelet α-MoO3 −films with wavelength-dependent in-plane and out-of-plane birefringence both in the visible and in the medium infrared is demonstrated. The films are prepared by a two-step process starting from structurally amorphous and continuous substoichiometric MoO3-X followed by isothermal annealing at low temperature (250 °C) to yield the formation of a dense network of large α-MoO3 2D nanoplatelets lying in-plane with no overlapping. These α-MoO3 crystals form a film with excellent thickness uniformity (20 nm) and are oriented with the [010] crystallographic direction parallel to the substrate normal. Finally, we report the film anisotropic optical complex refractive index, both parallel and perpendicular to the plane the full spectral range from 0.7 to 5 eV as determined by spectroscopic ellipsometry, and their characteristic in-plane phonon mode in the IR from FTIR measurements. These results show a promising pathway to the creation of highly functional anisotropic α-MoO3 2D films suitable for the development of integrated nanostructured photonic components.

Observation of Boyer-Wolf Gaussian modes

Stable laser resonators support three fundamental families of transverse modes: the Hermite, Laguerre, and Ince Gaussian modes. These modes are crucial for understanding complex resonators, beam propagation, and structured light. We experimentally observe a new family of fundamental laser modes in stable resonators: Boyer-Wolf Gaussian modes. By studying the isomorphism between laser cavities and quadratic Hamiltonians, we design a laser resonator equivalent to a quantum two-dimensional anisotropic harmonic oscillator with a 2:1 frequency ratio. The generated Boyer-Wolf Gaussian modes exhibit a parabolic structure and show remarkable agreement with our theoretical predictions. These modes are also eigenmodes of a 2:1 anisotropic gradient refractive index medium, suggesting their presence in any physical system with a 2:1 anisotropic quadratic potential. We identify a transition connecting Boyer-Wolf Gaussian modes to Weber nondiffractive parabolic beams. These new modes are foundational for structured light, and open exciting possibilities for applications in laser micromachining, particle micromanipulation, and optical communications.

Combining ultrahigh index with exceptional nonlinearity in resonant transition metal dichalcogenide nanodisks

Second-order nonlinearity in solids gives rise to a plethora of unique physical phenomena ranging from piezoelectricity and optical rectification to optical parametric amplification, spontaneous parametric down-conversion and the generation of entangled photon pairs. Monolayer transition metal dichalcogenides, such as MoS2, exhibit one of the highest known second-order nonlinear coefficients. However, the monolayer nature of these materials prevents the fabrication of resonant objects exclusively from the material itself, necessitating the use of external structures to achieve the optical enhancement of nonlinear processes. Here we exploit the 3R phase of a molybdenum disulfide multilayer for resonant nonlinear nanophotonics. The lack of inversion symmetry—even in the bulk of the material—provides a combination of massive second-order susceptibility, extremely high and anisotropic refractive index in the near-infrared region (n > 4.5) and low absorption losses, making 3R-MoS2 highly attractive for nonlinear nanophotonics. We demonstrate this by fabricating 3R-MoS2 nanodisks of various radii, which support resonant anapole states, and observing substantial (>100-fold) enhancement of second-harmonic generation in a single resonant nanodisk compared with an unpatterned flake of the same thickness. The enhancement is maximized at the spectral overlap between the anapole state of the disk and the material resonance of the second-order susceptibility. Our approach unveils a powerful tool for enhancing the entire spectrum of optical second-order nonlinear processes in nanostructured van der Waals materials, thereby paving the way for nonlinear and quantum high-index transition metal dichalcogenide nanophotonics.

Tunable giant Goos-Hänchen shift in Au-ReS2-graphene heterostructure.

Enhancing and flexibly controlling the Goos-Hänchen (GH) shift directly is a significant challenge. Here, we report a tunable giant GH shift in a Au-ReS2-graphene heterostructure. The GH shift of this heterostructure demonstrates strong anisotropy and a unique "sign inversion" feature as the graphene reaches a specific thickness. Flexible control and enhancement of the GH shift to the centimeter scale can be achieved by simply rotating the crystallization direction of the heterostructure. Utilizing this feature, we designed an anisotropic refractive index sensor with a high sensitivity of 1.31 × 108 µm/RIU. This marks an order of magnitude improvement over previous research and introduces a rotation-dependent sensitivity adjustment feature. The tunable giant GH shift provides a promising approach for future designs of optical sensing and modulation devices.

Liquid crystal waveguide film and its application to smart glasses

Liquid crystal waveguide film that we can control through wide visible wavelength has been proposed. Recently, small optical system is needed for various industrial application. Among the application, liquid crystal is the material for controlling optical ray. The application includes optical switch and display by controlling birefringence. These applications include AR/VR display, for manufacturing, inspection, and so on. In this study, we have investigated liquid crystal (LC) waveguide film that light is emitted from the waveguide film on glasses toward the eye when the power is turned on. The cut-off of waveguide film is based on liquid crystal switch using refractive index anisotropy. We select smart glasses as one of the applications for AR/VR display. The potential of LC waveguide films is demonstrated by simulating the propagation characteristics of waveguides film, LC reorientations, and diffraction gratings in the field of RGB wavelength range. By using the waveguide structure, the film thickness can be configured to be less than 1 mm. We can expect the liquid crystal waveguide has much application to optical integrated circuit as other application except for smart glasses.

Study on polymer dispersed liquid crystal films with wide viewing angle

ABSTRACT Polymer Dispersed Liquid Crystal (PDLC) films have been widely used as electronically controlled dimming technology, and solving the problem of narrow angle of PDLC is a cutting-edge challenge for their application. In this paper, a theoretical model of the viewing angle characteristics of PDLC is constructed, and the corresponding samples are experimentally fabricated. It is found that the simulation results of transmittance changing with the viewing angle are in good agreement with the experimental results. Furthermore, the technical approaches to improve PDLC viewing angle characteristics and achieve a wide viewing angle are discussed theoretically. The results show that the PDLC films for wide viewing angle can be obtained by using liquid crystal materials with smaller anisotropy of refractive index and polymer materials with refractive index slightly higher than the ordinary refractive index of liquid crystal.

Supersymmetric behavior of polarized electromagnetic waves in anisotropic media

A medium with specific anisotropic refractive indices can induce a supersymmetric behavior in the propagation of polarized electromagnetic waves, in an analog fashion to a quantum mechanical system. The polarizations of the wave are the ones which behave as superpartners from each other. For this to happen, the anisotropy of the medium must be transverse to the direction of propagation of the wave, with different refractive indices along the direction of each polarization, being in this way a biaxial medium. These refractive indices must be complex and follow a very specific relation in order to trigger the supersymetric response of the electromagnetic wave, each of them with spatial dependence on the longitudinal (propagation) direction of the wave. In this form, in these materials, different polarized light can be used to test supersymmetry in an optical fashion.

Controlling the Optical Properties of Transparent Auxetic Liquid Crystal Elastomers.

Determining the tunability of the optical coefficients, order parameter, and transition temperatures in optically transparent auxetic liquid crystal elastomers (LCEs) is vital for applications, including impact-resistant glass laminates. Here, we report measurements of the refractive indices, order parameters, and transition temperatures in a family of acrylate-based LCEs in which the mesogenic content varies from ∼50 to ∼85%. Modifications in the precursor mixture allow the order parameter, ⟨P2⟩, of the LCE to be adjusted from 0.46 to 0.73. The extraordinary refractive index changes most significantly with composition, from ∼1.66 to ∼1.69, in moving from a low to high mesogenic content. We demonstrate that all LCE refractive indices decrease with increasing temperature, with temperature coefficients of ∼10-4 K-1, comparable to optical plastics. In these LCEs, the average refractive index and the refractive index anisotropy are tunable via both chemical composition and order parameter control; we report design rules for both.

Determining the Anisotropic Refractive Indices of Ferroelectric Nematic Liquid Crystals by Renormalized Ellipsometry

Determining the Anisotropic Refractive Indices of Ferroelectric Nematic Liquid Crystals by Renormalized Ellipsometry

Birefringent optical responses of single-chirality carbon nanotube membranes

Membranes composed of single-walled carbon nanotubes (SWCNTs) with a specific chiral structure (chirality) possess unique optical properties dominated by thermally robust quasi-one-dimensional excitons. Therefore they have emerged as promising materials for photonic and/or thermo-optic applications. As SWCNTs in the membranes are usually aligned two-dimensionally in a plane, and the optical responses of individual SWCNTs are highly anisotropic, the macroscopic responses of membranes are expected to depend on the light-propagation direction. However, the complex refractive-index spectra of some membrane axes are unknown, hindering the optical design of SWCNT-based devices that respond to arbitrarily propagating light. Here, we report the anisotropic complex refractive index spectra of a single-chirality SWCNT membrane fabricated via vacuum filtration. The polarization- and angle-resolved reflection spectra were obtained in the near-infrared-to-visible region. All spectral features in different polarizations and incident angle configurations were consistent with a uniaxial birefringent membrane, reflecting the random in-plane orientations of the SWCNTs. The ordinary (in-plane) refractive index spectra presented a series of sharp resonances of parallel-polarized excitons. The extraordinary (out-of-plane) refractive index in the 0.8–2.4 eV range was determined as ∼1.9. The extraordinary spectra included small but indispensable contributions from the cross-polarized exciton resonance, which are necessary for properly predicting the angle-dependent optical responses of the membrane. By completely unveiling the birefringent optical responses of single-chirality SWCNT membranes, this work paves the way for integrating SWNCT membranes into precise and diverse photonic and/or thermo-optic devices.

Spectral control in the liquid crystal tunable filters

A liquid crystal is used as an electro-optical device with the advantages of low voltage driving, low power consumption, and thin flatness by utilizing its refractive index anisotropy (birefringence). In this research, a liquid crystal tunable filter (LCTF) with control of a main peak wavelength λ 0 and a full-width Δλ at half maximum is developed. The device contains two stacking liquid crystal layers with different wavelength dependences of birefringence Δn with their optical axes perpendicular to each other and controlling the light wavenumbers in a complementary manner (S-ECB).

Observation of Coherent Perfect Absorption in Oil Film on Water Surface and Sensitive Detection of Refractive Index Anisotropy in the Film.

Sharp reflection dips of 50% were observed when white light was incident from the side of a cell on a 1 μm thick film of silicone oil (polydimethylsiloxane, PDMS, nearly transparent in visible light, with the extinction coefficient κ ≈ 0.0001) above a water surface in the cell so that the total reflection condition was satisfied at the oil-air interface. This is the first observation of a coherent perfect absorption (CPA) phenomenon in liquid. The experimental results can be reproduced by the Fresnel reflectance of the monolayer film, but the wavelength positions at which the dip appears for s-polarized and p-polarized light are reversed if the refractive index of the oil film is assumed to be isotropic. The experimental results were correctly reproduced by assuming that the extraordinary-ray refractive index (light polarized perpendicular to the interface) is 1% larger than the ordinary-ray refractive index (light polarized parallel to the interface). This indicates that the polarization dependence of the CPA phenomenon is extremely sensitive to the difference between the in-plane and out-of-plane refractive indices of the thin film.

First-principles calculations to investigate the dielectric and optical anisotropy in two-dimensional monolayer calcium and magnesium difluorides in the vacuum ultraviolet

Anisotropic dielectric and optical properties of two-dimensional (2D) calcium and magnesium difluorides were investigated in the vacuum ultraviolet (VUV) region of the electromagnetic spectrum (EM) using the first principles density functional theory (DFT). The anisotropy between the in-plane and out-of-plane directions shows that these materials are uniaxial, exhibiting optical and dielectric anisotropy. The optical functions of these anisotropic materials-optical absorption, photoconductivity, refractive index, reflection and extinction coefficients, and electron energy loss (EEL) spectra-are calculated in the framework of DFT. The low refractive index values and relatively small extinction coefficient make these materials alternative low-index 2D materials for the long wavelengths in the VUV region of the EM spectrum. The reflection and transmission spectra indicate the antireflective property of these materials. The calculated EEL function shows less energy loss of fast-traveling electrons in the material's medium. The maxima in the EEL spectrum are the main feature of plasma oscillations. The dissipation in the incident light radiation energy propagating through the dielectric medium is estimated with the dielectric loss tangent (tanδ). The magnesium difluoride is identified as a less dielectric loss medium than calcium difluoride in the VUV region. The present results suggest that these 2D materials are promising in low refractive index, high reflective, and antireflective coating materials in optoelectronic device applications. Also, electronic studies revealed that these are excellent materials for gate insulators in field-effect transistors based on 2D electronic materials.

Bulk ReSe2: Record High Refractive Index and Biaxially Anisotropic Material for All-Dielectric Nanophotonics

We show that bulk rhenium diselenide, ReSe2, is characterized by a record high value of the refractive index exceeding 5 in the near-infrared frequency range. We use back focal plane reflection spectroscopy to extract the components of the ReSe2 permittivity tensor and reveal its extreme biaxial anisotropy. We also demonstrate the good agreement between the experimental data and the predictions of the density functional theory. The combination of the large refractive index and giant optical anisotropy makes ReSe2 a promising material for all-dielectric nanophotonics in the near-infrared frequency range.

The Influence of Nonlocal Metal-Dielectric Environments on Physical and Chemical Processes.

ConspectusPlasmonic nanolayers and laminar metallic/dielectric multilayers were originally developed for optical cloaking applications and lensing applications that could potentially image objects whose size was below the diffraction limit. These assemblies were initially formed from gold or silver nanorods grown within an alumina mesh. However, more recently, assemblies with similar properties have also been prepared by sequential thin-layer deposition of alternating layers of gold and magnesium fluoride (MgF2). These metal/dielectric composite materials enable control of the dielectric constant in the directions perpendicular to the layers and balance the real and imaginary dielectric constants of the assembly such that the speed and the amplitude of the waves traveling through the assembly are not attenuated.In this Account, we will also focus on a few of the applications ranging from surface wetting to fluorescence quenching to enhancement of photochemical reactions. First, we will share an introduction to processes used to create these materials, which are combinations of low refractive index metals and transparent higher index materials arranged in a scalable repeating fashion. Two fabrication methods were employed: an electrochemical deposition of Ag nanorods into an anodized alumina matrix which produced materials with an anisotropic negative refractive index material within the plane of the film and lamellar metal/dielectric layers in which the negative index perpendicular to the growth direction. These alternating layers of plasmonic metals and dielectric materials were ultimately chosen to prepare films for further testing, because of their relative ease of fabrication. We will continue with a discussion of a few of the applications of both of these nonlocal dielectric composite materials including more specialized plasmonic, composite, and hyperbolic metamaterials including fluorescence quenching, photochemical reactions, and surface wetting. In each of these applications, the unique response caused by the enhancement of the electric field and the interface between hyperbolic materials and plasmonic materials as they interact photophysically with their near neighbors is presented. In each of the applications, the enhanced electric field extends from the composite substrate layer to interact with its near neighbors and beyond. The presence of this extended interaction can be observed in the form of decreased emission lifetime, enhancement of photochemical reaction rates, and changes in the surface energies measured by contact angle goniometry. In this Account, all of these situations will be addressed. Finally, we will conclude with a summary and vision for the future as well as a discussion of the unique challenges and opportunities available as research active faculty at an HBCU.

Preparation of Cholesteric Polymer Networks with Narrow-Bandwidth Reflection and Memory Effect

A polymer network with a memory effect based on a polymer-stabilized narrow-bandwidth cholesteric liquid crystal (CLC) was prepared using the washing-out/refill method. The effects of different polymerization conditions on the reflection properties of CLC films were investigated. Meanwhile, the selective reflection property and narrow-bandwidth reflection memory effect of the polymer network were proved, and the response mechanism was provided. Furthermore, different materials from liquid crystals, with an anisotropic refractive index, to toluene, with an isotropic refractive index, were refilled to polymer scaffolds with helical structures, which originated from the periodic arrangement of CLCs. It was confirmed that the reflection bandwidth of these films can be dramatically narrowed by the reduced birefringence (Δn) of the refilled materials. The narrowest bandwidth of 22.5 nm refilling toluene with an isotropic refractive index (Δn = 0) was obtained. These results may provide a novel idea for flexible reflective displays, color filters, printing, and colored cladding of a variety of objects.

Temperature-Dependent Anisotropic Refractive Index in β-Ga2O3: Application in Interferometric Thermometers

An accurate knowledge of the optical properties of β-Ga2O3 is key to developing the full potential of this oxide for photonics applications. In particular, the dependence of these properties on temperature is still being studied. Optical micro- and nanocavities are promising for a wide range of applications. They can be created within microwires and nanowires via distributed Bragg reflectors (DBR), i.e., periodic patterns of the refractive index in dielectric materials, acting as tunable mirrors. In this work, the effect of temperature on the anisotropic refractive index of β-Ga2O3n(λ,T) was analyzed with ellipsometry in a bulk crystal, and temperature-dependent dispersion relations were obtained, with them being fitted to Sellmeier formalism in the visible range. Micro-photoluminescence (μ-PL) spectroscopy of microcavities that developed within Cr-doped β-Ga2O3 nanowires shows the characteristic thermal shift of red-infrared Fabry-Perot optical resonances when excited with different laser powers. The origin of this shift is mainly related to the variation in the temperature of the refractive index. A comparison of these two experimental results was performed by finite-difference time-domain (FDTD) simulations, considering the exact morphology of the wires and the temperature-dependent, anisotropic refractive index. The shifts caused by temperature variations observed by μ-PL are similar, though slightly larger than those obtained with FDTD when implementing the n(λ,T) obtained with ellipsometry. The thermo-optic coefficient was calculated.