Variation Of Refractive Index Research Articles

Long term measurements of aerosol mass concentration with optical particle counters: Discrepancies with plausible reasons

Gravimetry-based direct measurements of mass concentration require offline analysis which is not suited for field campaigns. Hence such campaigns rely on the estimation of mass concentration by indirect methods mostly calibrated in controlled laboratory conditions. Optical particle counter (OPC) employs algorithms converting the measured number concentration to mass concentration using appropriate conversion factors. The accuracy of such conversion has not been validated for widely varying atmospheric conditions. This study compares the mass concentration estimated by OPC with those directly obtained from gravimetry-based instruments for outdoor samples collected in Bathinda City, Punjab, India from January 2022 to November 2023. The difference in the gravimetrically measured and OPC predicted values quantified in terms of ratios (gravimetric to optically estimated mass concentration), came out to be 1.42 ± 0.77, 0.99 ± 0.51, and 1.17 ± 0.58 for PM10, PM2.5 and PM1, respectively. This difference when estimated with the back-up filter of OPC itself (C factor), was 1.37 ± 0.66. More than half of the samples showed ratios outside the 0.8-1.2 range thus indicating under or over-estimation in the OPC predicted values. The probable role of variation in density, shape, and refractive index of atmospheric aerosol particles towards the observed inaccuracy of estimated mass concentration has been highlighted. In the absence of clear guidelines and protocols, the study suggests ways to improve the accuracy via periodic measurement of the C factor and/or incorporating calibration factors in such measurements.

Modulation of classical non-separability of vector vortex beams using Brewster effect

The vector vortex beams are generated by the superposition of two orthogonal orbital angular momentum (OAM) beams having orthogonal polarization states using a stable common path technique. The degree of non-separability of the generated vector vortex beams is controlled by changing the input polarization state and Brewster effect using a dielectric prism. We have estimated the linear entropy (SL) of the classical non-separable beams by measuring the reflection coefficients for different input polarization states at each rotation angle of the prism. The shifting of the peak value of SL for different input polarization states at the particular rotation angles of the prism is observed. The usefulness of the proposed scheme is demonstrated in sensing the refractive index variations of the prism.

Electronic analogue of Fourier optics with massless Dirac fermions scattered by quantum dot lattice

Abstract The field of electron optics exploits the analogy between the movement of electrons or charged quasiparticles, primarily in two-dimensional materials subjected to electric and magnetic (EM) fields and the propagation of electromagnetic waves in a dielectric medium with varied refractive index. We significantly extend this analogy by introducing an electronic analogue of Fourier optics dubbed as Fourier electron optics (FEO) with massless Dirac fermions (MDF), namely the charge carriers of single-layer graphene under ambient conditions, by considering their scattering from a two-dimensional quantum dot lattice (TDQDL) treated within Lippmann-Schwinger formalism. By considering the scattering of MDF from TDQDL with a defect region, as well as the moir'{e} pattern of twisted TDQDLs, we establish an electronic analogue of Babinet's principle in optics. Exploiting the similarity of the resulting differential scattering cross-section with the Fraunhofer diffraction pattern, we construct a dictionary for such FEO. Subsequently, we evaluate the resistivity of such scattered MDF using the Boltzmann approach as a function of the angle made between the direction of propagation of these charge-carriers and the symmetry axis of the dot-lattice, and Fourier analyze them to show that the spatial frequency associated with the angle-resolved resistivity gets filtered according to the structural changes in the dot lattice, indicating wider applicability of FEO of MDF.

Broadband terahertz absorption and Q-switching behavior of 5-chloro-2-nitroaniline (5C2NA) crystal

Abstract Growth of transparent 5-chloro-2-nitroaniline (5C2NA) crystals was achieved through the slow evaporation technique. Analysis via X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy confirmed the crystalline nature of the organic crystals and revealing distinct molecular fingerprints of 5C2NA respectively. UV-VIS and Photoluminescence (PL) spectroscopy were employed to study the material's band gap and ground state absorption respectively. Thermogravimetric analysis (TGA) analysis indicated the stability of the grown crystals up to 211ºC. Dielectric measurements and Urbach plot suggested the presence of fewer defects in 5C2NA crystals. Time domain terahertz spectroscopy was utilized to observe the variation of absorption coefficient and refractive index in the terahertz frequency regime. Nonlinear optical effects such as saturable absorption (SA) and reverse saturable absorption (RSA), are pivotal in the development of all-optical logic gates. The transition between SA and RSA possesses a significant importance in optoelectronic applications. In this study, we investigate the 5C2NA crystal, revealing its ability to exhibit both SA and RSA under Z scan technique with varying pump intensities. The switching properties observed in 5C2NA can be harnessed for applications such as all-optical logic gates, rapid optical switching, optical limiting, mode storage, and more.

Diffractionless transmission of optical beams through a four-level atomic system affected by a plasmonic nanostructure

A nondiffracting propagation of an optical beam through a four-level double-V-type quantum system near a plasmonic nanostructure is investigated. We study the linear absorption and dispersion properties of the quantum system as it interacts with two laser fields. We discuss the effect of the control beam with a Laguerre-Gaussian (LG) profile on the focusing of the probe beam in the presence of a plasmonic nanostructure. An appropriately selected control beam excites one transition of the atomic system and generates a spatially varying refraction index modulation for a weak probe beam that couples to the other transition. We demonstrate that placing a plasmonic nanostructure at a nanometer distance from the atomic system and using a control field with the spatial structure leads to the diffraction-less propagation of the probe beam through the atomic system. Also, it is shown that the optical properties and probe beam focusing can be controlled by adjusting the distance of the plasmonic nanostructure from the atomic system. The proposed all-optical waveguide with high contrast and transmission can be used to implement applications such as image transfer through the medium and image processing.

1D Topological Photonic Crystal based Nanosensor for Tuberculosis Detection.

In this study, we present a nanosized biosensor based on the photobiological properties of one-dimensional (1D) topological photonic crystals (PCs). A topological structure had been designed by combining two photonic crystal structures (PC 1 and PC 2) comprised of functional material layers, Si and SiO2. These two, PC 1 and PC 2, differ in terms of the thickness and arrangement of these dielectric materials. We carried out a comparison between two distinct topological photonic crystals: one using random photonic crystals, and the other featuring a mirror heterostructure. Tuberculosis may be diagnosed by inserting a sensor layer into 1D topological photonic crystals. The sensing process is based on the refractive indexes of the analytes in the sensor layer. When the 1D-topological heterostructure-based PC and its mirror-image structures are stacked together, the sensor becomes more efficient for analyte detection than the conventional PCs. The random-based topological photonic crystal outperformed the heterostructure-based topological photonic crystal in analyte sensing. Photonic media witness notable blue shifts due to the analytes' variations in refractive index. The numerical results of the sensor are computed using the transfer matrix approach. Effective results are achieved by optimizing the thicknesses of the sensor layer and dielectric layers; number of periods and incident angle. In normal incident light, the developed sensor shows a high sensitivity of 1500 nm/RIU with a very low limit of detection in the order of 2.24E-06 RIU and a high-quality factor of 30659.54.

Refractive insensitive directional bend sensor based on specialty microstructure optical fiber with dumbbell shape core

With the development of information society, more and more special occasions are calling for a wide range of sensors with special properties. In this work, a refractive index insensitive directional bend sensor is proposed based on a specialty microstructure optical fiber with dumbbell shape core, which works as a dual core fiber (DCF). Firstly, this specialty microstructure optical fiber with dumbbell shape core has been fabricated with self-pressurised drawing technique. Then, it has been constructed as a Mach-Zehnder interferometer (MZI) sensor. Consequently, its bend sensing characteristics has been investigated from both theory and experiment. The response of the proposed sensor to the bending evidently displays the directional dependence. In the bending along x direction, the interference peak wavelength is almost insensitive to the curvature. But the transmission is sensitive, which almost obeys the exponential function. Correspondingly, in the bending along y direction the wavelength almost linearly varies with curvature, giving out an averaged sensitivity of 2.44 nm/m−1 in the curvature range of 0–3.79 m−1. But the peak transmission fluctuates without typical principle. In addition, the response of the proposed sensor to the refractive index variation has been checked. It is found that the peak wavelength is almost insensitive to the change of refractive index from 1.33 to 1.45, and the peak transmission fluctuates without clear principle. All these results demonstrate that the proposed sensor has great potential for future internet of things (IoT) applications.

Dynamic Interferometry With Retrace Error Correction Using Fringe Projection For High-Gradient Fields

This paper introduces an innovative approach aimed at advancing the characterization of high-gradient flow fields through the integration of dynamic interferometry and fringe projection techniques. Traditional interferometry offers a precise tool sensitive to optical path changes, typically used to measure refractive index variations induced by factors such as density changes in fluid flow. Rays are usually considered as straight lines, and refraction can be neglected. However, in scenarios involving high-gradient fields, such as supersonic flow with shock waves, optical path changes due to ray bending become significant. In these high-gradient fields, the trajectory of the ray is altered, leading to longer optical paths (resulting in incorrect values) and geometric distortions of the image (resulting in inaccuracies in position). Although retrace error corrections using numerical models of refractive index distribution can mitigate some effects, complete compensation is often challenging. To address this, our proposed technique integrates fringe projection, which is highly sensitive to ray deflections caused by variations in the index of refraction within a transparent medium. The method involves projecting a precisely defined multiplexed cosine fringe pattern across the measured area. Modifications in the pattern indicate ray bending, providing data for interferometric corrections. Real-time interferometry is seamlessly combined with the fringe projection technique to compensate for optical path length changes and mitigate mapping errors induced by ray deflections. The dynamic nature of this combined approach enables the instantaneous capture of both interferometric and fringe-projection data, making it a suitable tool for real-time measurements. Experimental verification of the technique demonstrates its efficacy, with a comparative study against traditional interferometry showcasing a significant reduction in mapping errors.

Dielectric crystal coupled with guided-mode resonance to enhance the magneto-optical Kerr effect and sensing behavior in transmission mode

In this study, we introduce a high-performance magneto-optical sensor that features enhanced amplitude and sensitivity, achieved through guided-mode resonance (GMR) excitations in an aluminum antimonide slab. This advancement is facilitated by a square, periodically arranged nanorod structure comprising dielectric materials. The electromagnetic field distributions were obtained along with the transmission spectra to elucidate the GMR mode in our proposed structure, thereby highlighting the enhancement of the transverse magneto-optical Kerr effect (TMOKE) signal in the transmitted mode and the corresponding sensing performance. The findings demonstrate that the position of the TMOKE resonance is remarkably responsive to variations in the background refractive index, yielding an impressive sensitivity of 188.7° per refractive index unit, coupled with a narrow linewidth owing to the inherent low loss of dielectric materials. This investigation lays the groundwork for novel advancements in linear optics, integrated optical circuits, and low-loss biosensing applications.

3D Laser Writing of Low-Loss Cross-Section-Variable Type-I Optical Waveguide Passive/Active Integrated Devices in Single Crystals.

Optical waveguides fabricated in single crystals offer crucial passive/active optical components for photonic integrated circuits. Single crystals possess inherent advantages over their amorphous counterpart, such as lower optical losses in visible-to-mid-infrared band, larger peak emission cross-section, higher doping concentration. However, the writing of Type-I positive refractive index modified waveguides in single crystals using femtosecond laser technology presents significant challenges. Herein, this work introduces a novel femtosecond laser direct writing technique that combines slit-shaping with an immersion oil objective to fabricate low-loss Type-I waveguides in single crystals. This approach allows for precise control of waveguide shape, size, mode-field, and refractive index distribution, with a spatial resolution as high as 700nm and a high positive refractive index variation on the order of 10-2, introducing new degrees of freedom to design and fabricate passive/active optical waveguide devices. As a proof-of-concept, this work successfully produces a 7mm-long circular-shaped gain waveguide (≈10µm in diameter) in an Er3+-doped YAG single crystal, exhibiting a propagation loss as low as 0.23dBcm-1, a net gain of ≈3dB and a polarization-insensitive character. The newly-developed technique is theoretically applicable to arbitrary single crystals, holding promising potential for various applications in integrated optics, optical communication, and photonic quantum circuits.

Insights into the structural and optical properties of (Cu,Ag)2ZnSnSe4 thin films: Role of pre-annealing and selenization times

The present study investigates the properties of Cu2ZnSnSe4 (CZTSe) absorber layers with silver substitution, scrutinizing how their characteristics improve by varying the pre-annealing and selenization times. The X-ray diffraction (XRD) analysis confirms the presence of tetragonal structured (Cu,Ag)2ZnSnSe4 (CAZTSe) thin films and reveals a peak shift towards lower angles, indicating silver substitution in place of copper within the kesterite lattice. Raman peaks observed at 195 cm−1, 172 cm−1, and 233 cm−1 further support these findings. Particularly, the CAZTSe thin film subjected to a 30-min pre-annealing at 275 °C, followed by a 2-min selenization at 450 °C, exhibits a compact surface morphology. Additionally, Secondary ion mass spectrometry (SIMS) analysis demonstrates a uniform elemental profile throughout the depth of the film. The optical analysis reveals variations in transmittance (T%), reflectance (R%), extinction coefficient (k), and refractive index (n) over a wavelength range of 400–2000 nm. The described CAZTSe thin film possesses a band gap of 1.06 eV. Furthermore, the electrical analysis reveals the highest carrier concentration of 1.12 × 1016 cm−3, a maximum mobility of 260 cm2/(Vs)−1, and the lowest resistivity of 2.15 Ωcm among these CAZTSe thin films.

A temperature sensor based on multi-beam capture and interference

In this study, we proposed and validated a temperature fiber sensor based on optical signal capture Mach-Zehnder Interferometer (OSC-MZI) with high-precision temperature measurement capabilities. By creating three air cavities on a continuously offset-spliced no-core fiber (NCF), the optical signal experiences reflection and divergence within these cavities, which enhances the optical path of propagating light. Gradually, filling these NCF cavities with polymethyl methacrylate (PMMA) microspheres to increase the light coupling efficiency and as a sensing unit. Due to the directly contact between the optical fiber waveguide and PMMA, the state of incident lights is easily influenced by the refractive index (RI) variations of PMMA. When the environmental temperature changes, RI of the PMMA with high thermal-optic coefficient shows marked change, which finally leads to the obvious wavelength shift of interference dips in the transmission spectrum. Experimental results show that MZI coated with the polymer materials achieves a temperature sensitivity of up to 2.04 nm/°C within the temperature range of 25–50 °C. Furthermore, as an interference-based MZI sensor, its total length is only 4.75 mm. The proposed OSC-MZI provides a new approach for combining optical fiber sensors with polymer materials.

High sensitivity terahertz biomedical sensing with graphene metamaterial

Identification of biomedical molecules by terahertz (THz) sensing is a quite promising noninvasive technique. Here, we present a THz graphene metamaterial with plasmon-induced transparency (PIT) for high sensitivity biosensing. The strong PIT effect is generated by the destructive interference between the plasmon modes excited by two graphene strips. For testing different biomedical analytes, a distinct frequency shift at PIT resonance can be observed due to the effect of surrounding refractive index variation. The metamaterial scheme for noninvasive biomedical sensing can achieve the sensitivity as high as 2.6 THz/RIU. Particular, it can not only detect the phase of red blood cell infected by malaria but also the concentration of ethanol aqueous solution. The underlying mechanism is discussed for understanding the THz sensing operation.

Spectral Characteristics and Displacement Sensing of U-Shaped Single-Mode-Multimode-Single-Mode Fiber Structure.

The U-shaped fiber configuration represents the elementary form of micro-displacement sensing, characterized by its exceptional freedom and flexibility. The study proposes the U-shaped bent single-mode-multimode-single-mode (SMS) fiber structure that integrates the multimode interference (MMI) effect for enhanced mode dispersion and the Mach-Zönder interference (MZI) effect for spectral sensitivity improvement. The transmission spectral properties of the U-shaped SMS fiber structure with a bent radius over 1 cm are experimentally measured as the change in displacement varied within the range of 5 mm in this work. As the radius decreases, the spectrum shows redshift, which is related to the central wavelength of the peak or dips-a smaller wavelength results in a stronger redshift for the same displacement change. The average sensitivity of micro-displacement measurement within a range of 5 mm is 5.41 pm/μm, and the linearity is 99.62%. The maximum sensitivity of U-shaped SMS fiber structure is 34.46 pm/μm, with the minimum displacement change of approximately 5.804 nm. The transmission spectral properties of the U-shaped SMS fiber structure within the ranges of 50 μm, 500 μm, and 5 mm are experimentally measured in this work. This experiment observed a relatively uniform spectral drift pattern in a large range of micro-displacement sensing. The measurement range is limited by the limited spectral range of the light source and the discontinuous variation in the effective refractive index. This provides an experimental reference for further understanding the characteristics of U-shaped fiber structures and applying its application in micro-displacement sensing.

Optical and fluorescent properties in Er, Yb:YAG double-line waveguide by femtosecond laser direct writing

This paper reports the double-line waveguides in Er,Yb:YAG prepared by femtosecond laser (fs-laser) writing at 1030 nm, 450 fs pulse and 100 kHz repetition rate. When light propagates at 632.8 nm along transverse electric (TE) and transverse magnetic (TM) polarization, efficient single-mode guidance is demonstrated in the double-line structures. The minimum transmission loss is as low as 0.98 dB/cm in waveguide with space of 18 μm. The structural modifications in damaged tracks and strain-affected regions were characterized by confocal micro-Raman spectroscopy, and the refractive index variations were analyzed. The corresponding fluorescence emissions of Er3+ and Yb3+ are investigated in the Er,Yb:YAG double-line structure.

Double Resonance Characteristic of Photonic Crystal Structure with Defect Metal Layer

The optical response of the photonic crystal structure has been analyzed using the Finite Difference Frequency Domain (FDFD) method. Simulation using this method aims to analyze the characteristics of light transmission when interacting with related structure. The results of the study of photonic crystal structures with two defects show that there are two transmittance bands called double resonances at two specific wavelengths. Changes in the resonance peak with respect to the variations of refractive index in the second defect layer resulted in a linear change with significant sensitivity. For photonic crystals with two defects, the first defect is a metal layer and the second defect is material sensing, which produces double resonance with a sensitivity of 7,825 and 3,2675, while for photonic crystals with the first defect, a dielectric layer, and the second defect is material sensing, produces single resonance with a sensitivity of 2,022. The choice of the first defect is that the metal layer is quite important, in addition to producing greater sensitivity, it also produces a field enhancement between the dielectric and the metal layer which is called surface plasmon resonance (SPR). The sensitivity value can also be set by determining the thickness of the second defect layer. These results can be used as a standard for fabrication and developed for sensor applications.

A comparative study of ZnO, AZO, TiO2, Sb2O3, and SnO2 nanomaterials coated SPR based fiber optic refractive index sensors for chemical sensing application

This paper presents a comprehensive investigation of Surface Plasmon Resonance (SPR) based fiber optic sensors tailored for precise detection of refractive index (RI) variations in acetone and ethanol solutions. The sensor probe design incorporates a nanostructure comprising a metal layer (Ag, Al, or Cu) and a metal oxide sensing layer (ZnO, AZO, TiO2, Sb2O3, SnO2) over the fiber core. Employing the Transfer Matrix Method (TMM), optimal layer thicknesses were determined, with a combination of 50 nm Ag and 20 nm metal oxide demonstrating maximum sensitivities. The sensor probes were fabricated using the Radio Frequency (RF) sputtering technique, and their uniformity, elemental composition, and optical properties were assessed through Field Emission Scanning Electron Microscope (FESEM) and ellipsometer analyses.The sensor response of the probes is 12 %, 7.1 %, 10.8 %, 6.08 %, and 11.34 % for acetone and 11.9 %, 9.2 %, 12 %, 7.89 %, and 12.48 % for ethanol. Sensitivities ranged from 2.7 nm/v% to 2.84 nm/v% for acetone and 1.90 nm/v% to 3.02 nm/v% for ethanol, with the SnO2-coated probe exhibiting the highest sensitivity. Further sensitivity enhancement was achieved by introducing a bimetal layer (50 nm Ag/5 nm Sb/20 nm SnO2), resulting in a 15 % increase for ethanol compared to the Ag/SnO2-based SPR probe.

Fabrication and Characterization of PDMS Waveguides for Flexible Optrodes.

With the growth of optogenetic research, the demand for optical probes tailored to specific applications is ever rising. Specifically, for applications like the coiled cochlea of the inner ear, where planar, stiff, and nonconformable probes can hardly be used, transitioning from commonly used stiff glass fibers to flexible probes is required, especially for long-term use. Following this demand, polydimethylsiloxane (PDMS) with its lower Young's modulus compared to glass fibers can serve as material of choice. Hence, the long-term usability of PDMS as a waveguide material with respect to variations in transmission and refractive index over time is investigated. Different manufacturing methods for PDMS-based flexible waveguides are established and compared with the aim to minimize optical losses and thus maximize optical output power. Finally, the waveguides with lowest optical losses (-4.8dB cm-1 ± 1.3dB cm-1 at 472nm) are successfully inserted into the optogenetically modified cochlea of a Mongolian gerbil (Meriones unguiculatus), where optical stimuli delivered by the waveguides evoked robust neuronal responses in the auditory pathway.

Highly sensitive refractive index sensing based on nanostructured porous silicon interferometers

In this study, we present the experimental evidence demonstrating the utility of electrical double layer (EDL)-induced ion accumulation, using sodium (Na+) ion in water as model substances, on a negatively charged nanostructured surface, specifically thermally grown silicon dioxide (SiO2). This novel approach, termed Ion Surface Accumulation (ISA), aims to enhance the performance of nanostructured porous silicon (PSi) interferometers in optical refractometric applications. The experimental results show that the electrical double layer-induced ion surface accumulation (EDL-ISA) on oxidized PSi interferometers enables remarkable amplification of the interferometer output signal (the spectral interferogram), even when the bulk refractive index variation is below 10-3 RIU. This substantial signal enhancement translates into an increase in sensitivity of up to two orders of magnitude, facilitating the reliable measurement of refractive index variations with both a detection limit (DL) and resolution (R) as low as 10-4 RIU. This achievement elevates the performance of PSi interferometers in photonics and plasmonics-based refractive index platforms.

Propagation characteristics of the Bessel-Gaussian beams in turbulent plasma sheath surrounding the hypersonic aerocraft

To investigate the transmission characteristics of the Bessel-Gaussian beams in a turbulent plasma sheath, the flow field around a hypersonic vehicle is numerically simulated and analyzed according to the Navier-Stokes (N-S) equation and turbulence model. Then, utilizing the characteristics of the thickness of the plasma flow field around the supersonic vehicle at the centimeters level, the double fast Fourier transform (D-FFT) algorithm based on the Fresnel diffraction integral is used for the first time to predict the propagation characteristics of the Bessel-Gaussian beams in the turbulent plasma sheath. The results show that the lower the height and the higher the velocity, the smaller the thickness of the plasma sheath shock layer. The refractive index variance, the wavelength and the waist-radius are essential factors affecting the transmission quality of the Bessel-Gaussian beams. When compared to the Gaussian beams, the Bessel-Gaussian beams have smaller drift and scintillation index, which have the advantage in suppressing turbulence.