Refractive Index Difference Research Articles

Dispersion-engineered InGaAsP/InP waveguide for high efficiency all-optical on-chip wavelength conversion

InGaAsP/InP waveguide platform offers superiority for all-optical on-chip wavelength conversion due to strong optical nonlinearity. However, a small refractive index difference between the core and cladding makes it challenging to achieve sufficient structural dispersion for phase matching. In this work, the InGaAsP multilayer waveguide and the SiN-cladded InGaAsP waveguide are investigated for optimized wavelength conversion at telecom wavelengths. Structural dispersion is effectively engineered through the incorporation of additional slot layers and a heterogeneous passivation layer. Through comparative analysis, the wavelength conversion efficiency of -4.7 dB is achieved by the SiN-cladded waveguide, which shows an improvement of 22 dB compared to traditional nanowire waveguide.

Thermal Shielding Gd3TaO7-Based Thermal Barrier Ceramic with Ultralow NIR Transmittance.

Most thermal barrier coating materials exhibit transparent/semi-transparent properties at higher temperatures, causing the surface heat flow to directly heat the substrate with infrared radiation, which significantly reduces the thermal barrier effectiveness. Herein, composite ceramic materials composed of GdFeO3 diffusely dispersed within the Gd3TaO7 are produced. Specifically, the 0.9Gd3TaO7/0.1GdFeO3 composition demonstrates an ultra-low near-infrared (NIR) transmittance of less than 0.1% across the 400-2500nm range. The introduction of variable-valence behavior and oxygen vacancies contribute to the narrow bandgap of GdFeO3. The incorporated GdFeO3 produces extra multimode vibrations, resulting in the strong infrared emissivity of Gd3TaO7/GdFeO3 composite ceramics. Besides, the refractive index difference between Gd3TaO7 and GdFeO3 can contribute to the potential photons scattering within the composites, severely impeding the infrared radiation penetration. The upward curvature of the thermal conductivity curve of 0.9Gd3TaO7/0.1GdFeO3 at high temperature is significantly suppressed, confirming its excellent IR radiation shielding properties. Moreover, the Gd3TaO7/ GdFeO3 composite ceramic exhibits thermal suitability, with a thermal expansion coefficient (9.47-9.67 × 10-6 K-1 at 1400°C) comparable to YSZ. These combined benefits position the Gd3TaO7/ GdFeO3 composite ceramic system as a promising candidate for the development of infrared radiation shielding and high-emissivity thermal radiation materials.

Beyond two-octave coherent OAM supercontinuum generation in air-core Ge-doped ring fiber

Orbital angular momentum (OAM) has emerged as a revolutionary technology for communication networks due to its ability to significantly increase the channel capacity. However, traditional optical fibers present significant hurdles to harnessing OAM’s full potential, including dispersion and limited bandwidth, which facilitates investigations on supercontinuum (SC) generation for OAM beams. In this paper, an air-core Ge-doped ring fiber is proposed and designed to support high-order OAM mode up to |l| = 24. To achieve this, the fiber has a high refractive index difference between the ring core and the cladding, enabling stable transmission of high-order OAM modes. A key feature of this design is the OAM24,1 mode, which exhibits near-zero and flat dispersion. This characteristic translates into high coherence and a remarkably broad SC generation. The generated SC spans over two octaves (2336 nm) within the infrared wavelength range (764 nm to 3100 nm) at a power level of −40 dB. Furthermore, by optimizing the structural parameters, we ensure near-zero and flat dispersion characteristics for the other OAM modes (|l| < 24), along with broad SC generation exceeding two octaves. This fiber design holds significant promise for future advancements in OAM beam transmission within the infrared spectrum.

Spatial and reconfigurable control of photoluminescence from single-layer MoS2 using a strained VO2-based Fabry–Pérot cavity

We investigated the photoluminescence (PL) from single-layer MoS2 on VO2 platelets grown on SiO2, where the insulating and metallic phases can coexist above a bulk transition temperature of 340 K, due to the inhomogeneous strain. We found that the intensity of PL from MoS2 on metallic VO2 is higher than that on the insulating counterpart, resulting in spatially varying PL even at the sub-micrometer scale. In contrast to the intensity, the PL peak energies were observed to be nearly identical on insulating and metallic VO2, indicating that the influences of charge transfer, strain, and dielectric screening on MoS2 are comparable, regardless of the phase state. Thus, the observed difference in PL intensity is due to the difference in refractive indices of insulating and metallic VO2, leading to the phase-dependent Fabry–Pérot interference effect. We performed numerical simulations for the emission from MoS2 supported on the VO2-based Fabry–Pérot interferometer. The calculated emission intensity ratio on insulating and metallic VO2 well reproduces the experimental observations. These results suggest a strategy for controlling PL from two-dimensional semiconductors in a spatial and reconfigurable manner.

High capacity performance signature of micro optical mechanical system switches based fiber Bragg grating scheme implementation in optical WDM data routed center networks

Abstract This paper demonstrated the high capacity performance signature of micro optical mechanical system switches based fiber Bragg grating scheme in optical wavelength division multiplexing data routed center networks. The FBG reflectivity is simulated versus near infrared operating wavelength region with relative refractive index difference of Δn = 0.1 %. Required optical pump power, temperature increment and FBG power dissipation or consumption are demonstrated versus FBG wavelength spacing at pumping wavelength of 1,480 nm for doped/undoped fiber. Required FBG mechanical switching voltage, and required FBG mechanical/thermal switching time are clarified at pumping wavelength of 1,480 nm for doped/undoped fiber. Grating period and Bragg wavelength is studied against effective refractive silica/polymer FBG index.

Optimization on the design of nano-patterned ZnS:Cu LED surface using FDTD simulation

Communication technology is one of the most important parts of human history and is fast developing. Visible Light Communication (VLC) can be categorized as a Light Fidelity (Li-Fi) communication that uses visible light through a Light Emitting Diode (LED) to transfer data and information. Even though LEDs are considered as very good light sources, they also have problems with effectiveness, which is influenced by their surface structures. The LEDs must also be able to focus the transmitted data and greatly reduce the signal-to-noise ratio. One of the problems encountered with LEDs is the presence of Total Internal Reflection (TIR) due to the large difference in refractive index between the LED and air, which causes photons to be trapped in the LED. Some trapped photons' energy may change into heat, thus reducing the LED's Light Extraction Efficiency (LEE). Using nanopatterns on the surface of LEDs is one way to reduce TIR on LEDs and enhance photon extraction from LEDs. Many parameters can influence the performance of nanopatterns on LEDs, so modelling efforts are needed before fabrication to save time and cost. Ansys Lumerical FDTD can be used to simulate and model the effects of nanopatterns on LED surfaces on LEE and beam focusing effect. In this study, simulations were carried out using Ansys Lumerical FDTD with variations in nanopattern shape parameters in the form of grating, blaze grating, triangular grating, and hemisphere. Apart from that, variations were also made to the height and width parameters of the grating to see the effect of these parameters on the efficiency of the LED. The FDTD simulation codes were validated using the Ansys database. The optimization results show that the most optimum shape is the grating nanopattern with a width of 216 nm and a height of 300 nm, which produces a peak wavelength of 480 nm in the far field pattern and has the highest increase in the LEE of 300 times in ±15° and 5500 times in ±5°. The huge spike in LEE enhancement at ±5° indicates that the nanopattern caused a focusing effect. The simulation results were compared using previous experimental data of Fabricated ZnS:Cu LED. It is shown that the simulation results are in line with the experimental data. The result shows that the simulation is very useful in designing the nano-patterned ZnS:Cu LED surface to achieve the best performance in the LEE and LED focusing effect for a specific application such as the VLC.

INNV-05. DEVELOPMENT OF A LABEL-FREE PLASMONIC OPTICAL BIOSENSOR FOR INTRAOPERATIVE DETECTION OF TUMOR TISSUE IN GLIOBLASTOMA

Abstract Safe maximal resection of patients with glioblastoma remains a surgical challenge due to the invasive nature of the tumor and the difficulty in distinguishing tumor margins during resection. Several tools are currently used for this purpose, such as, confocal laser endomicroscopy and fluorescence-guided surgery. In this context, biosensors based on plasmonic technology using the extraordinary optical transmission (EOT) phenomenon can help to distinguish between tumor and surrounding tissue based on their optical properties. We developed a biosensor utilizing EOT through a nanostructured gold matrix with 220 nm diameter holes and conducted a prospective study in 35 GBM patients. Paired samples of tumor and peritumoral tissue were collected during surgery. Each sample’s optical imprint was analyzed, revealing significant differences in refractive indices (RI). Tumor tissue showed higher RI values (1.350, IQR 1.344–1.363) compared to peritumoral tissue (1.341, IQR 1.339–1.349) with a p-value of 0.0047. The receiver operating characteristic (ROC) curve indicated the biosensor’s ability to differentiate the tissues (AUC = 0.8779, p &amp;lt; 0.0001). An optimal RI cut-off point of 0.003 was determined, with the biosensor achieving 81% sensitivity and 80% specificity. Our results indicate that this plasmonic technology-based biosensor could have a significant impact on the surgical treatment of glioblastoma by providing a label-free tool for tumor margin discrimination and improving the accuracy of glioblastoma surgery. Future research will focus on optimizing the nanoarchitecture of the biosensor to further improve its sensitivity and specificity, as well as exploring its applicability to other cancer types by analyzing the biomarkers responsible for the different RIs of the footprints.

Ultra-broadband multimode fiber-to-chip edge coupler based on periodically segmented waveguides.

Mode-division multiplexing (MDM) technology demonstrates a bright outlook for enhancing the capacity of chip-scale or fiber-based optical communication. Nevertheless, the fiber-to-chip MDM optical interconnects are hindered by the considerable mode mismatch and inter-modal cross talk between the few-mode fiber (FMF) and on-chip few-mode waveguide (FMW). In this Letter, a new, to the best of our knowledge, multimode coupling solution based on periodically segmented waveguides for the MDM system is proposed, which achieves efficient conversion between LP01, LP11a, and LP11b modes in FMF and E11, E21, and E12 modes in FMW with low refractive index difference. The simulation results show that the coupling loss is less than 0.41, 0.27, and 0.90 dB for the three modes, over the wavelength range of 1100-1800 nm. The fabricated device based on a polymer platform shows low fiber-to-chip coupling losses of less than 1.8, 1.7, and 3.0 dB, respectively, over a 130 nm wavelength. The presented scheme provides a competitive solution for realizing the ultra-efficient integration of prospective fiber-chip optical interconnections and communications.

Gradient refractive index tellurite glass-ceramics for miniaturized and lightweight MWIR imaging

Tellurite glasses are highly anticipated for creating gradient refractive index (GRIN) lenses, which have the potential to be applied in the lightweight and miniaturization of optical imaging systems to improve their performance. However, it is still challenging to precipitate pure crystalline phases in tellurite glasses after heat treatment to control the refractive index via spatial variation of nanocrystal distribution. Here, we present a novel method for fabricating axial telluride GRIN glass via a gradient heat-treatment technique. Through the selection of glass composition and control of the heat treatment process, pure phase Bi4TeO8 nanocrystals with a high refractive index (n) can be precipitated in telluride glass with a lower refractive index. This allows the axial telluride GRIN structure of maximum refractive index difference Δn ∼ 0.1 to be obtained by modulating the spatial distribution and concentration of Bi4TeO8 nanocrystals. This work not only demonstrates the feasibility of producing GRIN lenses from tellurite glass but also lays the foundation for the future development of medium wave infrared (MWIR) imaging.

Numerical Analysis of Optical Fiber Refractive Index in the Miniaturized Graded‐Index Fiber Probe

ABSTRACTWe analyze the impact of the refractive index of optical fibers on the focusing properties of a miniaturized graded‐index (GRIN) fiber probe. The ABCD ray transfer matrix and characteristic parameters are employed to characterize the working distance, focusing spot size, and the depth of field effectively. The three‐dimensional (3‐D) function diagrams and two‐dimensional (2‐D) graphs are used to illustrate the impact of the no‐core fiber (NCF) refractive index and the center refractive index of GRIN fiber on the focusing properties. Numerical analysis results show that when the length of the NCF is 0.36 mm and the GRIN fiber is 0.1 mm, the variation of the refractive index of the NCF between 1.44 and 1.52 leads to the variation of the working distance in the range of 0.02 mm, while the focusing spot size varies in the range of 5 μm. A comparison of 12 probe samples reveals that a 0.02 difference in the center refractive index of GRIN fiber could result in a 0.16 mm variation in working distance and a variation in focusing spot size of over 3 μm. The experimental results indicate that alterations in the length of the fibers have a considerable effect on the focusing properties of the probe. In contrast, the range of variation in focusing properties with fiber refractive index is relatively limited.

Forward design method for the design of panda polarization-maintaining few-mode optical fiber based on artificial neural network.

Panda polarization-maintaining few-mode optical fiber (PPMFMOF) has important research significance in the short distance optical transmission field owing to its advantages of weak nonlinear effects, which is benefit to reduce the use of digital signal processing equipment. Designing a high-performance PPMFMOF quickly and efficiently is expected and yet challenging. In this article, we demonstrated a forward design method for the design of PPMFMOF based on artificial neural network (ANN) to solve the problems of inefficient and time-consuming PPMFMOF design in traditional design method. By studying the influence of different ANN models on the fiber performance, the approximate range of the optimal value was obtained in advance, then the minimum effective refractive index difference (Δneff,min) between adjacent LP modes was used as the optimization object, finally design of PPMFMOF supporting 10 LP modes in C + L band was successfully realized. This method provided low time-consuming, high-efficiency and high-accuracy for the fast design of PPMFMOF and the maximum mean absolute percentage error (MAPE) of the ANN model to predict the effective refractive index (neff) of 10 LP modes is only 3.2211 × 10-7. We believe that the proposed method could also be quickly and accurately applied to other functional optical fiber designs.

Measuring linear birefringence via rotating-sample transmission Stokes spectropolarimetry

Linear birefringence is a fundamental property of optically anisotropic media, defined by the difference in refractive index experienced by light polarized along orthogonal directions. It is usually manifested in microscopically aligned molecular systems, where a preferential direction of light–matter interaction is created. For instance, the anisotropic structure of calcite crystal causes the famous double-refraction phenomenon. Another common example is commercial adhesive tapes, which are polymeric materials possessing birefringent properties due to their manufacturing processes. The intrinsic relation between birefringence and molecular alignment forges a new analytical route to study materials such as polymeric thin films. Therefore, the capacity of measuring linear birefringence and its fast axis is of paramount importance for the science of anisotropic molecular systems. In this contribution, a comprehensive approach to acquire linear birefringence using rotating-sample transmission Stokes spectropolarimetry is presented and applied to transparent adhesive tapes as a case study. The experimental setup comprises a thermal light source and a spectropolarimeter capable of determining wavelength distributions of Stokes parameters. The samples are carefully aligned in a rotating mount and subjected to a fixed broadband vertically polarized light beam. Then, the transmitted light is analyzed using a rotating retarder type of spectropolarimeter. Through systematic variation of the sample’s angular position, the Stokes parameters of transmitted light are measured for each transmitted wavelength as a function of the sample’s angular position. The linear retardance and fast axis direction relative to the tape’s long axis are then determined from the modulation of Stokes parameters over sample rotation. The model derivation, experimental procedure, and signal processing protocol are described in detail, and the approach is verified with a simple correlation between linear retardance and the number of stacked layers of tape.

Ultrahigh-Reflectivity Circularly Polarized Mirrors Based on the High-Contrast Subwavelength Chiral Metasurface

The circularly polarized laser sources are core components for many optical applications such as biomedicine, quantum technology, and AR/VR. However, conventional techniques make it difficult to further diminish the size of circularly polarized lasers. Thus, the high-contrast subwavelength chiral metasurface (HCCM) with a 980 nm operating wavelength is numerically investigated. The HCCM is composed of chiral metasurfaces modulating the circular dichroism of reflectance and 6 pairs of Distributed Bragg Reflectors (DBR) with 55% reflectivity. The reason that the HCCM has an ultra-high reflectivity (99.9%) at the operating wavelength of 980 nm is the combination of the optical refractive index difference between the GaAs metasurface and the AlOx substrate and weak destructive interference in the AlOx support layer. In addition, the circular dichroism of the chiral metasurfaces (2.1%) is mainly caused by the displacement of two square air holes in opposite directions, thus transforming the unit cell of the metasurface from C2 symmetry to chiral symmetry. The reflector has the advantages of a simple structure and miniaturization, which is expected to greatly reduce the fabrication difficulty and cost of the circular polarization VCSELs.

Unidirectional and highly dispersive grating emitters for silicon photonic beam steerers.

Grating emitters are the key component used in silicon photonic beam steerers to create light emission and allow dispersion-based steering. Conventional grating emitters are inefficient due to bidirectional emission, and their angular dispersion is only on the order of 0.1°/nm. In this paper, we develop unidirectional grating emitters by choosing a top cladding material with a refraction index higher than that of the bottom cladding. The refractive index difference allows the phase-matching condition to be satisfied only for the upward diffraction. We demonstrate experimentally grating emitters with a directionality of 83% and an angular dispersion of 0.79°/nm. These grating emitters have a simple device structure compatible with the standard silicon photonic platform.

A double-layer radiative cooling coating that utilizes the refractive index difference between layers to achieve extremely high solar reflectivity

A double-layer radiative cooling coating that utilizes the refractive index difference between layers to achieve extremely high solar reflectivity

Design and investigation of a terahertz hollow-core fiber with double-layer ring core for OAM mode transmission

A terahertz hollow-core fiber having a double-layer ring core is presented and investigated to support OAM mode transmission. These two annular cores will work as two separate OAM transmission spaces that are not interfered with and can support more OAM mode transmissions. A numerical model has been developed to assess the transmission performances of EH and HE modes in the 0.3∼0.4 THz frequency band. Using only a single material to design the suggested fiber, 22 (in) + 58 (out) OAM modes can be stably transmitted at 0.36 THz. Neighboring EH and HE modes in the identical OAM family can obtain effective refractive index differences of more than 10−4. Furthermore, the designed fiber demonstrates excellent transmission properties, with the highest mode purities in the inner and outer rings being 92.22% and 93.78%, respectively. In the inner ring the effective mode field area is between 5.37 × 106 and 1.79 × 107 μm2, while in the outer ring, it is between 8.95 × 106 and 1.58 × 107 μm2. The HE1,1 mode has the smallest change in dispersion, with values of 2.76 and 0.87 ps THz−1 cm−1 in two high refractive index transmission ring cores, respectively. Additionally, the isolation (ISO) parameter is also analyzed. These mode characteristics indicate that the fiber structure has significant potential for applications in high-capacity terahertz communication systems.

Liquid-immersion inclined-rotated exposure system for fabricating three-dimensional microstructures with large inclination angles

Abstract This study utilized liquid-immersion inclined-rotated ultraviolet lithography to fabricate three-dimensional (3D) microstructures. The maximum achievable inclination angles obtained through conventional inclined-rotated exposure (IRE) methods were limited by the significant refractive index differences in material. We proposed an IRE with liquid-immersion and adjustable mirrors, which enabled greater inclination angles with improved adjustability. Using liquid as a medium helped minimize the refractive index disparities between materials. We fabricated polydimethylsiloxane molds for micro suction cup (MSC) array sheets to evaluate the performance of the developed liquid-immersion IRE. The resulting MSC array sheets (10 mm2) with a suction cup diameter of 500 μm, achieved inclination angles up to 51°, approximately double those obtained with the conventional IRE method. In addition, the suction force of the fabricated MSC arrays were evaluated by pulling along the vertical, horizontal, and edge directions under wet conditions. The maximum measured suction force was 0.15 N, confirming the effectiveness of the proposed liquid-immersion IRE in fabricating 3D microstructures, as demonstrated by the fabricated MSC array sheets.

Relative intensity noise management and thermal/shot noise control for high speed ultra high bandwidth fiber reach transmission performance

Abstract This work has clarified the relative intensity noise management and thermal/shot noise control for high speed ultra high bandwidth fiber reach transmission performance. Total link losses variations are clarified against ambient temperature and relative refractive index difference variations at 1,550 nm wavelength, 20 km fiber link length, and 45 % fluoride dopant ratio inside the fiber material cable. Besides the total link losses variations are demonstrated versus fiber link length and relative refractive index difference variations at 1,550 nm wavelength, room temperature, and 45 % fluoride dopant ratio inside the fiber material cable. Required optical signal per noise ratio (OSNR), electronic signal to noise ratio (SNR), and optimum gain variations are measured and analyzed versus the fiber link length and relative refractive index difference variations at 1,550 nm wavelength, room temperature, and 45 % fluoride dopant ratio inside the fiber material cable.

Designing an OAM fiber with a two-layer seven-core structure to support 322 OAM-mode transmissions

In this paper, a two-layer seven-core structural fiber (TLSCSF) is proposed to support and improve the propagation of more orbital angular momentum (OAM) mode beams, thus increasing the transmission capacity and spectrum efficiency (SE) of an optical communication system. The TLSCSF is composed of seven sub-cores, each containing two inner and outer layers of core rings and a central air hole. The two core rings are prepared using two materials with different doping concentrations. The supported OAM modes can be propagated in the fiber core ring. A finite element method (FEM) is used to analyze the designed fiber, and the calculated results show that the TLSCSF can stably transmit 322 OAM modes without higher order radial modes in the wavelength range of 1.5∼1.6 um. The corresponding effective refractive index difference between two adjacent vector modes (HE/EH) is more than 10−4. The measured effective mode areas (Aeff) and the confinement losses (CL) of HE or EH modes are larger than 54 um2 and smaller than 10−10 dB m−1, respectively. Moreover, the calculated dispersion variations and the mode qualities are lower than 230 ps nm−1 km−1 and more than 90%, respectively. Finally, the 10ps walk-off lengths of all vector modes supported by TLSCSF at a wavelength of 1.55 um are also evaluated, where the measured results show that except for HE2,1 and EH1,1, all other modes achieve a 10ps walk-off length of the order of 103 to 105. Consequently, the TLSCSF can contributes to improving the channel capacity and SE by supporting 322 fundamental radial OAM modes with robust performances in an optical communication system.

A novel heterogeneous sixteen-core four-mode fiber with two types of refractive index profiles

To this day, the inter-core crosstalk and intra-core crosstalk are potential factors limiting optical fiber transmission over long distances and serious degradation of signal quality in communication systems of multi-core few-mode fibers. To this end, this work proposes a heterogeneous sixteen-core four-mode fiber with two types of refractive index profiles is proposed in this paper, which consists of a graded-index core and a step-index core. After optimization, the four modes can transmit independently and steadily at 1550 nm, and the effective refractive index difference between adjacent modes is more than 1 × 10−3. Under the optimal fiber parameters, the inter-core crosstalk results can be obtained from the research results and are less than −69 dB/km, which reflects the significant effect of the proposed fiber in reducing the inter-core crosstalk. The proposed fiber is the first to use the combination of step-index core and graded-index core so that the inter-core crosstalk of heterogeneous fibers is successfully suppressed at a low level, while providing new ideas and solutions for future research on novel low-crosstalk multi-core few-mode fibers that can be applied to terrestrial and undersea communication systems.