High Reflectivity Band Research Articles

Proposal of Technique for Analysis of Complementary Frequency Selective Surfaces

AbstractA specific class of frequency selective surface (FSS) is the complementary frequency selective surface (CFSS) that has interesting characteristics such as high angular stability, multiple transmission and/or reflection bands, and the possibility of miniaturization. The analysis and design of this sort of structure are commonly performed using commercial software, which demands a high computational effort, impacting a longer optimization time. The equivalent circuit model combined with a cascading technique emerges as an alternative method to the use of these softwares, in the optimization of the physical dimensions of these structures, as they model the behavior of a CFSS with low computational effort and optimization time, in addition to being able to be implemented in various programming languages. Thus, this work proposes a CFSS analysis technique that combines the equivalent circuit method with the ABCD matrix. To the best of our knowledge, this is the first reported research on approximate techniques for CFSS analysis. The chosen geometry was the circular ring, due to high angular stability and polarization independence. The modeling of the equivalent circuit for patch and aperture geometries is presented. Some structures are simulated, and the results are compared with results obtained with the HFSS software. Finally, two prototypes are built to validate the analyses performed.

Violet phosphorus with high-index and low-loss: A promising candidate for nanophotonics applications

The recent synthesis and characterization of violet phosphorus (VP) demonstrate that it is the most stable allotrope of phosphorus, which could become a promising 2D layered semiconductor with potential applications in optoelectronic and electronic devices. However, experiments exploring the complex refractive index or permittivity have yet to be performed, hindering further application access. Due to the small size of the VP crystal sample available by the current growth method, the measurement for the complex refractive index is challenging. In this work, coherent acoustic phonon spectroscopy was performed first in the a–b plane using polarization-resolved pump-probe spectroscopy, revealing negligible anisotropy in the refractive index in the VIS–NIR regime (400–1000 nm). Then, the non-polarized reflectance spectra of VP films with different thicknesses on Si in the VIS–NIR regime were collected. The complex refractive index spectra were obtained by fitting the reflectance spectra with Fresnel's law considering multiple reflections in thin films. It was found that VP has high refractive indices and low extinction coefficients in the spectral range of 560–1000 nm, based on which a metasurface with VP nanodisks could achieve a high reflection band in the VIS–NIR regime by simulation. These results unveil the striking potential of VP for nanophotonics applications.

Reconfigurability analysis of an all‐dielectric thermal microfluidic‐based metasurface

AbstractMetasurface tuning is performed using different ways for a wide range of applications. This study presents the design of a thermally‐tuned all‐dielectric reconfigurable metasurface. A microfluidic channel, filled with different concentrations of tellurium–selenium (Te‐Se) alloy, is added on the top of the elliptical dielectric resonator (EDR) unit cell of the considered metasurface. The electrical properties of used semiconductor alloy are varied in the range of 400 to 700°C (steps size of 100°C). The impact of thermal tuning on the reflection and transmission characteristics of the designed metasurface is analyzed in the frequency range of 20–30 GHz using COMSOL Multiphysics. Obtained results demonstrated that the realized metasurface exhibits reconfigurable behavior in terms of variations in the reflection and transmission characteristics with a change in either temperature or concentrations of selenium and tellurium. The wider bands with high reflection and low transmission frequency bands are obtained with lower concentrations of selenium and tellurium for all operating temperatures.

Silver split nano-tube array as a meta-atomic monolayer for high-reflection band

In this work, an ultra-thin silver film-coated grating as a split silver nanotube array exhibits not only high TE polarized reflectance as a conventional subwavelength grating but also high TM polarized reflectance that is close to or higher than TE reflectance at certain wavelength range. The TM reflectance peak shifts with the morphology of the silver covering. The near-field analysis reveals that the silver nanotube array is an ultra-thin optical double negative metamaterial. The negative permeability associated magnetic field reversal is induced within the grating that is surrounded by a split current loop at the TM reflectance peak wavelength. The near field simulation is used to retrieve the equivalent electromagnetic parameters and optical constants that cause the anomalous TM high reflection. It is demonstrated that the TM impedances have a low magnitude and high magnitude with respect to unity for light incident onto the top and bottom of the grating at the peak wavelength, respectively.

Charge and Lattice Dynamics in Excitonic Insulator Ta2NiSe5 Investigated Using Ultrafast Reflection Spectroscopy

An excitonic insulator, in which an optical gap is formed by exciton condensation, is expected to exhibit large photoresponses. Here, we report the results of femtosecond reflection spectroscopy covering a wide infrared region on a typical excitonic insulator, Ta2NiSe5, which gives direct information not only about a change of excitonic gap but also a metallization. With a weak excitation, the excitonic gap decreases due to the weakening of excitonic effect by photocarriers. With a strong excitation, a Drude-like high-reflection band appears, showing an excitonic-insulator-to-metal transition. Analyses of spectral and temporal changes of reflectivity revealed that the metallization is accompanied by large structural changes. This demonstrates an important role of electron–lattice interaction on the stabilization of excitonic phase.

Design of highly reflective film for smart radiation device

Smart radiation device (SRD) based on the asymmetrical Fabry-Perot cavity automatically tune its IR emittance depending on the ambient temperature, making it an ideal choice for thermal control system of spacecrafts. The low solar absorption is desirable for SRD to prevent the spacecraft from overheating by sun light. In this paper, a multilayer highly reflective film with LHk stacking layers is designed to reduce the solar absorptance (As) of the Ag/Al2O3/VO2 structured SRD. The reflective film achieves a high reflection band in ~220 nm bandwidth from 460 nm to 680 nm, which results in a reduction of the solar absorptance by 25.56 % for SRD working at low temperature and 24.27 % at high temperature as the stacking factor k= 5. The simulation results indicate that an economic reflective film with k= 3 can achieve effective suppression of As of SRD, demonstrating the promising potential of the proposed reflective film in thermal control application of spacecrafts.

Influence of top mirror on performance of GaN-based resonant cavity light-emitting diode

In this paper, two kinds of distributed Bragg reflectors (DBRs) with high-reflective-film structure and filter structure are designed and evaporated on the top of GaN-based resonant cavity light emitting diode (RCLED), respectively. Firstly, the reflectivity spectra of the two kinds of DBRs are simulated. Then, the differences in performance including optical longitudinal modes, spectral linewidth, and output light intensity between the two kinds of RCLED devices with different top mirrors, are compared and analyzed. Finally, the influence of the top mirror reflection characteristics on the output spectrum of the RCLED is studied in detail. The results show that the top mirror is an important part of RCLED, and its reflection characteristics determine the optical performance of the device. For the conventional DBR with high-reflective-film structure, its reflectivity spectrum has a wide high-reflection band. Accordingly, the spectral linewidth of the RCLED can be effectively narrowed by using the conventional DBR as the top mirror. However, the spectrum still consists of multi-longitudinal modes. For the DBR with filter structure, its reflectivity spectrum has a narrow high-transmittance band at the central wavelength. Depending on the modulation effect of the high-transmittance band to the output light, single longitudinal mode light emission is realized for the RCLED with the specially designed DBR as the top mirror, which shows a broad application prospect in optical communication and optical fiber sensing. Moreover, the spectral characteristics of the RCLED can be further optimized to meet its application requirements in much more fields, by designing the top mirror structure and changing its reflectivity spectrum characteristics.

Analysis of Optical Coherence Tomography in Acute versus Chronic Central Serous Chorioretinopathy.

Central serous chorioretinopathy (CSC) is a common retinal disease that causes vision loss worldwide. Studying the imaging characteristics of CSC is helpful for the differential diagnosis of diseases. This study analyzed the differences between acute and chronic CSC and provide related information. The aim of this study was to describe the optical coherence tomography features in patients with acute and chronic CSC. Sixty-two eyes of 56 patients with CSC were included in the study. Optical coherence tomography was performed to observe the image features. The photoreceptor outer-segment (PROS) thickness above the pigment epithelium detachment (PED) coinciding with the leakage point in fundus fluorescein angiography was measured and compared with the mean PROS thickness outside the PED in acute cases. The SPSS 23.0 software (IBM Co., Chicago, IL) was used for statistical analysis. Of our 56 patients (62 eyes), 41 (73.21%) were male and 15 (26.79%) were female. There were 53 eyes (85.48%) with acute CSC and 9 eyes (14.52%) with chronic CSC. Besides other common features, the new feature of a high-reflection band in the outer nuclear layer was found to be limited to the macular detachment area. In acute cases, the mean PROS thickness above the retinal pigment epithelium layer protuberance coinciding with the leakage point was 22.7 ± 8.8 μm, which was less than the mean PROS thickness outside the PED at 64.3 ± 21.3 μm (P ≤ .001). The high-reflection band in the outer nuclear layer within the serous neurosensory detachment limited to the macular area was a new finding in CSC patients. This finding can be used as an imaging feature to aid in the diagnosis of CSC.

Giant and tunable Goos-Hänchen shift with a high reflectance induced by PT-symmetry in atomic vapor.

The Goos-Hänchen (GH) shifts of light beams reflected from conventional passive optical systems could be enhanced using the Brewster angle effect or resonance effect, but the maximum GH shift is located at the reflectance minima, which is difficult for experimental detection. In this paper, we present an efficient and flexible scheme to realize complex parity-time (PT)-symmetric periodic optical potentials (complex crystals) in helium atomic vapor. The GH shifts of probe light reflected from the complex crystal are theoretically investigated and large GH shifts could be obtained inside the high-reflection band. When the complex crystal is operated near the coherent perfect absorption-laser point, the maximum GH shift of probe light is exactly located at the reflectance peak. Moreover, the GH shifts could be easily controlled by adjusting the intensity of control light.

Optical Platform to Analyze a Model Drug-Loading and Releasing Profile Based on Nanoporous Anodic Alumina Gradient Index Filters.

In this work, a methodology that exploits the optical properties of the nanoporous anodic alumina gradient index filters (NAA-GIFs) has been developed and applied to evaluate in real time the release dynamics of a cargo molecule, acting as a model drug, filling the pores. NAA-GIFs with two photonic stopbands (PSBs) were prepared with one of its stop bands in the same absorption wavelength range of the cargo molecule, whereas the second stopband away from this absorption range. Numerical simulation and experiments confirm that the relative height of the high reflectance bands in the reflectance spectra of NAA-GIFs filled with the drug can be related to the relative amount of drug filling the pores. This property has been applied in a flow cell setup to measure in real-time the release dynamics of NAA-GIFs with the inner pore surface modified by layer-by-layer deposition of polyelectrolytes and loaded with the cargo molecule. The methodology developed in this work acts as a tool for the study of drug delivery from porous nanostructures.

Understanding the structural, electronic and optical properties of CuXY2 (X = Si, Ge, Y = P, As): A DFT +U approach

In this study, the structural, electronic and optical properties of CuXY2 (X = Si, Ge, Y = P, As) materials were computed via first-principles DFT calculations. The obtained results are quite consistent with the experimental data and previous theoretical data. CuSiP2, CuSiAs2, CuGeP2 and CuGeAs2 are direct and narrow band gap semiconductors with band gap energies of 0.80 eV, 0.62 eV, 0.53 eV and 0.35 eV, respectively. In the upper and lower bands, the s, p and d atomic orbitals were the main contributors. The s, p and d bonding orbitals contributed substantially to the total and partial density of states in the formation of the valence band and conduction band. The electronic charge density revealed that the bonding nature was a mixture of covalent and ionic bonds. The optical features of these materials were studied and discussed in terms of the dielectric function, refractive index, optical conductivity, reflectivity and loss function. The high reflectivity and direct band gap of these compounds in the ultraviolet and visible ranges of electromagnetic energy (photon) revealed the potential utilization of these materials in optoelectronic applications.

Photonic crystal films with high reflectance based on mesoporous silica in the extreme ultraviolet range

In the present work, mesoporous silica films with hexagonally packed perpendicular nanochannels treated as a 2D photonic crystal in extreme ultraviolet (EUV) range are explored by combining theories and experiments. The simulation results show that this film structure exhibits high reflectivities in a wide band range from 5 nm to 11 nm and at broad incidences angle range. The better reflective properties can be ascribed to its excellent photonic crystal structure corresponding to short spectral region. Experimentally, highly ordered mesoporous silica films with vertical mesochannels were fabricated by electrochemically assisted self-assembly method. Moreover, the reflectances of the fabricated film at 5–11 nm region were also tested using synchrotron radiation, which can reach up to 80.4% at 2°from grazing incidence direction. Meanwhile, a relative high reflectance can be maintained at 5–11 nm at incidence angles of 2–7°. This research results offer a new strategy to developing a promising extreme ultraviolet optical film with high reflectance and broad band range.

High-efficiency, low-cost distributed Bragg reflector based on Al2O3/PbZr0.4Ti0.6O3 multilayer

Ferroelectric oxides are kinds of materials for construction of tunable photonic crystals with remarkable electro-optical properties. However, it is a challenge to prepare high quality dielectric mirrors consisting of ferroelectric and dielectric materials. Herein, we report a simple and inexpensive PVP-based sol–gel technique for fabricating crack-free Al2O3/PbZr0.4Ti0.6O3 (PZT) Bragg reflectors. A high-reflection band with a 200-nm bandwidth and an average optical reflectivity of more than 75% is achieved by a periodic stack of only four Al2O3/PZT bilayers, making the multilayer structure potentially applicable in tunable dielectric mirrors, optical cavities, and selective filters.

Effect of Zr Content on Formation and Optical Properties of the Layered PbZrxTi1–xO3 Films**Supported by the Frontier Science Research Project of Chinese Academy of Sciences (No. QYZDJ-SSW-SLH018), the National Natural Science Foundation of China (Nos. 11174307 and 11933006), and the National Key Basic Research Program of China (No. 2016YFB0402405).

PbZrxTi1–xO3 (PZT) films are fabricated on F-doped tin oxide (FTO) substrates using chemical solutions containing PVP polymer and rapid thermal annealing processing. The dependence of the layered PZT multilayer formation and their optical properties on the Zr content x are examined. It is found that all the PZT films are crystallized and exhibit 110-preferred orientation. When x varies in the region of 0–0.8, the PZT films display lamellar structures, and a high reflection band occurs in each optical reflectance spectrum curve. Especially, those PZT films with Zr/Ti atomic ratio of 35/65–65/35 show clearly layered cross-sectional morphologies arranged alternatively by porous and dense PZT layers, and have a peak optical reflectivity of >70% and a band width of >45 nm. To obtain the optimal Bragg reflection performance of the PZT multilayers, the Zr content should be selected in the range of 0.35–0.65.

Optical Properties of Planar and Annular Ternary Superconducting Photonic Crystals in Near-Zero-Permittivity Operation Range

Optical properties of ternary superconducting planar/annular photonic crystals composed of a high-temperature superconductor and two dielectrics were theoretically investigated based on transfer matrix methods in Cartesian and cylindrical coordinates. Electrodynamics of superconductor were modelled using two-fluid model. It is of interest to observe that, for a planar structure, there exists an additional high reflectance band termed as superpolariton gap near the superconducting threshold wavelength for the TM wave at oblique incidences and some reflection dips also were found in the TM reflectance. However, the superpolariton gap and reflection dips were seen for an annular geometry at higher order azimuthal number. For an annular structure, the size of superpolariton gap can be controlled by simply adjusting the starting radius. Moreover, the superpolariton gap in both structures strongly depends on the operating temperature, different combinations of dielectric refractive indices, and thicknesses of dielectric materials.

Porous Silicon Bragg Reflector/Carbon Dot Hybrids: Synthesis, Nanostructure, and Optical Properties.

Carbon dots (C-dots) exhibit unique fluorescence properties, mostly depending upon their physical environments. Here we investigate the optical properties and nanostructure of Carbon dots (C-dots) which are synthesized in situ within different porous Silicon (PSi) Bragg reflectors. The resulting hybrids were characterized by photoluminescence, X-ray photoelectron, and Fourier Transform Infrared spectroscopies, as well as by confocal and transmission electron microscopy. We show that by tailoring the location of the PSi Bragg reflector photonic bandgap and its oxidation level, the C-dots emission spectral features can be tuned. Notably, their fluorescence emission can be significantly enhanced when the high reflection band of the PSi host overlaps with the confined C-dots' peak wavelength, and the PSi matrix is thermally oxidized at mild conditions. These phenomena are observed for multiple compositions of PSi Bragg reflectors/C-dots hybrids.

Fabrication and properties of high quality InGaN-based LEDs with highly reflective nanoporous GaN mirrors

Distributed Bragg reflectors (DBRs) are essential components for the development of optoelectronic devices. In this paper, we first report the use of the nanoporous GaN (NP-GaN) DBR as a template for regrowth of InGaN-based light-emitting diodes (LEDs). The wafer-scale NP-GaN DBR, which is fabricated by electrochemical etching in a neutral solution, has a smooth surface, high reflectivity (>99.5%), and wide spectral stop band width (>70 nm). The chemical composition of the regrown LED thin film is similar to that of the reference LED, but the photoluminescence (PL) lifetime, PL intensity, and electroluminescence intensity of the LED with the DBR are enhanced several times compared to those of the reference LED. The intensity enhancement is attributed to the light reflection effect of the NP-GaN DBR and improved crystalline quality as a result of the etching scheme, whereas the enhancement of PL lifetime is attributable to the latter.

Metal Nanoparticles/Porous Silicon Microcavity Enhanced Surface Plasmon Resonance Fluorescence for the Detection of DNA.

A porous silicon microcavity (PSiMC) with resonant peak wavelength of 635 nm was fabricated by electrochemical etching. Metal nanoparticles (NPs)/PSiMC enhanced fluorescence substrates were prepared by the electrostatic adherence of Au NPs that were distributed in PSiMC. The Au NPs/PSiMC device was used to characterize the target DNA immobilization and hybridization with its complementary DNA sequences marked with Rhodamine red (RRA). Fluorescence enhancement was observed on the Au NPs/PSiMC device substrate; and the minimum detection concentration of DNA ran up to 10 pM. The surface plasmon resonance (SPR) of the MC substrate; which is so well-positioned to improve fluorescence enhancement rather the fluorescence enhancement of the high reflection band of the Bragg reflector; would welcome such a highly sensitive in biosensor.

Electrical and optical properties of a kind of ferroelectric oxide films comprising of PbZr0.4Ti0.6O3 stacks

A type of ferroelectric oxide films, consisting of three PbZr0.4Ti0.6O3 stacks with different periodic thicknesses, has been designed and fabricated on F-doped transparent conductive tin oxide substrates by using one single precursor solution and spinning-coating process. These films exhibit superior ferroelectric, dielectric, and optical performance. Each PbZr0.4Ti0.6O3 multilayer has a high reflectivity band with ∼110 nm photonic band width and average reflectivity of >80%, a dielectric constant of 530 and dielectric tunability of ∼28% at 1 MHz, a remnant polarization of 36 μC/cm2, and a polarization loss of <5% after 109 polarization switching cycles, rendering their perspective application in photonic band-gap engineering, microwave tunable devices, and integrated optoelectronics.

Comparison between high- and zero-contrast gratings as VCSEL mirrors

This study presents a comparison between high-contrast gratings (HCGs) and zero-contrast gratings (ZCGs) for high-speed vertical-cavity surface-emitting lasers (VCSELs). Both types of gratings exhibit high reflectivities beyond 99.5% due to the destructive interference at the output plane, but the HCG has a broader high reflectivity band. The HCG has a lower reflection delay time and smaller energy penetration length than the ZCG. The HCG has poorer mode selectivity for the VCSEL than the ZCG. The fabrication of the ZCG is less complex but with tight fabrication tolerances.