Large Refractive Index Research Articles

Numerical study of microstructured fiber with concentric dual rings for supporting orbital angular momentum mode propagation

A microstructured fiber with two concentric high-refractive-index rings is proposed to support more orbital angular momentum (OAM) beam propagations and OAM-based mode-division multiplexing (MDM) for improving the channel capacity and spectral efficiency. The two ring cores can be considered as two independent OAM channels without interference for enhancing more OAM channels. A finite element method (FEM) is employed to numerically study the properties of the designed fiber. A large effective refractive index difference between the adjacent OAM modes is achieved, which indicates that the propagated OAM modes can be well separated without multiple input, multiple output in each of the two rings. The calculated results demonstrate that a total of 80 OAM modes with a mode quality above 70% is achieved without any high-order radial modes from 1.5 to 1.6 μm, which contains 18 inner-ring OAM modes and 62 outer-ring OAM modes. Moreover, the designed fiber possesses low confinement loss (<2.2 × 10−7 dB m−1) and can be potentially utilized for high-capacity communications by OAM-based MDM.

Extreme ultraviolet metalens by vacuum guiding.

Extreme ultraviolet (EUV) radiation is a key technology for material science, attosecond metrology, and lithography. Here, we experimentally demonstrate metasurfaces as a superior way to focus EUV light. These devices exploit the fact that holes in a silicon membrane have a considerably larger refractive index than the surrounding material and efficiently vacuum-guide light with a wavelength of ~50 nanometers. This allows the transmission phase at the nanoscale to be controlled by the hole diameter. We fabricated an EUV metalens with a 10-millimeter focal length that supports numerical apertures of up to 0.05 and used it to focus ultrashort EUV light bursts generated by high-harmonic generation down to a 0.7-micrometer waist. Our approach introduces the vast light-shaping possibilities provided by dielectric metasurfaces to a spectral regime that lacks materials for transmissive optics.

A Hybrid Cladding Ring-Core Photonic Crystal Fibers for OAM Transmission with Weak Spin–Orbit Coupling and Strong Bending Resistance

A hybrid cladding ring-core photonic crystal fiber (PCF) for transmitting orbital angular momentum (OAM) modes is proposed, which breaks the circular symmetry of the fiber structure to suppress the spin–orbit coupling and promotes bending resistance. Through the optimization of fiber structure parameters, the designed fiber can support 22 OAM modes (6 OAM mode groups) over a 200-nm wide bandwidth (covering the whole C + L band) with large effective refractive index separation between adjacent modes (&gt;10−4) and mode groups (&gt;3.6 × 10−3), low confinement losses (&lt;3.5 × 10−9 dB/m), and high mode purity (&gt;98.3%). Meanwhile, the phase of the OAM modes varies periodically and uniformly with an increase in the azimuth angle, and the polarization of OAM modes maintain nearly circular polarization in the designed fiber, which also demonstrates that the fiber has weak spin–orbit coupling. Moreover, the confinement losses of all vector modes are less than 10−7 dB/m when the bending radius is larger than 0.8 mm, indicating strong bending resistance. Furthermore, the fiber also exhibits large differential group delay, relatively low and flat dispersion, and low nonlinear coefficients (&lt;2.0 W−1/km). Therefore, the novel fiber structure has great potential in the application of mode division multiplexing (MDM) based on OAM modes.

Excitonic Beam Steering in an Active van der Waals Metasurface.

Two-dimensional transition metal dichalcogenides (2D TMDCs) are promising candidates for ultrathin active nanophotonic elements due to the strong tunable excitonic resonances that dominate their optical response. Here, we demonstrate dynamic beam steering by an active van der Waals metasurface that leverages large complex refractive index tunability near excitonic resonances in monolayer molybdenum diselenide (MoSe2). Through varying the radiative and nonradiative rates of the excitons, we can dynamically control both the reflection amplitude and phase profiles, resulting in an excitonic phased array metasurface. Our experiments show reflected light steering to angles between -30° and 30° at different resonant wavelengths corresponding to the A exciton and B exciton. This active van der Waals metasurface relies solely on the excitonic resonances of the monolayer MoSe2 material rather than geometric resonances of patterned nanostructures, suggesting the potential to harness the tunability of excitonic resonances for wavefront shaping in emerging photonic applications.

All-optical polarization tuning based on an intensity-dependent nonlinear metasurface

Active control of light polarization at the nanoscale is essential for integrated photonic devices. Here, an all-optical approach is proposed to tune the polarization state of near-infrared light using a nonlinear metasurface. Based on the large intensity-dependent refractive index change of epsilon-near-zero (ENZ) materials, a phase difference between two orthogonal electric fields at ENZ wavelength can be continuously tuned in a range larger than 0.61π by varying the incident light power. The polarization state of the reflected light can thus be actively tuned from linear to circular state via an all-optical approach. Notably, abundant polarization states can be obtained by altering the polarization angle of incident light. The proposed all-optical approach is promising for tunable photonic functionalities of practical applications.

Topologically optimized concentric-nanoring metalens with 1 mm diameter, 0.8 NA and 600 nm imaging resolution in the visible.

Metalenses can achieve diffraction-limited focusing via localized phase modification of the incoming light beam. However, the current metalenses face to the restrictions on simultaneously achieving large diameter, large numerical aperture, broad working bandwidth and the structure manufacturability. Herein, we present a kind of metalenses composed of concentric nanorings that can address these restrictions using topology optimization approach. Compared to existing inverse design approaches, the computational cost of our optimization method is greatly reduced for large-size metalenses. With its design flexibility, the achieved metalens can work in the whole visible range with millimeter size and a numerical aperture of 0.8 without involving high-aspect ratio structures and large refractive index materials. Electron-beam resist PMMA with a low refractive index is directly used as the material of the metalens, enabling a much more simplified manufacturing process. Experimental results show that the imaging performance of the fabricated metalens has a resolution better than 600 nm corresponding to the measured FWHM of 745 nm.

Understanding the conditions for the optimum nonlinear refraction of epsilon-near-zero films based on transparent conducting oxides.

Transparent Conducting Oxides (TCOs) exhibit a large and ultrafast intensity-dependent refractive index in their Epsilon-Near-Zero (ENZ) spectral region, which depends dramatically on the material properties and measurement arrangement conditions. Therefore, attempts to optimize the nonlinear response of ENZ TCOs usually involve extensive nonlinear optical measurements. In this work, we show that significant experimental work can be avoided by carrying out an analysis of the material's linear optical response. The analysis accounts for the impact of thickness-dependent material parameters on the absorption and field intensity enhancement under different measurement conditions and estimates the incidence angle required for achieving the maximum nonlinear response for a given TCO film. We perform measurements of angle-dependent and intensity-dependent nonlinear transmittance for Indium-Zirconium Oxide (IZrO) thin films with different thicknesses and demonstrate a good agreement between the experiment and theory. Our results also indicate that the film thickness and the excitation angle of incidence can be adjusted simultaneously to optimize the nonlinear optical response, allowing a flexible design of TCO-based highly nonlinear optical devices.

Tm3+/Ho3+ co-doped tellurate glass with high emission cross section for 2μm fiber lasers

A modified melt quenching technique was used to prepare new Tm3+/Ho3+ co-doped TeO2-Ta2O5-La2O3 (TTL) glass with high emission cross section. By standing the traditional tube furnace sideways, the melt quenching method is optimized to produce tellurate glass efficiently and simply. The performance of the glass system is studied in detail, and the possibility of this glass system as a 2.0 μm fiber laser material is obtained. The glass system was obtained by test analysis to have large refractive index (nd>2.01) and density (>5.28 g/cm3), excellent mechanical properties (>4.5Gpa). TTL5 glass was obtained with excellent glass transition temperature (Tg>439 °C) and good resistance to crystallization based on thermodynamic analysis(ΔT>170 °C). In addition, the bonding structure was studied and maximum phonon energy of TTL5 glasses was determined to be ∼669 cm−1 by Raman. The SEM micrographs show well the surface micromorphology of TTL3 and TTL5, and EDAX verifies that the results of elemental analysis of this system of glasses are reliable. Fluorescence spectroscopy demonstrated a TTL5 series of samples with 2.0 μm strong emission. Finally, the absorption emission cross-sections of Ho3+ were calculated to be as high as7.92×10−21 (at 1963 nm) and 12.301×10−21 (at 2061 nm), respectively, with a maximum gain factor of 2.75 cm−1. The results show that this new TTL series glass has high thermal stability, low phonon energy, and efficient 2.0 µm fluorescence, extending the tellurite glass system and being a good candidate for obtaining efficient 2.0 µm emission.

Broadband single-cell-driven multifunctional metalensing

Metasurfaces are artificially engineered ultrathin photonic components that can be freely designed to exhibit unprecedented capabilities of highly-efficient electromagnetic wave manipulation. The ever-growing demand for miniaturized photonic devices for emerging applications, like imaging, spectroscopy, biosensing, and quantum information processing, consistently requires broadband multifunctional and highly-efficient meta-devices. Recent years have witnessed tremendous advancements in metasurfaces; however, investigating the novel platform to realize broadband metasurfaces that integrate multiple functionalities in a single-layered structure would be an obvious technological extension. Here, we present a broadband single-cell-driven multifunctional metasurface platform capable of manipulating electromagnetic waves over a wide range of visible wavelengths (475−650 nm). A lossless zinc sulfide material exhibiting a sufficiently large refractive index and negligible extension coefficient across the visible spectrum is exploited to demonstrate the state-of-the-art meta-devices. Furthermore, a well-known spin-decoupling technique is implemented to multiplex different optical phenomena into a single-cell-driven structure. For proof of the concept, we demonstrate two meta-devices that provide transverse and longitudinal splitting of different optical phenomena for the visible wavelengths. The presented zinc sulfide material and unique design philosophy to achieve broadband multifunctional meta-devices may find potential applications in polarization and dispersion analyzers, sensing, optical communication, and many more.

Interdigitated electrodes enhanced photosensitive monolayer WS2 field effect transistor on glass substrate

Recently, 2D-material-based optoelectronic devices are integrated on optical-fiber as a substrate. However, silicon substrate, usually for optoelectronic device integration, impedes fiber-optic integration due to its much larger refractive index. Here, we fabricated a highly transparent photosensitive field effect transistor (FET) using monolayer WS2 on a fiber-compatible glass substrate with interdigitated electrodes. Experiments shows that the current switching ratio for the FET reach up to 105. Additionally, the interdigitated electrodes could improve photocurrent collection capability, leading to a relative high responsivity of 1.48 mA/W. Meanwhile, the difference in the photocurrent induced by light illumination from the front and back to the device is only 1.25%. This indicates light beam can pass through the device, facilitating online monitoring of optical power. Moreover, it is also shown that the transmitted light beam could be reflected to retransmit thought the device, thus increasing photocurrent by 6.8%. Such glass-substrate provide a simple and feasible strategy for future incorporation of optical-fiber with on-chip device.

Ultrafast All-Optical Switching Using Doped Chromoprotein Films

Next-generation communication networks require > Tbit/s single-channel data transfer and processing with sub-picosecond switches and routers at network nodes. Materials enabling ultrafast all-optical switching have high potential to solve the speed limitations of current optoelectronic circuits. Chromoproteins exhibit a large light-controlled refractive index change alleviating the driving energy requirements for optical switching. Here, we report femtosecond transient grating experiments demonstrating the feasibility of < 200 fs all-optical switching by hydrated thin films of photoactive yellow protein and compare the results with those obtained using bacteriorhodopsin. Possibilities for the practical utilization of the scheme in extremely high-speed optical modulation and switching/routing with nominally infinite extinction contrast are discussed.

Modulation of epsilon-near-zero wavelength and enhancement of third-order optical nonlinearity in ITO/Au multilayer films

We report the modulation of epsilon-near-zero (ENZ) wavelength and enhanced third-order nonlinearity in indium tin oxide (ITO)/Au multilayer films. The samples consisting of five-layer 40 nm ITO films spaced by four-layer ultrathin Au films of different thickness, i.e., ITO(40 nm)/[Au(x)/ITO(40 nm)]4, were prepared by magnetron sputtering at room temperature. The ENZ wavelength in the multilayer films is theoretically calculated and experimentally confirmed. The nonlinear refractive index and nonlinear absorption coefficient of the samples of x=0, 2, 3, 4 nm were determined using the Z-scan method at a wavelength of 1.064 µm. The large nonlinear refractive index n2=1.12×10-13 m2/W and nonlinear absorption coefficient β=-1.78×10-7 m/W in the sample of x = 4 nm are both four times larger than those in the single-layer ITO film. The large optical nonlinearity due to the ENZ enhancement and carrier concentration is discussed. The results indicate that the ITO/Au multilayer films are promising for advanced all-optical devices.

Design and analysis of the biconvex liquid lens with circular hole plate electrode structure

In this paper, based on the research of zoom liquid lens with parallel plate electrode and the principle of dielectrophoresis, a model of the biconvex liquid lens with circular hole plate electrode structure is proposed, which is a novel three-layer liquid lens structure. The dielectrophoretic effect refers to the phenomenon that free dielectric molecules will be polarized and moved by the force in a non-uniform electric field, thus deforming the dielectric liquid. In the dielectrophoretic liquid lens, only two insulating liquid materials with large refractive index difference and dielectric constant difference need to be selected, which can increase the selection range of liquid materials. The liquid lens structure mainly consists of a piece of double-sided conductive flat plate ITO glass with a circular hole and two pieces of single-sided conductive flat plate ITO glass, which respectively form two sets of flat electrode structures to control the upper and lower interfaces of the liquid droplet. In this structure, the influence of the intermediate glass plate on the focus and imaging is reduced by using the flat plate electrode with circular hole. The theoretical analysis of the structure is carried out with simulation software. Firstly, the models of the biconvex liquid lens with circular hole plate electrode under different voltages are built with Comsol software, the data of upper and lower interfaces of the liquid droplet are exported. Then by using Matlab, the surface shapes of the upper and lower interfaces of the droplet are fitted and the corresponding aspheric coefficients are obtained. Finally, the optical models are built with Zemax software, the imaging optical paths and the variation range of focal length under different voltages are analyzed. On the basis of the simulation, the corresponding device is manufactured, and the specific experimental analysis is carried out. The surface pattern of the upper and lower interfaces of the droplet of the biconvex liquid lens under different voltages are recorded, the focal length and imaging resolution of the liquid lens are measured. When the operating voltage is 0V-260V, the focal length varies from 23.8mm to 17.5mm, which is basically consistent with the simulation results(22.6mm-15.9mm). The feasibility of the structure of the biconvex liquid lens with circular hole plate electrode structure is verified by experiments. The imaging resolution can reach 45.255 lp/mm. The results show that this proposed novel three-layer liquid structure of the biconvex liquid lens has the characteristics of simple structure, easy to realize and good imaging quality. Therefore, the research of this biconvex liquid lens can provide a new idea for expanding the high-resolution imaging research of liquid lenses and their applications.

U-Grooved Selectively Coated and Highly Sensitive PCF-SPR Sensor for Broad Range Analyte RI Detection

Point-of-care (POC) diagnostic medical devices require simple but sensitive and inclusive detection capabilities. This study develops a unique photonic crystal fiber-based surface plasmon resonance (PCF-SPR) sensor that uses a u-grooved selective coating for extremely sensitive remote and biosensing applications. Scale-down (SCD) air holes control the path that incoming light takes through the sensor in order to activate the free electrons that are present there. To reduce outward radiation, perfectly matched layers (PML) of the absorbing boundary are integrated in addition to optimizing the structural geometry and material thickness. The Finite Element Method (FEM) is employed to quantitatively tune the properties of light directing and sensitivity of the proposed sensor. The proposed sensor has the maximum amplitude sensitivity (AS) of 1189 RIU <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , the highest wavelength sensitivity (WS) of 12,500 nm/(RIU), and an outstanding resolution of 8.0×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> RIU. The constructed model covers a large refractive index (RI) range of unknown samples, ranging from 1.29 to 1.40. This range allows this sensor to cover the detection of a wide range of bio-analytics, including viruses, cancer cells, carbohydrates, proteins, and DNA/RNA etc. We strongly believe that the suggested sensor will play an extremely useful role in the detection of POC bio-sample detection.

Flexible omnidirectional reflective film for CO2 laser protection

In this Letter, we presented a flexible omnidirectional reflective film made of polymer substrates and multiple alternating layers of two chalcogenide glasses for full-angle CO2 laser protection. The structure parameters of the device were simulated for theoretical prediction of best device structure. The reflector was fabricated by alternate thermal evaporation of two chalcogenide glasses with large refractive index contrast. The reflectivity was greater than 78% at 10.6 µm. The flexible reflective film can provide an effective solution for full-angle CO2 laser protection of the moving targets, such as laser operators and mobile optical components, with potential applications for wearable laser protective clothing.

Measurement of scattering intensity distribution of single microparticles/nanoclusters based on laser levitation

The scattering measurement of particulates in gaseous medium is helpful in understanding light transmission, laser detection, combustion radiation and atmospheric environment. In order to explore the scattering characteristics of micron-/nano-sized particles, this paper proposes a method of accurately measuring the scattering intensity distribution of a single micron-sized particles/nanoclusters by combining laser levitation and scattering measurement. An experimental apparatus is first built based on the counter-propagated bi-Bessel beams levitation system and scattering test system. The microparticles/nanoclusters of various matters and sizes are then levitated and their stabilities are evaluated. Finally, the scattering intensity distribution of levitated particles within 2π scattering angle is accurately measured with an angular resolution of 9.2″. The forces acting on particles under laser irradiation and the scattering intensity distribution of different particle parameters are simulated and calculated, and compared with experimental results. The influence of noise on the uncertainty of the scattering measurement system is analyzed in depth, including background light, laser beam, and reflected light from the walls. The results show that the signal-to-noise ratio of scattering measurement for metallic magnesium and aluminum, whether single particles or clusters, are both greater than 20 dB and their maximum values are both 94.6 dB in a range of 2π angle. For graphite nanoclusters, the signal-to-noise ratio in the backscattering direction is relatively poor. The influence of levitation instability on the scattering measurement results is estimated in detail, verifying that the influence of levitation instability in the test system on the scattering measurement is ignorable. Metallic magnesium, aluminum, and graphite particles can be stably levitated by the counter-propagated bi-Bessel beams, with a relative instability of less than 0.15. During the levitation, the photophoretic force plays a dominant role. The scattering intensity distribution of a single micron-sized particles and nanoclusters both conform to the scattering characteristics of Mie particles. Microparticles with large refractive index imaginary parts have stronger forward scattering characteristics. The larger the particle size parameter, the stronger the forward scattering effect becomes. The accurate measurement of the scattering intensity distribution of a single microparticles confirms the versatility and reliability of the levitation scattering test system, providing a new research method for in-depth understanding of the scattering characteristics of substances.

Synthesis of atomically thin yellow pearl: An impetus for nonlinear optical effect assisted light scattering application

Synthesis of 2D materials from natural resources, and having unique functionalities have created enormous research interest among material scientists. Such 2D materials can be used as impetus for light scattering to generate continuous wave (CW) pumped random laser (RL) emission at room temperature. In this work, defects-free 2D yellow pearl (2D-YP) has been synthesized from bulk south sea pearl using liquid-phase exfoliation (LPE) technique. Thereafter, 2D-YP has been employed as a passive scatterer to achieve CW pumped RL emission from a conventional gain molecule. Compared to other semiconductor (TiO2) and 2D (Graphene and hBN) passive scatterers, ∼25 times improvement in gain volume is observed for the disordered system consisting of 2D-YP, and is due to the formation of a large refractive index gradient via thermal nonlinear optical (NLO) interaction. Hence, incoherent RL (ic-RL) emission ∼591 nm is reported at a lowest threshold input pump power and a linewidth of 65 mW and 3 nm, respectively. Additionaly, a clear transition from ic-RL to mode tunable coherent RL (c-RL) emission is also demonstrated by altering the pump configuration. Therefore, the newly designed van der Waals heterostructure, i.e., 2D-YP with intrinsic photon trapping capability may pave an avenue towards developing exciting optoelectronic devices.

Large birefringence in the quasi-one-dimensional material Sb2Se3: A DFT investigation of electronic and optical anisotropy

Investigation of the optical and electronic properties of the quasi-one-dimensional semiconductor Sb2Se3 is performed with density functional theory, which takes into consideration the spin–orbit coupling and the on-site Coulomb self-interaction. Sb2Se3 has an orthorhombic crystal structure and belongs to the Pnma space group. The pronounced dispersion along the R−U, Z−Γ, Γ−Y, and S−X high-symmetry directions and low dispersion along the U−Z and Y−S directions confirm the layered structure, with strong bonding interactions along the [001] direction, as well as along the [010] direction, along which the SbSe6 and SbSe7 chains are aligned, and weak van der Waals interactions along the [100] direction. The material is a semiconductor with an indirect bandgap of 1.2 eV. Sb2Se3 exhibits appreciable carrier mobility and low carrier recombination rates in the plane of the layers, indicating its applicability in solar and photovoltaic devices. Simulated X-ray photoelectron spectra show that the photoelectron emission occurs from the Se p, Sb s, and Se s atomic orbitals, while for high-energy X-rays, photoelectron emission is mainly from Sb s orbital. Sb2Se3 possesses appreciable anisotropy in its refractive index, absorption coefficient, reflectivity, extinction coefficient, and optical conductivity. Sb2Se3 is a negative biaxial crystal, in which the principal optical axis X→a, Y→b, and Z→c. It possesses uniform birefringence in the IR region until the optical cutoff edge at 1030 nm. The material shows strong unidirectional linear dichroism in the wavelength range from 300 nm to 620 nm, with linear dichroism conversion at 275 nm. The energy loss is isotropic for slow electrons below 4.0 eV, but for highly energetic electrons it varies along different crystal directions. Sb2Se3 is a promising material for optoelectronic and polarimetric applications because of its desirable bandgap of 1.2 eV, linear dichroism conversion, and large optical anisotropy, refractive index, dielectric constant, and absorption coefficient.

Giant Second Harmonic Generation from Membrane Metasurfaces.

Metasurfaces have emerged as a fascinating framework for nonlinear optics, which have advantages of a compact footprint and unprecedented flexibility in manipulating light. But their nonlinear responses are generally limited by the short interaction lengths with light. Therefore, further enhancement is highly desired for building high-efficiency nonlinear devices. Here, we experimentally demonstrate a record high second harmonic generation (SHG) efficiency of 2.0 × 10-4 using lithium niobate (LN) membrane metasurfaces. Benefiting from the large refractive index contrast in the vertical direction and high fabrication quality, distinct spectral resonances and tight field confinements in the LN layer were achieved. Strong SHG peaks resulting from pump resonances of the metasurfaces were observed. Our nonlinear efficiency is more than 2 orders of magnitude larger than previously reported LN metasurfaces. The results inspire a way to improve the efficiency of nonlinear metasurfaces for ultracompact nonlinear light sources in applications of nonlinear holography, Li-Fi, beam shaping, etc.

Review: Elaboration, structure, and mechanical properties of oxynitride glasses

AbstractOxynitride glasses are glasses where threefold coordinated nitrogen atoms substitute for twofold oxygen ones, hence resulting in a larger interatomic cross‐linking degree. Such glasses were first observed at the grain boundary in silicon nitride ceramics, where they govern the high‐temperature behavior. Later, they were prepared as bulk materials and motivated numerous researches, thanks to their large viscosity, glass transition range, elastic moduli, hardness, and fracture toughness among inorganic and non‐metallic glasses. In different chemical systems that were investigated, the synthesis routes and the sources for these exceptional mechanical properties are reviewed. Oxynitride glasses are not easy to process and suffer from the loss of transparency as nitrogen is incorporated over some critical content. Nevertheless, they are attractive “specialty” glasses in various niche areas, thanks to their large refractive index and dielectric constant, improved chemical durability, high softening point, etc., and majorly to their exceptional mechanical properties.