High Refractive Index Materials Research Articles

Design and fabrication of highly efficient antireflective coating in MWIR on germanium using ion-assisted e-beam deposition

We present the design, fabrication, and characterization of a multilayer anti-reflective (AR) coating on a Germanium (Ge) substrate for mid-wavelength infrared (MWIR) applications using the ion-assisted electron-beam evaporation (IAPVD) technique. The AR coating structure comprises five layers of thin films, alternating between low (nL) and high (nH) refractive index materials, specifically silicon dioxide (SiO2) and Ge. The total thickness of the multilayer structure is maintained below 2 μm. The design was meticulously optimized for the 3.6–4.9 μm MWIR range using OptiLayer software, achieving an impressive average transmission of 99.492 % and an average reflectance of merely 0.508 % over a broadband spectral width of 1300 nm. Following the fabrication process using the IAPVD technique, the empirical results demonstrated an average transmission of 98.56 % and a reflectance value of 0.177 % within the 3.6–4.9 μm range. A strong correlation was observed between the simulated and the fabricated results, with a deviation of only 0.331 % on the reflectance value, proving the accuracy and reliability of the design and fabrication process. This work introduces a highly efficient solution for AR coatings on Ge surfaces, enhancing the performance of electro-optical applications in the MWIR region. To the best of our knowledge, the newly designed Ge/SiO2 multilayer structure and the results achieved have not been previously documented in the literature, and offer substantial improvements in transmission efficiency and reflectance reduction for Ge-based optical systems.

Optical properties of germania and titania at 1064 nm and at 1550 nm

One of the main noise sources in current gravitational-wave detectors is the thermal noise of the high-reflectivity coatings on the main interferometer optics. Coating thermal noise is dominated by the mechanical loss of the high-refractive index material within the coating stacks, Ta2O5 mixed with TiO2 . For upgrades to room-temperature detectors, a mixture of GeO2 and TiO2 is an interesting alternative candidate coating material. While the rather low refractive index of GeO2 increases with increasing TiO2 content, a higher TiO2 content results in a lower threshold temperature before heat treatment leads to crystallisation, and potentially to a degradation of optical properties. For future cryogenic detectors, on the other hand, a higher TiO2 content is beneficial as the TiO2 suppresses the low-temperature mechanical loss peak of GeO2. In this paper, we present the optical properties of coatings—produced by plasma-assisted ion-beam evaporation—with high TiO2 content at 1550 nm, a laser wavelength considered for cryogenic gravitational-wave detectors, as a function of heat-treatment temperature. For comparison, the absorption of pure GeO2 was also measured. Furthermore, results at the currently-used wavelength of 1064 nm are presented.

Diamond-lattice photonic crystals assembled from DNA origami.

Colloidal self-assembly allows rational design of structures on the micrometer and submicrometer scale. One architecture that can generate complete three-dimensional photonic bandgaps is the diamond cubic lattice, which has remained difficult to realize at length scales comparable with the wavelength of visible or ultraviolet light. In this work, we demonstrate three-dimensional photonic crystals self-assembled from DNA origami that act as precisely programmable patchy colloids. Our DNA-based nanoscale tetrapods crystallize into a rod-connected diamond cubic lattice with a periodicity of 170 nanometers. This structure serves as a scaffold for atomic-layer deposition of high-refractive index materials such as titanium dioxide, yielding a tunable photonic bandgap in the near-ultraviolet.

Ar transport and blister growth kinetics in titania-doped germania-based optical coatings

Blistering is a phenomenon sometimes observed in sputtered-deposited thin films but seldom investigated in detail. Here, we consider the case of titania-doped germania (TGO)/silica multilayers deposited by ion beam sputtering. TGO is a candidate as high refractive index material in the Bragg mirrors for the next iteration of gravitational waves detectors. It needs to be annealed at 600 ∘C for 100 h in order to reach the desired relaxation state. However under some growth conditions, in 52-layer TGO/silica stacks, blistering occurs upon annealing at a temperature near 500 ∘C, which corresponds to the temperature where Ar desorbs from TGO. In order to better understand the blistering phenomenon, we measure the Ar transport in single layers of TGO and silica. In the case of < 1 µm-thick TGO layers, the Ar desorption is mainly limited by detrapping. The transport model also correctly predicts the evolution of the total amount of Ar in a 8.5 µm stack of TGO and silica layers annealed at 450 ∘C, but in that case, the process is mainly limited by diffusion. Since Ar diffusion is an order of magnitude slower in TGO compared to silica, we observe a correspondingly strong accumulation of Ar in TGO. The Ar transport model is used to explain some regimes of the blisters growth, and we find indications that Ar accumulation is a driver for their growth in general, but the blisters nucleation remains a complex phenomenon influenced by several other factors including stress, substrate roughness, and impurities.

Development of a large volume line scanning, high spectral range and resolution 3D hyperspectral photoluminescence imaging microscope for diamond and other high refractive index materials

Hyperspectral photoluminescence (PL) imaging is a powerful technique that can be used to understand the spatial distribution of emitting species in many materials. Volumetric hyperspectral imaging of weakly emitting color centers often necessitates considerable data collection times when using commercial systems. We report the development of a line-scanning hyperspectral imaging microscope capable of measuring the luminescence emission spectra for diamond volumes up to 2.20 × 30.00 × 6.30 mm with a high lateral spatial resolution of 1–3 µm. In an single X-λ measurement, spectra covering a 711 nm range, in a band from 400–1100 nm, with a spectral resolution up to 0.25 nm can be acquired. Data sets can be acquired with 723 (X) × 643 (Y) × 1172 (λ) pixels at a rate of 6 minutes/planar image slice, allowing for volumetric hyperspectral imaging with high sampling. This instrument demonstrates the ability to detect emission from several different color centers in diamond both at the surface and internally, providing a non-destructive method to probe their 3D spatial distribution, and is currently not achievable with any other commonly used system or technique.

Enhanced terahertz high-harmonic generation from high-Q quasi-bound states in the continuum empowered by permittivity-broken metasurface

The extraordinary emergence of all-dielectric resonant meta-photonics underpinned by high refractive index and low optical loss materials promises a standout platform for unprecedented manipulation and subwavelength control of light. Recent significant breakthroughs in meta-photonics have to do with the exploration of the non-radiative eigenmodes that lie inside the light cone, called the bound states in the continuum (BICs), which have demonstrated considerable potential to achieve high quality (Q) factors but require ultra-small structure asymmetry, complicating fabrication and hindering precise control of Q factors. Here, we propose a scheme to excite quasi-BICs by breaking symmetry in the permittivity of the comprising meta-atoms where small permittivity perturbation renders the same effect in the conventional geometrically asymmetric design. Empowered by the insignificant permittivity perturbation introduced from chemical doping and carrier injection, arbitrarily small permittivity asymmetry, that is, infinitely high-Q factor, can be precisely and dynamically tailored. As an example of application, we numerically show the THz range, exceptionally high conversion efficiency for both third-harmonic and fifth-harmonic generation from the permittivity-broken metasurface (MS) benefiting from the extreme field confinement at high-Q quasi-BICs resonance. Our results present a feasible and efficient strategy to mitigate the shortcomings of previous photonic platform and shall shed light on the advances of nonlinear all-dielectric THz-MS.

Wavelength Scale Singlet Achromatic Microlenses Based on High Refractive Index Materials

AbstractThe common methods used for correcting chromatic aberration are typically based on multi‐lens and multi‐material systems, resulting in lens thicknesses that are several orders of magnitude greater than the wavelength and complex combination designs. A method to achieve the singlet achromatic microlens of the wavelength‐scale thickness by utilizing high refractive index materials with an aspherical profile is proposed. A theoretical model based on the dispersion effect is developed to guide the selection of materials and the design of thicknesses for achieving chromatic aberration correction in singlet microlenses of a given diameter and numerical aperture. H‐ZLaF68N (68N) glass, sapphire, and fused silica with relatively high to low refractive index are selected to prepare the singlet achromatic microlenses to verify the validity of the model. The thicknesses of three microlenses are 573, 737 nm, and 1.27 µm, respectively, and all of them have achieved achromatic correction as designed. This indicates that the high refractive index material not only achieves achromatic aberration but also reduces the thickness by ≈50% compared with the conventional low refractive index material of silica glass. The presented wavelength‐scale singlet achromatic microlens hold significant promise for compact wearable devices, dynamic holography, and color projection displays.

Simulation of laser-induced ionization in wide bandgap solid dielectrics with a particle-in-cell code.

Modeling the laser-plasma interaction within solids is crucial in controlling ultrafast laser processing of dielectrics, where the pulse propagation and plasma formation dynamics are highly intricate. This is especially important when dealing with nano-scale plasmas where specific phenomena of plasma physics, such as resonance absorption, can significantly impact the energy deposition process. In this article, we report on adapting of a Particle-In-Cell code, EPOCH, to model the laser-plasma interaction within solids. This is performed by implementing a background permittivity and by developing and validating adapted field ionization and impact ionization modules. They are based on the Keldysh ionization theory and enable the modeling of ionization processes within solids. The implementation of these modules was validated through comparisons with a hydrodynamic code and existing literature. We investigate the necessary number of super-particles per cell to model realistic ionization dynamics. Finally, we apply the code to explore the dynamics of plasma formation in the regime of of quantized structuring of transparent films. Our study elucidates how a stack of nano-plasma layers can be formed by the interference of a pulse with its reflection on the exit surface of a high refractive index material.

Recent Progress in Sulfur-Containing High Refractive Index Polymers for Optical Applications.

The development in the field of high refractive index materials is a crucial factor for the advancement of optical devices with advanced features such as image sensors, optical data storage, antireflective coatings, light-emitting diodes, and nanoimprinting. Sulfur plays an important role in high refractive index applications owing to its high molar refraction compared to carbon. Sulfur exists in multiple oxidation states and can exhibit various stable functional groups. Over the last few decades, sulfur-containing polymers have attracted much attention owing to their wide array of applications governed by the functional group of sulfur present in the polymer repeat unit. The interplay of refractive index and various other polymer properties contributes to successfully implementing a specific polymer material in optical applications. The focus on developing optoelectronic devices induced an ever-increasing need to integrate different functional materials to achieve the devices' full potential. Several devices that see the potential use of sulfur in high refractive index materials are reviewed in the study. Like sulfur, selenium also exhibits high molar refraction and unique chemical properties, making it an essential field of study. This review covers the research and development in the field of sulfur and selenium in different forms of functionality, focusing on the chemistry of bonding and the optical properties of the polymers containing the heteroatoms mentioned above. The strategy and rationale behind incorporating heteroatoms in a polymer matrix to produce high-refractive-index materials are also described in the present review.

Elimination of reflection losses in gradient metasurface optical elements

Optical metasurfaces can achieve arbitrary wavefront control using subwavelength nanostructures, enabling imaging, sensing, and display applications using ultrathin structures. Metasurface optical elements that rely on propagation phase achieve this control using varying fill fractions of high refractive index materials, resulting in surface regions with significantly different effective refractive index. The intrinsic need for index variation makes it a non-trivial task to eliminate reflection losses across a complete optical element. Here, we investigate the optical performance of high-index metasurfaces, modified to suppress surface reflections. We demonstrate that a drastic reduction of surface reflections across an entire gradient metasurface is possible through the application of a single optimized anti-reflective layer with a thickness and refractive index that are substantially different from the traditionally expected values.

First-principles to explore the hydrogen storage properties of XPtH3 (X=Li, Na, K, Rb) perovskite type hydrides

Based on the first-principles, the hydrogen storage, mechanical, electronic, optical, thermodynamic and kinetic properties of XPtH3 (X = Li, Na, K, Rb) perovskites are investigated for the first time in this work. The lattice constants of the optimized LiPtH3, NaPtH3, KPtH3, and RbPtH3 are 3.54, 3.63, 3.80, and 3.90 Å, respectively. The Cauchy's pressure and the ratio of the bulk modulus to shear modulus (B/G) consistently prove that the XPtH3 hydrides all exhibit ductility. The investigation of electronic properties reveals that XPtH3 compounds exhibit metallic nature. According to Poisson's ratio and charge density distribution, the bonding type of XPtH3 hydrides is mainly ionic. The Bader partial net charges are also calculated to analyze the charge transfer in XPtH3 perovskites. The results of the optical properties of XPtH3 compounds reveal that they are high dielectric constant and high refractive index materials. It is found that the XPtH3 perovskites exhibit thermodynamic, mechanical, and dynamic stability. The gravimetric hydrogen storage capacities are 1.45, 1.35, 1.26, and 1.06 wt% for LiPtH3, NaPtH3, KPtH3, and RbPtH3, respectively. The hydrogen desorption temperatures of LiPtH3, NaPtH3, KPtH3 and RbPtH3 are 237.77, 225.39, 162.43, and 80.11 K, respectively. Our results suggest that LiPtH3 hydride is the most suitable material for hydrogen storage applications among the four compounds.

3D-printed multilayer structures for high-numerical aperture achromatic metalenses.

Flat optics consisting of nanostructures of high-refractive index materials produce lenses with thin form factors that tend to operate only at specific wavelengths. Recent attempts to achieve achromatic lenses uncover a trade-off between the numerical aperture (NA) and bandwidth, which limits performance. Here, we propose a new approach to design high-NA, broadband, and polarization-insensitive multilayer achromatic metalenses (MAMs). We combine topology optimization and full-wave simulations to inversely design MAMs and fabricate the structures in low-refractive index materials by two-photon polymerization lithography. MAMs measuring 20 μm in diameter operating in the visible range of 400 to 800 nm with 0.5 and 0.7 NA were achieved with efficiencies of up to 42%. We demonstrate broadband imaging performance of the fabricated MAM under white light and RGB narrowband illuminations. These results highlight the potential of the 3D-printed multilayer structures for realizing broadband and multifunctional meta-devices with inverse design.

UV filters and high refractive index materials based on carboxymethyl cellulose sodium and CuO@ZnO core/shell nanoparticles

Nanoparticles have substantially contributed to the field of skincare products with ultraviolet (UV) filters to preserve human skin from sun damage. Thus, the current study aims to develop new polymer nanocomposites for the efficient block of UV light that results from the stratospheric ozone layer loss. Co-precipitation method was used to successfully synthesis CuO@ZnO core/shell NPs with a well-crystalline monoclinic CuO core and wurzite ZnO shell. Using the casting method, core/shell NPs were successfully introduced to carboxymethyl cellulose sodium (CMC). The CMC nanocomposites displayed considerably broader optical response extending from near-ultraviolet to visible light, which was likely due to heterojunction between the p-CuO core and n-ZnO shell and defects originating from the synthetic process. The transmittance of pure CMC in the UV, visible, and near IR regions is significantly reduced with the addition of 2 and 4 wt% of CuO@ZnO core/shell NPs to CMC. 99% of UV light is absorbed when 4 wt% of CuO@ZnO core/shell NPs are added. The addition of different concentrations of CMC nanocomposite to one of the sunblock in Egyptian market were studied and showing the highest Sun Protection Factor of 22. Moreover, optical dispersion parameters and refractive index were improved strongly with core/shell NPs addition.

β-FeSi2: A high refractive index candidate material for infrared bandpass filters

Bandpass filters (BPFs) are optical filters with significantly high transmittance in a specific wavelength range and low transmittance on both sides. Infrared BPFs can reduce system losses and overheating caused by other light wavelengths owing to their ability to selectively transmit infrared light of the desired wavelength. This article discusses the potential of using a high refractive index material, β-FeSi2, in BPFs. To the best of our knowledge, no studies have applied β-FeSi2 to infrared BPFs. Simulation results showed that its high refractive index allows the excellent performance of the BPF to be achieved using a multilayer thin film structure with only three layers. Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy results showed that the β-FeSi2 thin film exhibited the lowest absorptance of approximately 0 when the correct stoichiometry (Fe:Si = 1:2) was achieved through co-sputtering. Based on these findings, a β-FeSi2/SiO2/β-FeSi2 multilayer thin film was designed to fabricate the BPF. The fabricated BPF exhibited a narrow peak and achieved a peak transmittance exceeding 80%. This suggested that β-FeSi2 is a promising material for fabricating infrared BPFs. Utilizing these filters is expected to yield significant efficiency improvements and reduce losses across various applications, including thermophotovoltaics and infrared heaters.

Revelation of a tunable near infrared metamaterial and its sensing properties

All-dielectric metasurfaces based on high refractive index material are becoming important components of optoelectronic devices. However, the achived functions are often fixed by the traditional all-dielectric metasurfaces due to the resonant properties are primarily excited based on the design strategies. Optically behaviors of these metasurfaces are solidified at the manufacturing stages. It is of great significance to design and verify an all-dielectric metasurface with dynamic modulation and reconfigurable properties. Here, the silicon and Ge2Sb2Te5 (GST) blocks are combined in a hybrid arrays to achieve a dynamically modulated and reconfigurable all-dielectric metasurface. Based on the amorphous state of GST, two absorption peaks are excited by this metasurface: P1 peak (amplitude 0.87, resonance wavelength 1302 nm) and P2 peak (amplitude 0.78, resonance wavelength 2008 nm). Since the GST block is in the crystalline state, only the P1 peak (the measured amplitude is 0.71 and the resonant wavelength is 1311 nm) is obtained. This all-dielectric metasurface is highly sensitive to human body fluids (glucose solution, sweat, and urine) and ambient temperature, which can be applied in health information and temperature sensors in travel or healthy care hotel, etc. Based on optimization the crystallinity of the GST block, the dynamic modulation characteristics is achieved by this all-dielectric metasurface. The measurement results show that P2 peak is insensitive to the incidence angle of electromagnetic wave. On the contrary, with the increase of incidence angle, the P1 peak is gradually divided into two new absorption peaks (regardless of whether the GST block is in amorphous state or crystalline state).

Near visible pump high efficiency supercontinuum generation spanning from high-order-mode within Ta2O5 waveguides

Broadband optical sources emitting low pulse energy have attracted significant attention owing to their remarkable potential for quantum technology. One of the obstacles to achieving this objective is the weak anomalous dispersion. In this study, supercontinuum generation (SCG) from 645.3 nm to 851 nm pumped by a Ti:sapphire femtosecond laser at a center wavelength of 800 nm is demonstrated by using a high-order mode (transverse electric TE20) and engineering the dispersion within the high nonlinear refractive index material, the tantalum pentoxide (Ta2O5) waveguide. The total on-chip average pulse energy of around few pJ used to excite SCG shows clear future for applications.

Multichannel filtering properties of one-dimensional graphene photonic crystals with high refractive index media layer

Graphene photonic crystals have been attracting increasing attention due to their distinctive properties. In this work, the multichannel filtering performance of a one-dimensional graphene photonic crystal structure is theoretically investigated in the optical communication band. The graphene photonic crystal composed of a stack of alternating layers of dielectric material with high refractive index and graphene sheet. The influence of several structural parameters on the filtering performance of this structure was analyzed using the transfer matrix method. It is shown that the channel number and channel spacing of the multichannel filter can be adjusted by changing the period number and the dielectric constant and thickness of the high refractive index material. The multichannel filtering properties of this graphene structure have potential applications in wavelength division multiplexing systems.

Residual dispersion compensation photonic crystal fiber based on simulated annealing algorithm and Livelink data interaction

An octagonal photonic crystal fiber (O-PCF) structure with equal air hole diameter of the cladding is proposed for residual dispersion compensation, and a high refractive index material SF57 is introduced in the core region to make it have the advantages of high negative dispersion, low attenuation loss, low fabrication difficulty. Due to the symmetric structure, no additional polarization control is required in the dispersion compensation process, which further reduces the production cost. Using Livelink data interaction not only accelerates the modeling of O-PCF, but also enables the effective refractive index of HE11 to be unattended continuously searched. Furthermore, simulated annealing algorithm is used to reverse design the structural parameters of cladding air hole diameter and central solid hole diameter. The results show that, based on the simulated annealing algorithm interacting with Livelink data, the optimize efficiency, design accuracy and dispersion characteristics are improved. The ultra-flat high negative dispersion O-PCF can be reverse designed in the S + C + L and S + C + L + U bands which shows the average dispersion of −847.34 ps/(nm·km) and −752.90 ps/(nm·km) with the absolute dispersion of only ±7.25 ps/(nm·km) and ±10.28 ps/(nm·km), respectively. The designed RDCF is tolerant to ±1% and ±2% variation of the central solid hole diameter and the cladding air hole diameter during fabrication, respectively. The combination of simulated annealing algorithm and Livelink data interaction provides an efficient method for reverse design of PCF.

High Sensitivity and Wide Range Refractive Index Sensor Based on Surface Plasmon Resonance Photonic Crystal Fiber.

In order to detect the refractive index (RI) of high refractive index materials such as trichlorobenzene and aniline in the near-infrared and mid-infrared spectra and expand the detection range of the refractive index, a surface plasmon resonance (SPR) photonic crystal fiber (PCF) sensor based on an elliptical sensing channel is proposed for high refractive index detection. The fiber core and the analyte channel are surrounded by two types of air holes with different sizes. When the surface plasmon resonance effect appears at the interface between the fiber core and the elliptical sensing layer, obvious resonance peaks appear in the near-infrared and mid-infrared bands. The full vector finite element method (FEM) is used to study the sensing characteristics of the sensor and the influence of structural parameters on the resonance peak. The results demonstrate that the sensor achieves detection in the refractive index range of 1.41-1.58, in the wavelength range of 1600-3200 nm. The average wavelength sensitivity is 9217.22 nm/RIU, and the refractive index resolution is 10.85 × 10-6 RIU. The proposed sensor realizes high refractive index detection in the near-infrared and mid-infrared bands, and obtains an ultra-wide detection range and higher sensitivity. The sensor has broad application prospects in chemical detection, biomedical sensing and other fields, and provides a theoretical reference for the design of a photonic crystal fiber surface plasmon resonance sensor.

Uric acid is a major chemical constituent for the whitish coloration in the medaka leucophores.

In whitish parts of teleost skin, the coloration is attributed to a light scattering phenomenon within light-reflecting chromatophores, namely leucophores and iridophores, which contain high refractive index materials in their cytoplasmic organelles, leucosomes and light-reflecting platelets, respectively. Previous chemical examinations revealed that guanine is a major constituent of the materials in the platelets of the iridophores, while, in leucophores, the detailed chemical nature of the materials contained in the leucosomes has not been reported. Here, using liquid chromatography-tandem mass spectroscopy, we investigated the chemical features of materials eluted from scales, larvae, and single chromatophores of the medaka. Results of the liquid chromatography-tandem mass spectroscopy suggested that uric acid is a major constituent of the high refractive index materials in medaka leucophores and is a unique marker to investigate the presence of leucophores in the fish. The whitish appearance of the medaka leucophores may be attributed to the light-scattering phenomenon in leucosomes, which contain highly concentrated uric acid.