Refractive Index Change Research Articles

Scalable Photonic Digital-to-Analog Converters

This work introduces a novel architecture for implementing a parallel coherent photonic digital-to-analog converter (PDAC), designed to transform parallel digital electrical signals into corresponding analog optical output, convertible to analog electrical signals using photodiodes. The proposed architecture incorporates microring resonator-based modulators (MRMs), phase shifters, and symmetric multimode interference couplers. Efficient modulation is achieved by MRMs utilizing carrier depletion-induced refractive index changes, while metal heaters facilitate tuning of the ring resonator resonance wavelength. The proposed architecture is scalable to higher bit resolutions and exhibits a dynamic range limited by MRM’s sensitivity to applied bias and noise levels. Experimental results of the fabricated chip in the silicon-on-insulator (SOI) platform showcase the successful realization of a 4 GSample/sec conversion rate in a 2-bit resolution operation, along with a stationary conversion of four parallel DC digital signals into 16 analog intensity levels in a 4-bit PDAC configuration. The study encompasses a proof-of-concept experimental demonstration of 8 Gbps data conversion, along with a 50 Gbps data conversion rate using the optimized design in the simulation, affirming the accuracy and quality of the PDAC architecture. These findings contribute to the advancement of PDAC technology, providing insights into performance characteristics, limitations, and potential applications.

Three-dimensional imaging of acoustic pressure by tomography applied to optical feedback interferometry

This paper presents a novel experimental technique for three-dimensional imaging of acoustic pressure waves by means of an optical feedback interferometer associated to standard Radon transformation algorithm for tomography. The imaging system is based on the sensing of the optical refractive index changes induced by the pressure variation in the medium where the acoustic wave propagates. Thanks to the compactness allowed by the laser optical feedback interferometer, this technique allows to image in three dimensions the variations of pressure of any acoustic source without any displacement of the source.

Sophisticated Attenuated Total Reflection Correction Within Seconds for Unpolarized Incident Light at 45°.

The most common mid-infrared (MIR) attenuated total reflection (ATR) accessory has a nominal angle of incidence of 45° and does not have a polarizer. A spectrum recorded with such an accessory does not hold enough information for the sophisticated ATR correction of MIR spectra with strong peaks, which are often strongly affected by refractive index changes due to anomalous dispersion. Here we show that a 45° ATR spectrum recorded without a polarizer and the polarization angle for the same ATR Fourier transform infrared spectroscopy system provide enough information to determine the ATR s-polarized spectrum. Further analysis with an improved non-iterative Kramers-Kronig analysis immediately yields the complex refractive index function. The analysis is about two orders of magnitude faster than iterative formalism and runs within seconds on a typical office PC. The effectiveness of our advanced ATR correction formalism is showcased through its application to water, employing diamond, ZnSe, and Ge ATR crystals, along with two distinct ATR accessories. Additionally, the formalism is applied to octadecane spectra. Potential sources of errors such as incidence angle spread, dispersion of the polarization angle, and the influence of reflection at the air/ATR crystal interface are investigated by simulations.

Design and fabrication of indium tin oxide for high performance electro-optic modulators at the communication wavelength

In recent years, transparent conductive oxides(TCO) based electro-optic modulators has garnered significant attention due to its epsilon-near-zero(ENZ) effect to enhance the local optical field and produce a large refractive index change in the vicinity of ENZ wavelength, thereby enabling substantial optical field modulation at micrometer size. The modulators manipulate the light field by altering the carrier concentration in TCO through an electric field. Therefore, the modulator's performance is closely related to the TCO's material behavior. In this paper, the effect of carrier concentration and mobility of indium tin oxide(ITO) on the modulation efficiency, depth and bandwidth was analyzed and it is found that ITO with the carrier concentration near 1 × 1020 cm−3 and mobility as high as 80 cm2/Vs is beneficial to the electro-optic modulators with large modulation depth and bandwidth. Specifically, our research shows that the carrier concentration and mobility of ITO films can be tunned over a wide range from 2.2 × 1019cm−3 to 7.7 × 1020cm−3 and 7.2 cm2/Vs to 82.3 cm2/Vs respectively, by changing doping concentration, deposition conditions and annealing parameters. As a result, ITO film integrated on a 3.5-μm-long silicon waveguide with an MOS structure can achieves a high modulation bandwidth of over 40 GHz at the optical communication wavelength.

Correlation of refractive index based and THz streaking arrival time tools for a hard X-ray free-electron laser.

To fully exploit ultra-short X-ray pulse durations routinely available at X-ray free-electron lasers to follow out-of-equilibrium dynamics, inherent arrival time fluctuations of the X-ray pulse with an external perturbing laser pulse need to be measured. In this work, two methods of arrival time measurement were compared to measure the arrival time jitter of hard X-ray pulses. The methods were photoelectron streaking by a THz field and a transient refractive index change of a semiconductor. The methods were validated by shot-to-shot correction of a pump-probe transient reflectivity measurement. An ultimate shot-to-shot full width at half-maximum error between the devices of 19.2 ± 0.1 fs was measured.

Enhanced Fiber Long‐Range Surface Plasmon Sensing Enabled by Resonance Coupling to Surface Plasmon Polaritons

A tunable fiber optic (FO) long‐range surface plasmon (LRSP) sensor with strong coupling is developed and demonstrated theoretically in this article. The sensor consists of a square lattice array of Ag nanodisks resting on the FO end face. Utilizing nanodisks with small diameters leads to the pronounced excitation of two distinct and independent resonant modes: surface plasmon polaritons (SPP) and LRSP. A systematic investigation is performed to evaluate the sensing performance and capabilities of the sensor, focusing on its bulk and surface sensitivity. Significantly, the LRSP mode demonstrates high sensitivity and favorable linearity in response to refractive index (RI) changes, with an exceptionally high figure of merit (FOM). On the contrary, the SPP mode is regarded as an ideal self‐referencing mode due to its immunity to RI fluctuations. The enlargement of nanodisks diameters results in a swift redshift in the LRSP wavelength, leading to a strong coupling with the SPP mode. This coupling facilitates the transfer of electric fields within the SPP mode, promotes sensing capabilities, and enables the realization of dual‐channel sensing functionality. The occurrence of strong coupling phenomena along with the use of FO substrates provides an innovative option for achieving multifunctionality and miniaturization in sensor platforms.

Enhanced Temperature Sensing by Multi‐Mode Coupling in an On‐Chip Microcavity System

AbstractThe microcavity is a promising sensor platform, but any perturbation will disturb its linewidth, cause resonance shift or splitting. However, such sensing resolution is limited by the cavity's optical quality factor and mode volume. Here, an on‐chip integrated microcavity system is proposed and demonstrated that the resolution of a self‐referenced sensor can be enhanced with multi‐mode coupling. In experiments, inter‐mode coupling strength is carefully optimized with a pulley waveguide and observed a resolution improvement of nearly 3 times in the frequency domain. While experiencing small refractive index change tuned by temperature, mode‐coupled system shows a 7.2 times sensitivity enhancement that is than the uncoupled system on the same chip and a very significant lineshape contrast ratio change as great reference for minor frequency shifts. This approach will help design microcavity sensors to improve detection sensitivity and resolution under limited manufacture precision.

Clinical Aspects of Transient Myopia after Blunt Eye Trauma

Purpose: To investigate the clinical aspects of transient myopia after blunt eye trauma.Methods: The medical records of 32 patients treated at our hospital for from 2020 to 2022 traumatic microhyphema were analyzed retrospectively. The visual acuity, intraocular pressure, anterior chamber cells, anterior chamber depth, axial length, lens thickness, and refractive index were analyzed immediately after injury and after treatment. Patients with additional diseases such as vitreous and retinal hemorrhage, traumatic retinal detachment, iridodialysis, or orbital fracture were excluded. Correlations between the myopic changes and the amount of myopic refractive change caused by trauma were analyzed.Results: The 32 patients included 24 (75%) males and 8 (25%) females. The average myopic change was -3.03 ± 0.92 (range -1.00 to -5.25) diopters (D). The average change in anterior chamber depth due to trauma was -0.22 ± 0.11 mm, the change in lens diameter was 0.20 ± 0.09 mm, and the change in axial length was -0.07 ± 0.05 mm. The average change in refractive index due to ciliary spasm was -1.63 ± 0.65 D. The correlations between the change in refractive index and anterior chamber depth (r = 0.475, <i>p</i> = 0.008), traumatic ciliary spasm (r = 0.649, <i>p</i> < 0.001), and lens thickness (r = -0.184, <i>p</i> = 0.330) were determined.Conclusions: Factors such as ciliary spasm, change in the anterior chamber depth, anterior shift of the lens-iris diaphragm, and increased lens thickness affect the myopic changes caused by trauma. Of these, a shallower anterior chamber depth and ciliary spasm are highly correlated with the transient myopic changes.

Electrically tunable dual polarization states of light using lithium niobate-based nanograting.

Tuning polarization states of light electrically has unique advantages in the integration of optoelectronic devices. Here, a lithium niobate-based nanograting is proposed to dynamically tune the polarization state of both the reflected and transmitted lights simultaneously in the near-infrared range. By judiciously designing the nanograting, a quasi-bound state in the continuum (qBIC) is excited under the excitation of an obliquely incident plane wave. The excited mode with a high quality-factor and enhanced local electric field can respond to a refractive index change in nanograting structures as small as 10-4 level, which can be generated with a low external voltage via the electro-optic effect. As a result, both the polarization states of reflected and transmitted lights can be dynamically tuned from a right circular polarization to a linear polarization state. The proposed lithium niobate-based nanograting for tuning dual polarization states of light with a qBIC mode suggests a promising electrical scheme for achieving high speed optoelectronic devices.

Single line of sight frame camera based on the RadOptic effect of ultrafast semiconductor detector

A new optical beam splitting method is proposed, based on which the optical frame camera capable of capturing multiple frames in a single exposure is designed and experimentally verified. The operation of the frame camera is based on an ultra-fast response semiconductor detector. It is equipped with an optical beam splitter and an optical imaging module. The ultrafast semiconductor detector receives an optical pulse that produces a transient refractive index change, and ultrafast physical processes are recorded by diffracting the probe laser through the transient phase grating. The interaction of an X-ray pulse with a semiconductor detector to produce a phase grating is simulated, based on the Monte Carlo method. The optical beam splitting mode separates a laser into two optical pulses with a certain time difference in the direction of polarization perpendicular to each other. The imaging module filters the diffracted probe laser in the spectral plane and then images multiple frames. The frame camera was used to record the temporal and spatial distribution characteristics of femtosecond laser pulses with a temporal resolution of 4.1 ps. This frame camera has great potential and value for applying to experimental studies of inertial confinement fusion.

Development of chrono-spectral gold nanoparticle growth based plasmonic biosensor platform

Plasmonic sensor platforms are designed for rapid, label-free, and real-time detection and they excel as the next generation biosensors. However, current methods such as Surface Plasmon Resonance require expertise and well-equipped laboratory facilities. Simpler methods such as Localized Surface Plasmon Resonance (LSPR) overcome those limitations, though they lack sensitivity. Hence, sensitivity enhancement plays a crucial role in the future of plasmonic sensor platforms. Herein, a refractive index (RI) sensitivity enhancement methodology is reported utilizing growth of gold nanoparticles (GNPs) on solid support and it is backed up with artificial neural network (ANN) analysis. Sensor platform fabrication was initiated with GNP immobilization onto solid support; immobilized GNPs were then used as seeds for chrono-spectral growth, which was carried out using NH2OH at varied incubation times. The response to RI change of the platform was investigated with varied concentrations of sucrose and ethanol. The detection of bacteria E.coli BL21 was carried out for validation as a model microorganism and results showed that detection was possible at 102 CFU/ml. The data acquired by spectrophotometric measurements were analyzed by ANN and bacteria classification with percentage error rates near 0% was achieved. The proposed LSPR-based, label-free sensor application proved that the developed methodology promises utile sensitivity enhancement potential for similar sensor platforms.

Investigation of linear and nonlinear optical properties in quantum dots: Influence of magnetic and AB flux fields

In the present work, analytical expressions for the energy eigenvalues and eigen functions are obtained by solving the radial Schrödinger equation for inverse quadratic Hellmann plus Yukawa (IQHY) potential using Nikiforov-Uvarov functional analysis method. As an application part of this work, expressions for the linear and third-order nonlinear absorption coefficients and refractive index changes are computed to study the optical properties of the spherical GaAs quantum dots. We extend the investigation of the IQHY potential by incorporating the effects of magnetic and Aharonov–Bohm (AB) flux fields. Our results reveal that the optical properties of the GaAs quantum dots are strongly influenced by variations in the dot radius, potential height and intensity of incident photons. Specifically, we observe that the introduction of magnetic and AB flux fields significantly alters the energy eigenvalues and eigenfunctions, leading to distinct optical characteristics. The results of present study are found in consistent with other theoretical studies available in the literature.

Performance analysis of PLC-based 32-channel arrayed waveguide grating used for FBG interrogator

The Fiber Bragg grating (FBG) is an optical fiber component that reflects or diffracts optical signals of specific wavelengths through periodic refractive index changes inside it. Currently, the use of arrayed waveguide grating (AWG) to interrogate such optical signals is a popular research topic at present. Using the planar lightwave circuit, we designed and fabricated a 32-channel AWG. The AWG has an insertion loss of less than 4 dB, a 3 dB bandwidth of about 1.76 nm, and a crosstalk of non-adjacent channels of less than −28 dB. Additionally, we integrated the AWG with a photodetector (PD) array and built a corresponding wavelength interrogation system, which was tested using three different interrogation methods. The test results indicate that when using the linear fitting, this AWG provides the best results using the dual-channel edge filtering method with an interrogation range of 1.6 nm and an interrogation accuracy of 18.9–20.1 pm. The interrogation accuracies of the relative intensity method are 9.4 pm and 3.7 pm when Gaussian and Lorentzian fittings are used.

Silver-TiO2 coated D-shaped photonic crystal fiber based SPR sensor for ultrasensitive refractive index detection: design and FEM analysis

Photonic crystal fiber (PCF) based surface plasmon resonance (SPR) sensors are emerging as a promising technology for ultrasensitive detection of various biological and chemical analytes. This paper presents a novel D-shaped PCF based SPR sensor, which has shown great potential for highly sensitive detection of refractive index (RI) changes. The D-shaped configuration is achieved through the polishing of the upper side of the PCF fiber. To enhance sensitivity, a 0.1 μm silver layer is strategically placed between the fiber and analyte, intensifying light–matter interactions. Additionally, a 0.05 μm titanium dioxide (TiO2) layer is employed not only to further boost sensitivity but also to shield the metal from oxidation, ensuring the longevity and stability of the sensor. The finite element method (FEM) is employed to optimize the structural parameters of the sensor design. The findings demonstrate that the proposed SPR sensor is sensitive to RI changes in the 1.31–1.35 range, achieving a peak wavelength sensitivity of 30000 nm RIU−1 and an amplitude sensitivity of −185.33 RIU−1. The sensor holds promise for diverse applications, including chemical and biological sensing, making it a versatile tool with promising implications for advancing sensing technologies in various domains.

Experimental demonstration of a Fano resonant hybrid plasmonic metasurface absorber for the O and E bands of the optical communication window

Plasmonic metasurface absorbers are capable of absorbing the incident light at wavelengths corresponding to the excitation of Fano resonant modes. Absorption of the incident light is possible because of its confinement near the edges of the plasmonic nanostructure. Confinement of light takes place because of the coupling of superradiant and subradiant modes near the edges of the plasmonic metasurface. Superradiant and subradiant modes are excited for the oblique angle incidence of transverse magnetic (TM)-polarized light. The incidence of TM-polarized light supports the excitation of surface plasmon modes at the metal–dielectric interface. For the oblique angle incidence, surface plasmon modes couple with the incident light and generate the superradiant and subradiant modes near the plasmonic metasurface. We experimentally demonstrate the absorption of near-infrared light in the O and E optical communication band by a one-dimensional (1D) hybrid plasmonic metasurface. A low-cost, and flexible, 1D hybrid plasmonic metasurface absorber (HPMA) was obtained by extracting an Ag-coated, flexible, and 1D patterned polycarbonate layer from a digital versatile disc (DVD). The DVD consists of an Ag layer sandwiched between two 1D patterned polycarbonate layers. A large-area HPMA of 3cm2 in size was fabricated for optical characterization. Control experiments on the variation of the angle of incidence of light were performed to achieve the maximum light absorption of 79%. The effect of transverse electric (TE)- and TM-polarized light on the HPMA was studied. The effect of the thickness of the polymer layer on the HPMA, and per unit change of refractive index (RIU) of the analyte medium, were also investigated. HPMA supports refractive index sensing characteristics with a maximum sensitivity of 954 nm/RIU. Electric field profiles at different incidence angles were simulated using the finite element method on COMSOL Multiphysics software to explain the underlying physics of Fano resonance. HPMA can be used to develop cost-effective photonic devices such as sensors, spectral filters, photodetectors, heat-absorbing protective photonic covers, etc.

Superalkali nature of the Si9M5 (M = Li, Na, and K) Zintl clusters: a theoretical study on electronic structure and dynamic nonlinear optical properties.

Zintl clusters have attracted widespread attention because of their intriguing bonding and unusual physical properties. We explore the Si9 and Si9M5 (where M = Li, Na, and K) Zintl clusters using the density functional theory combined with other methods. The exothermic nature of the Si9M5 cluster formation is disclosed, and the interactions of alkali metals with pristine Si9 are shown to be noncovalent. The reduced density gradient analysis is performed, in which increased van der Waals interactions are observed with the enlargement of the size of alkali metals. The influence of the implicit solvent model is considered, where the hyperpolarizability (βo) in the solvent is found to be about 83 times larger than that in the gas phase for Si9K5. The frequency-dependent nonlinear optical (NLO) response for the dc-Kerr effect is observed up to 1.3 × 1011 au, indicating an excellent change in refractive index by an externally applied electric field. In addition, natural bonding orbitals obtained from the second-order perturbation analysis show the charge transfer with the donor-acceptor orbitals. Electron localization function and localized orbital locator analyses are also performed to better understand the bonding electrons in designed clusters. The studied Zintl clusters demonstrate the superalkali character in addition to their remarkable optical and nonlinear optical properties.

Experimental investigation of density shift through glass surface modification with ultrashort pulsed laser

In recent years, the use of innovative and sustainable materials such as glass has become increasingly important, especially to eliminate the use of polymers in lab-on-a-chip applications. Transparent material processing in the field of micro/nano structuring using ultrashort pulse lasers contributes to this trend. The induced change in refractive index can be related to the glass materials, with differences between e.g. soda-lime glass, BF33 and fused silica. In this work, the change of surface modifications below the ablation threshold has been experimentally investigated for these different glass types. The results show that surface structuring is possible by modifying the density and the corresponding volume without material removal. The size of the modification depends on the pulse energy and could be measured from a height of about 1 nm. This can be used for nano-structuring of moving components, as in lab-on-a-chip applications, and for inducing a self-cleaning surface combining micro/nano-structures.

Enhanced photonic spin Hall effect and its sensing application via optical Tamm state based on nanocomposites

In this contribution, by combining the nanocomposite with distributed Bragg reflector, a simple multilayer structure to enhance the photonic spin Hall effect (PSHE) via optical Tamm state (OTS) for both horizontal (H) and vertical (V) polarized lights is presented. The structural parameters of nanocomposite layer (e.g., thickness and filling factor) have a significant influence on PSHE. The maximal spin shifts of 16.2 and 17.8 μm can be achieved for H and V polarized lights, respectively, which are attributed to strong excitation of OTS. Moreover, the spin shift is extremely sensitive to the refractive index changes of sensing medium, and a gas sensor with superior sensing performance is theoretically proposed for both H and V polarized beams. These findings provide an effective method to enhance the PSHE for both H and V polarized lights and therefore open an opportunity for developing PSHE refractive index sensors.

Pattern-illumination time-resolved phase microscopy and its applications for photocatalytic and photovoltaic materials.

Pattern-illumination time-resolved phase microscopy (PI-PM) is a technique used to study the microscopic charge carrier dynamics in photocatalytic and photovoltaic materials. The method involves illuminating a sample with a pump light pattern, which generates charge carriers and they decay subsequently due to trapping, recombination, and transfer processes. The distribution of photo-excited charge carriers is observed through refractive index changes using phase-contrast imaging. In the PI-PM method, the sensitivity of the refractive index change is enhanced by adjusting the focus position, the method takes advantage of photo-excited charge carriers to observe non-radiative processes, such as charge diffusion, trapping in defect/surface states, and interfacial charge transfer of photocatalytic and photovoltaic reactions. The quality of the image sequence is recovered using various informatics calculations. Categorizing and mapping different types of charge carriers based on their response profiles using clustering analysis provides spatial information on charge carrier types and the identification of local sites for efficient and inefficient photo-induced reactions, providing valuable information for the design and optimization of photocatalytic materials such as the cocatalyst effect.

Giant static refractive index gradient induced by strong ultrasonic wave

Spatiotemporal modulation of refractive index in an optical path medium allows for control of light propagation. This refractive index change (Δn) can be achieved by external stimulation such as temperature or density change, but there is a limit to the extent to which Δn can be controlled by external stimulation acting on a single medium. Here, we demonstrate a technique to form a giant refractive index gradient (Δn = 0.06) in a small region of water (< 10 mm) using a high-frequency, high-intensity (in the 100-megahertz-range, on the order of megapascals) ultrasonic wave. Ultrasonic radiation in water can statically modulate the refractive index in water from the initial value (n = 1.33) toward that of air.