High Refractive Index Research Articles

Tantalum pentoxide on a fused quartz substrate platform for advanced photonic integrated circuits

Integrated photonics is a growing field in optics and microelectronics. In particular, tantalum pentoxide (Ta2O5) is a promising material for advancing integrated photonic circuits. Ta2O5 exhibits favorable characteristics, such as a high refractive index, wide transparency window, and low autofluorescence. Therefore, this study develops low-loss Ta2O5 waveguide-based microring resonators optimized for telecom band operations on fused quartz substrates. The experiments demonstrated the excellent optical properties of Ta2O5 for fabricating high-performance photonic structures. Moreover, we explored integrating diamond-inverted nanocones with Ta2O5 waveguides for single-photon emission. The findings provide insights into using Ta2O5 to develop single-photon emitters integrated into photonic circuits.

Quantitative Investigation of Layer-by-Layer Deposition and Dissolution Kinetics by New Label-Free Analytics Based on Low-Q-Whispering Gallery Modes

A new instrument for label-free measurements based on optical Low-Q Whispering Gallery Modes (WGMs) for various applications is used for a detailed study of the deposition and release of Layer-by-Layer polymer coatings. The two selected coating pairs interact either via hydrogen bonding or electrostatic interactions. Their assembly was followed by common Quartz Crystal Microbalance (QCM) technology and the Low-Q WGMs. In contrast to planar QCM sensor chips of 1 cm, the WGM sensors are fluorescent spherical beads with diameters of 10.2 µm, enabling the detection of analyte quantities in the femtogram range in tiny volumes. The beads, with a very smooth surface and high refractive index, act as resonators for circular light waves that can revolve up to 10,000 times within the bead. The WGM frequencies are highly sensitive to changes in particle diameter and the refractive index of the surrounding medium. Hence, the adsorption of molecules shifts the resonance frequency, which is detected by a robust instrument with a high-resolution spectrometer. The results demonstrate the high potential of the new photonic measurement and its advantages over QCM technology, such as cheap sensors (billions in one Eppendorf tube), simple pre-functionalization, much higher statistic safety by hundreds of sensors for one measurement, 5–10 times faster analysis, and that approx. 25, 000 fewer analyte molecules are needed for one sensor. In addition, the deposited molecule amount is not superposed by hydrated water as for QCM. A connection between sensors and instruments does not exist, enabling application in any transparent environment, like microfluidics, drop-on slides, Petri dishes, well plates, cell culture vasculature, etc.

Nanoring Tamm cavity in the telecommunications O band

Quantum and classical telecommunications require efficient sources of light. Semiconductor sources, owing to the high refractive index of the medium, often exploit photonic cavities to enhance the external emission of photons into a well-defined optical mode. Optical Tamm States (OTSs) in which light is confined between a distributed Bragg reflector and a thin metal layer have attracted interest as confined Tamm structures are readily manufactureable broadband cavities. Their efficiency is limited however by the absorption inherent in the metal layer. We propose a nanoring Tamm structure in which a nanoscale patterned annular metasurface is exploited to reduce this absorption and thereby enhance emission efficiency. To this end, we present designs for a nanoring Tamm structure optimized for the telecommunications O band and demonstrate a near doubling of output efficiency (35%) over an analogous solid disk confined Tamm structure (18%). Simulations of designs optimized for different wavelengths are suggestive of annular coupling between the Tamm state and surface plasmons. These designs are applicable to the design of single photon sources, nano-LEDs, and nanolasers for communications.

High-refractive-index glass device fabricated by room temperature bonding

Abstract Wafer bonding is an essential technology for attaching various materials, including glass. Glass-glass bonding has been applied to fabricate glass micro/nanofluidic devices that work as analytical tools for chemical and biological research. Here, we report the fabrication of micro/nanofluidic devices consisting of a high refractive index (HRI) glass as a component of the device for the first time. Although HRI glass has been used for high-sensitivity and high-resolution imaging in microscopy, it has not been used for glass fluidic devices due to the lack of bonding method for HRI glass. The glass fluidic devices were made through glass-glass bonding at room temperature, which could avoid severe damage to bonded substrates of two glass types due to different rates of thermal expansion. The pressure endurance of these devices was measured to find the correlation with the surface properties of glass substrates. The measured results indicated that chemical groups other than SiOH enhanced the pressure endurance of the devices. Thus, these chemical groups helped to form the fluidic devices consisting of HRI glass and another glass type, borosilicate or fused silica, to achieve practical pressure endurance up to 270 kPa. The HRI glass-incorporating device showed 8-fold higher sensitivity and 30 % narrower spatial resolution of fluorescence signals than the other devices with non-HRI glass.

Comparison between InAs-based and GaSb-based interband cascade lasers with hybrid superlattice plasmon-enhanced claddings

We compare InAs-based and GaSb-based interband cascade lasers (ICLs) with the same 12-stage active region designed to emit at a wavelength of 4.6 µm. They employ a hybrid cladding architecture with the same geometry and inner claddings consisting of InAs/AlSb superlattices but different outer claddings: The InAs-based ICL employs plasmon enhanced n-type doped InAs layers while the GaSb-based ICL employs plasmon-enhanced n-type doped InAs0.915Sb0.085 claddings lattice matched to GaSb. Due to the lower refractive index of n + -InAsSb (n = 2.88) compared to n+ -InAs (n = 3.10) and higher refractive index of separate confinement layers, the GaSb-based ICL shows a 3.8% higher optical mode confinement in the active region compared to the InAs-based ICL. However, the InAs-based ICL has 15.3% lower free carrier absorption losses than the GaSb-based ICL, resulting in approximately equal threshold gains. Experimentally operated in pulsed mode and at room temperature, the GaSb-based ICL shows a 15.3% lower threshold current density, but also 12.8% higher threshold voltage resulting in comparable threshold power densities. Also presented is the influence of geometry and doping variation on confinement factors and calculated free carrier absorption losses in the GaSb-based ICL.

Linear and nonlinear optical properties of femtosecond laser inscribed waveguides into GLS glass

Gallium lanthanum sulfide (GLS) glass is a promising material for mid-infrared photonics due to its wide transmission window and its high nonlinear refractive index that is almost three orders of magnitude higher than that of fused silica. In this paper, we present the results of a detailed study into the linear and nonlinear optical properties of waveguides fabricated in GLS glass via ultrafast laser direct-inscription using three different techniques: cumulative heating in the thermal regime as well as multi-scan and half-scan in the athermal regime. Using quadriwave lateral shearing interferometry, we fully characterized the refractive index profiles of such inscribed waveguides and found no difference between half-scan and multi-scan writing which indicates the absence of laser-induced stress in this soft glass in stark contrast to fused silica. In terms of nonlinearity, we utilized self-phase modulation (SPM)-induced spectral broadening experiments at mid-IR wavelengths to demonstrate that waveguides fabricated in the athermal regime preserve the high intrinsic nonlinearity of the GLS bulk material, outperforming those written in the thermal regime based. These findings pave the way for the fabrication of fiber-coupled optical waveguide chips for nonlinear mid-infrared photonics.

Fabrication of silicon microparticle dispersion as terahertz wave refractive index control material

Optical systems utilizing terahertz waves hold significant potential across various fields; however, the materials for the optical elements that constitute such systems are currently scarce and the range of refractive indices that can be handled is limited. In the visible light region, there are refractive index control materials made of nanocomposites with dispersed dielectric nanoparticles. However, few technologies for refractive index control materials exist in the terahertz wave region. In this study, we developed an Si microparticle dispersion in which Si microparticles are dispersed in a cycloolefin polymer (COP) matrix as a high refractive index material for terahertz waves. Results showed that a material with a refractive index between that of Si and COP can be designed by changing the volume fraction of the Si cube, a subwavelength structural particle, in the dispersion. A method in which Si cubes werefabricated by dicing with COP and using a mold was implemented to produce four types of Si microparticle dispersions with Si concentrations of 1 vol%, 5 vol%, 15 vol%, and 25 vol%. The effective refractive index was evaluated, and results demonstrated that in the 0.1–0.2 THz range, including the D band of the Beyond 5G/6G communication band, the effective refractive index values were according to the concentration, achieving control in the range of 1.53–1.96. Additionally, the Si cubes were uniformly dispersed in the dispersion with random orientations in three dimensions, making them an optical material with minimal polarization dependence. The proposed Si microparticle dispersion can be used as a new refractive index control material in the terahertz band.

Aromatic Poly(dithioacetal)s: Spanning Degradability, Thermostability, and High Refractive Index Towards Eco-friendly Optics.

In the quest for eco-friendly optics, high refractive index polymers (HRIPs) with degradability have been one of the desirable optical materials for realizing eco-friendly and efficient lighting technologies. However, it has been challenging for HRIPs to simultaneously realize thermostability, high refractive index (RI), visible transparency, and efficient degradability, all of which are essential for their practical use. In this context, we herein focus on aromatic poly(dithioacetal)s, composed of visible-transparent yet degradable dithioacetal moieties and rigid phenylene sulfide spacers, exhibiting moderately high Tg (> 60 °C), high RI (> 1.7), and colorless film features. In addition, poly(dithioacetal)s can balance (1) high stability under the operating conditions even upon heating and (2) quantitative degradability that can selectively yield cyclic low-molecular-weight products that can be further repolymerized upon further addition of an acid catalyst. These results provide a key concept for high refractive index polymers that allow on-demand degradability and recyclability without compromising their high potential thermal and optical properties.

Enhanced absorption in SnS/SnSe, SnS/ZnS, and SnS/ZnSe vdW heterostructures for optoelectronic applications: DFT insights

Abstract The electronic and optical properties of monolayers of tin monochalcogenides and zinc monochalcogenides are elucidated by utilizing density functional theory. The calculated results indicate that the monolayers of tin monochalcogenides (SnS and SnSe) have low bandgap and significant absorption in some segments of the visible region (∼400 nm to ∼500 nm). However, the monolayers of zinc monochalcogenides (ZnS and ZnSe) have wide bandgap and negligible absorption in the visible region, which limits their optical performance. Despite low absorption in visible region, ZnS and ZnSe exhibit fascinating properties such as wide band gap, cheapness, low toxicity, earth abundance, structural stability, and high refractive index. To identify the combined potential of zinc and tin, the van der Waals heterostructures SnS/SnSe, SnS/ZnS, and SnS/ZnSe are formed, and their optical and electronic properties are calculated. The calculated results illustrate that the formed heterostructures exhibit bandgap lowering and enhanced visible light absorption. The optical absorption is entirely shifted towards the visible region due to the formation of heterostructure (redshift). The enhanced visible light absorption and narrowed bandgap of the formed heterostructures make them a potential candidate for the fabrication of optoelectronic devices and solar cells.

All-dielectric transmissive narrow-band filters adjustable within wide spectral range

The challenge of obtaining transmission spectra with both a wider tunable spectral range and high resolution persists. In this paper, a novel all-dielectric transmissive grating structure based on Mie resonance is proposed. The structure excites the Mie resonance by embedding a high refractive index contrast grating in a KF-Si film system band-stop filter. The dipole resonance energy is confined within the high refractive index silicon grating material, expanding the cutoff bandwidth of the KF-Si film system to over 400 nm and achieving single-peak transmission in the visible range. And the continuous transmission spectrum has an adjustable range of 300 nm with the highest spectral resolution (FWHM) (∼20 nm) can be achieved by adjusting the grating period. To suppress the coupling energy excitation between the higher-order magnetic dipole and the substrate, a silicon-rich nitride (SRN) matching layer is introduced between the KF-Si film system and the substrate. This layer enhances the intensity of the resonance peaks while simultaneously suppressing the short-wavelength sidebands, thereby improving the saturation and purity of the spectrum. In addition, we obtained a large angular tolerance of up to 15° by stacking the silicon film system. This work is of great significance for the advancement of hyperspectral imaging and display technology.

Enhanced optical and electrical properties of PEO/PMMA/TiO2 nanocomposites for optoelectronic applications

In this study, we synthesized titanium oxide (TiO2) nanoparticles (NPs) via the sol-gel process and incorporated them into a polymer blend of polyethylene oxide (PEO) and polymethyl methacrylate (PMMA) to form PEO/PMMA/TiO2 nanocomposites. The TiO2 NPs were integrated into the polymer matrix using the casting method at varying concentrations (1, 2, and 3 wt%). The structural, morphological, optical, and electrical properties of the prepared TiO2 and nanocomposite films were thoroughly characterized using techniques such as transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, ultraviolet–visible (UV–Vis) spectroscopy, and electrical impedance spectroscopy (EIS). TEM analysis revealed predominantly spherical TiO2 NPs with an average particle size of 15 nm. XRD and FTIR analyses confirmed the successful incorporation of TiO2 NPs and their interaction with the polymer chains, leading to modifications in the crystalline structure and chemical bonding of the nanocomposites. UV–Vis spectroscopy demonstrated a redshift in the absorption edge and increased absorbance with higher TiO₂ content, indicating enhanced optical properties. The indirect optical bandgap was observed to decrease from 4.19 eV to 2.21 eV as the TiO2 concentration increased, enhancing the material's photoresponsiveness. Additionally, the refractive index increased from 2.11 to 2.91, further supporting the potential for optical applications. EIS results showed a decrease in bulk resistance with increasing TiO2 concentration, suggesting improved electrical conductivity. These findings highlight the potential of PEO/PMMA/TiO2 nanocomposites for optoelectronic applications, where enhanced optical properties, such as improved light absorption, higher refractive index, and improved charge transport, are critical for device performance. The study offers an understanding of the PEO/PMMA/TiO2 nanocomposites, laying a foundation for future research in their applications in optoelectronic devices.

Light Harvesting Materials: A Study on Förster Resonance Energy Transfer and Optoelectronic Properties of Potential Nerium oleander Flowers.

The present study focused on extracting the anthocyanin dyes in ethanol, acidic ethanol, methanol, and acidic methanol solvents from Nerium oleander flowers by a simple maceration extraction process. FTIR spectroscopy and vibrational studies have confirmed the existence of polyphenolic groups in 2-phenyl chromenylium (anthocyanin) dyes. The optoelectronic results show the least direct bandgap (2.89 eV), indirect bandgap (1.98 eV), Urbach energy (0.120 eV), high refractive index (1.654), dielectric constant (3.294) and high optical conductivity (1.813 103 S/m) for the anthocyanin dye extracted found in acidic ethanol solvent. The photoluminescence properties such as Stokes' shift, quantum yield, and lifetime results show that anthocyanin dyes are promising candidates for red-LEDs and optical materials. The excellent correspondence between the absorption and emission spectra reinforces that the anthocyanins are efficient (89%) FRET probes. Further, the donor and acceptor undergo redshift in excitation and emission spectra in all studied solvents. The photometric properties such as CIE, CRI, CCT and color purity results of anthocyanins in all studied solvents revealed that this material exhibits pink to red shades (x = 0.40 → 0.50 and y = 0.46 → 0.39) and is well suitable for have great potential in the manufacturing of Organic-LEDs and other optoelectronic device applications.

Scalable SOA-based Banyan optical space switch on InP membrane on silicon (IMOS).

Optical switches (OS) are vital for meeting modern data centers' capacity and latency needs, overcoming the bandwidth and speed limitations that electronic switches encounter. To this end, this work proposes a semiconductor optical amplifier (SOA)-based OS on the IMOS, a unique platform with a combination of high refractive index contrast for compactness and monolithic active-passive integration for active functionalities, thereby enabling the creation of compact SOA-based OSes. An 8 × 8 Banyan SOA-based OS is integrated on a 4 × 4 mm2 area on the standard 4 × 6 mm2 IMOS cell. This marks the first application of the IMOS technology platform for SOA-based OSes and sets the stage for compact and large-scale monolithic photonic integrated OS circuits. The basic 2 × 2 switch module delivers a high optical signal-to-noise ratio (OSNR) and an extinction ratio (ER) above 45 dB. Employing NRZ-OOK routing on the 2 × 2 basic OS module, a 15 dB input power dynamic range (IPDR) is achieved within a 1 dB power penalty at 12.5 Gb/s. Data routing at higher data rates of 25 Gb/s and 40 Gb/s incurs power penalties of 0.8 and 1.4 dB, respectively. Furthermore, data routing for a 3-stage 8 × 8 switch operating at 25 Gb/s results in a power penalty of 1.2 dB. Compared to the 1-stage 2 × 2 switch, only a 0.4 dB additional penalty is observed despite incorporating 2 additional SOAs in the cascade. This indicates that the switch can scale to higher radix to accommodate multi-stage switches with numerous ports, large bandwidth, and fast speed, which is essential in modern large-scale data centers.

The effect of photo-thermo-induced crystallization on the properties and microstructure of transmission volume Bragg gratings

Volume Bragg gratings (VBGs) recored on photo-thermo-refractive (PTR) glass are extensively utilized in high power lasers, optical communication, holographic display fields, etc. In the majority of application fields, high refractive index modulation (RIM) value and high transmittance are the required parameters for transmission Volume Bragg gratings (TVBGs) devices. This study indicates that the RIM values and transmittance of TVBGs strongly depend on the size and concentration of sodium fluoride (NaF) crystals within the grating periodic structure. Increasing the UV exposure dose causes a reduction in the size of NaF crystals, leading to a finitely rise in the RIM value of TVBGs. Prolonging the crystallization heat treatment time and elevating the temperature effectively enhance the RIM value of TVBGs, which also significantly enlarge the size of NaF crystals in the grating periodic structure. By fine-tuning the process parameters of photothermal induced crystallization, a RIM value of 20–320 ppm is freely controlled. For 1.5 mm thick TVBGs, the above RIM value range can meet the application requirements of achieving high diffraction efficiency in the visible to near-infrared range. Following UV exposure at a dose of 5 J/cm2 and nucleation at 480 °C for 120 min, crystallization treatments at 550 °C for 30–60 min and at 570 °C for 60–120 min are determined as optimal for fabricating TVBGs with high diffraction efficiency and transmittance in the visible (532 nm) and near-infrared (1053 nm) bands, respectively.

Silicon rich nitride: a platform for controllable structural colors

Abstract High refractive index dielectric materials like silicon rich nitride (SRN) are critical for constructing advanced dielectric metasurfaces but are limited by transparency and complementary metal oxide semiconductor (CMOS) process compatibility. SRN’s refractive index can be adjusted by varying the silicon to nitride ratio, although this increases absorption, particularly in the blue spectrum. Dielectric metasurfaces, which utilize the material’s high dielectric constant and nano-resonator geometry, experience loss amplification due to resonance, affecting light reflection, light transmission, and quality factor. This study explores the impact of varying the silicon ratio on structural color applications in metasurfaces, using metrics such as gamut coverage, saturation, and reflection amplitude. We found that a higher SRN ratio enhances these metrics, making it ideal for producing vivid structural colors. Our results show that SRN can produce a color spectrum covering up to 166 % of the sRGB space and a resolution of 38,000 dots per inch. Fabricated samples vividly displayed a parrot, a flower, and a rainbow, illustrating SRN’s potential for high-resolution applications. We also show that SRN can provide a better CIE diagram coverage than other popular metasurfaces materials. These findings highlight the advantages of SRN for photonic devices, suggesting pathways for further material and application development.

Ultrafast Laser-Fabricated Fluoride Glass Waveguides with Exceptionally High Positive Refractive Index Change for Mid-Infrared Integrated Optics

Ultrafast Laser-Fabricated Fluoride Glass Waveguides with Exceptionally High Positive Refractive Index Change for Mid-Infrared Integrated Optics

Fabrication of Ripple Structured Silicon Carbide (SiC) Films for Nano‐Grating and Solar Cell Applications

AbstractIn the present study, we aim to investigate the self‐organization of unexplored silicon carbide (SiC) film surfaces under 30 keV oblique Ar+ ions irradiation and hence unprecedented tailoring of optical and electrical characteristics with view of their uses in solar cells, gratings and nano‐ to micro‐scale devices. The surface morphology mainly consisted of triangular shaped nanoparticles which evolves into nanoscale ripple structures with an alignment parallel to the projection of ion beam direction. For the first time, we have demonstrated the fabrication of highly‐ordered ripple patterns with wavelength in visible region over SiC films and applicable as nano‐gratings. The underlying mechanism relies on the structural rearrangement due to transition of film microstructure from amorphous to mixed phase (crystalline, nano‐crystalline and amorphous) and lowering of C=C and C−C vibration modes by the heavier Si atoms. These nanostructured silicon carbide film shows unparalleled optical (energy gap decreases from 4.60±0.4 eV to 3.16±0.2 eV) & electrical characteristics (conductivity increases from 6.6×10−11 to 1.12×10−3 S/m with linear I−V behavior). Thus, we propose that ripple structured SiC films with wide band gap, high refractive index and high electrical conductivity with ohmic behaviour are promising candidates for application as window layer in solar cells and opto‐electronics.

Highly oriented as-grown beta-phase bismuth oxide for optical MEMS

AbstractReactive sputtering of bismuth has been used to form highly oriented beta-phase Bi2O3 thin films at 120 °C. Notably, the films consist of nearly vertical ~ 50 nm diameter nano-filaments of highly aligned β phase pure bismuth oxide. The optical characteristics of the films include a direct optical bandgap of 3.95 eV at 300 K and nearly negligible optical absorption in the wavelength range from 500 to 3000 nm. Nanoindentation experiments show that the mechanical characteristics of the films include reduced modulus and hardness values of 90 and 4.5 GPa, respectively. As fabricated, the films exhibit tensile stress which in combination with a relatively high refractive index value in the range of 2.4–2.5 makes the realized Bi2O3 films structurally and optically suitable for fabrication of optical microelectromechanical (MEMS) devices with applications in the visible to mid-infrared wavelength region, such as MEMS-based Fabry–Pérot microspectrometers, which is demonstrated through simulations. Graphical Abstract

Fabrication of slanted gratings with high refractive index starting from master nanoimprint mold.

Recently, nanoimprinting has attracted a new round of attention in the industry due to the boom in demand for augmented reality/virtual reality (AR/VR), metalens and microlens, and even semiconductors. Slanted gratings have great application prospects in AR/VR displays because of their high efficiency in light coupling. UV-Nanoimprint lithography (UV-NIL) has been identified as one of the most feasible routes for mass manufacture of high refractive index (RI) slanted gratings. This paper presents a fabrication of high RI slanted gratings based on UV-NIL. A comprehensive study on the optical principles of slanted gratings is conducted, followed by simulation-based optimization of the grating parameters. The key element for applying nanoimprint to fabricate slanted gratings is the master mold, which is acquired by a tilted angle etching of metal gratings as an etching mask on silicon wafers with F-based plasma. The influence of experimental parameters, such as the etching power and etching mask thickness on the morphology of the slanted gratings on the master mold are investigated. The working mold was simply duplicated from the master mold by UV-NIL with a low surface energy working mold material. The high RI slanted gratings were achieved by imprinting a UV-curable resin with high RI. Finally, experimental verification was performed to assess the optical performance of the slanted gratings.

The Preparation of a Novel Silicone Optical Adhesive with Low Hardness and High Refractive Index

The Preparation of a Novel Silicone Optical Adhesive with Low Hardness and High Refractive Index