Refractive Index Change Research Articles

Tapered fiber optic refractive index sensor using speckle pattern imaging

In this study, a low-cost and compact refractive index (RI) sensor based on speckle pattern imaging has been proposed and demonstrated numerically and experimentally. We have proven that speckle pattern images recorded with a CCD camera placed at the end of the tapered optical fiber (TOF) are highly sensitive to the RI of the medium to be measured. The effects of the waist diameter and length of the tapered region, which is the sensing part of the fiber, on sensitivity were investigated using the beam propagation method (BPM). It has been seen that the numerical results have compatible with the experimental results. Tapered fiber sensor was fabricated by electric arc discharge technique using step index multimode fiber (SI-MMF) with core and cladding diameters of 105 and 125 μm. The sensor sensitivity increases as the waist diameter decreases and the waist length increases. The sensor, which waist diameter is reduced to 10 μm, is sensitive to RI changes of the order of 10-4. It has been clearly seen that the proposed sensor based on speckle pattern imaging is highly sensitive to RI change.

D-shaped fiber optic plasmonic sensors using planar and grating structures of silver and gold: design and analysis.

In this paper, a D-shaped optical fiber plasmonic sensor using planar and grating structures of silver and gold metals is simulated using the finite element method under the wave optics module of COMSOL Multiphysics. Performance defining parameters are based on (i) the transmittance curve, viz., resonance wavelength (λ r), shift in resonance wavelength (Δ λ r), minimum transmittance (T m i n ), and bandwidth (BW), and (ii) on electric field distribution of a surface plasmon wave, viz., penetration depth (PD) and propagation length (PL) obtained for the considered sensor structures. It is found that gold gives wider BW than silver (e.g.,at 1.39 refractive index of the sample: 480% for the planar case and 241% for the grating case), which deteriorates sensor performance by degrading detection accuracy. However, gold gives higher Δ λ r than silver (at 1.40-1.39=0.01 change in refractive index of the sample: 18.33% for the planar case and 16.39% for the grating case), which improves sensor performance and enhances sensitivity. A grating slightly increases the BW and Δ λ r for both gold and silver. Further, with respect to silver, the sensor that contains gold demonstrates higher PD (e.g.,22.32% at 1.39 refractive index of the sample for the planar case) and lower PL (e.g.,22.74% at 1.39 refractive index of sample for the planar case). A grating increases the PD (e.g.,10% for silver at 1.39 refractive index of the sample), whereas it decreases the PL (e.g.,8.73% for silver at 1.39 refractive index of the sample). Lower PL signifies the localization of the field, whereas higher PD enables the sensor to detect larger molecules. Therefore, the sensor with grating metals provides better sensitivity with reduced detection accuracy for the detection of comparatively larger molecules.

Optical Properties of Electrochemically Gated La1−xSrxCoO3−δ as a Topotactic Phase‐Change Material

AbstractMaterials with tunable infrared refractive index changes have enabled active metasurfaces for novel control of optical circuits, thermal radiation, and more. Ion‐gel‐gated epitaxial films of the perovskite cobaltite La1−xSrxCoO3−δ (LSCO) with 0.00 ≤ x ≤ 0.70 offer a new route to significant, voltage‐tuned, nonvolatile refractive index modulation for infrared active metasurfaces, shown here through Kramers–Kronig‐consistent dispersion models, structural and electronic transport characterization, and electromagnetic simulations before and after electrochemical reduction. As‐grown perovskite films are high‐index insulators for x < 0.18 but lossy metals for x > 0.18, due to a percolation insulator‐metal transition. Positive‐voltage gating of LSCO transistors with x > 0.18 reveals a metal‐insulator transition from the metallic perovskite phase to a high‐index (n > 2.5), low‐loss insulating phase, accompanied by a perovskite to oxygen‐vacancy‐ordered brownmillerite transformation at high x. At x < 0.18, despite nominally insulating character, the LSCO films undergo remarkable refractive index changes to another lower‐index, lower‐loss insulating perovskite state with Δn > 0.6. In simulations of plasmonic metasurfaces, these metal‐insulator and insulator‐insulator transitions support significant, varied mid‐infrared reflectance modulation, thus framing electrochemically gated LSCO as a diverse library of room‐temperature phase‐change materials for applications including dynamic thermal imaging, camouflage, and optical memories.

Highly sensitive cholesterol concentration trace detection based on a microfiber optic-biosensor enhanced specificity with beta-cyclodextrin film

A beta-cyclodextrin (β-CD) based optic-fiber microfiber biosensor for the detection of cholesterol concentration is propose and experimentally demonstrated. As an identifying substance, β-CD is immobilized on the fiber surface for cholesterol reaction to form an inclusion complex. When the surface refractive index (RI) change is cause because of capturing the complex cholesterol (CHOL), the proposed sensor translates RI change into a macroscopic wavelength drift in the interference spectrum. The microfiber interferometer has a high RI sensitivity of 1251 nm/RIU and a low-temperature sensitivity of −0.019 nm/°C. This sensor can rapidly detect cholesterol in the concentration range of 0.001 to 1 mM and has a sensitivity of 12.7 nm/(mM) in the low concentration range of 0.001 to 0.05 mM. Finally, the characterization by infrared spectroscopy shows that the sensor can indeed detect cholesterol. This biosensor has a few strong advantages of high sensitivity and good selectivity, which expects great potential in biomedical applications.

Effect of CuO on physical, structural and optical properties of lithium borosilicate glasses

This paper reports the effect of copper on the physical, structural and optical properties of lithium borosilicate glasses with the composition of 50B2O3-30SiO2-(20-x) Li2O-xCuO (x = 2, 4, 6, 8, 10 mol %) synthesized by the conventional melt-quench technique. The prepared samples were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and UV–visible spectroscopy to study their structural and optical properties. Various physical parameters such as density, molar volume, oxygen packing density, ionic concentration and their inter-ionic distance are investigated. The band gap decreases from 3.64 to 2.70 eV with CuO doping into Li2O. The BO4 structural units change into BO3 units with the addition of CuO. The change in dielectric constant, extinction coefficient, refractive index and skin depth as a function of wavelength is also discussed. The theoretical optical basicity and oxide ion polarizability are found to increase with Li2O doping in place of CuO.

Resolving Molecular Size and Homologues with a Self-Assembled Metal–Organic Framework Photonic Crystal Detector

Traditional environmental photonic sensing based on the balanced reflection of a photonic structure can hardly be applied to distinguish molecular size and homologues, since the refractive index change introduced by these analytes can be too similar to be unambiguously distinguished. Here, a sensing method based on the pore size selectivity and organics adsorption–desorption capacity of self-assembled ZIF-8 photonics crystal is demonstrated, which can tell apart different molecular sizes, homologues, and organics with similar structures and physical properties. Specificity is improved by the inherent relationships between the pore size of ZIF-8 metal–organic frameworks and the associated molecular diffusion rates in the pores, so that the combination of the observed peak shift and recovery time are unique to each analyte. Using this method, selective detection of molecular size is also demonstrated in the case of linear vs cyclic molecules, since only the former can penetrate inside the ZIF-8 pores to induce a large wavelength shift. The reflection peak shift caused by the linear molecules was found to be about 30–40 nm, an order of magnitude larger compared to those for the cyclic molecules. A relationship between the diffusion rate of linear molecules in ZIF-8 pore and the recovery time of photonic crystal reflective peak is further established, linking the recovery time and the linear molecular diffusion coefficient. Finally, different linear molecules are identified by their associated recovery time to detect homologues or organics with similar refractive index.

Integrated Reconfigurable Photon-Pair Source Based on High-Q Nonlinear Chalcogenide Glass Microring Resonators.

Chalcogenide glasses (ChGs) have recently emerged as enabling materials for building reconfigurable nanophotonic devices by employing their refractive index changes associated with photosensitive effects. In particular, the availability of low-loss thin-film ChGs and the realization of high-Q microresonators provide exciting opportunities for integrated photonics. So far, the ChG photonic devices are predominately operated in the classical optics regime. In this work, we present the realization on-chip bright photon-pair quantum light sources via spontaneous four-wave mixing in a high-Q microring resonator fabricated on the newly developed ChG Ge25Sb10S65 platform. The emission wavelength of the photon-pair source can be continuously tuned across a double-free spectral range in a reconfigurable manner. Our work serves as a starting point to fully unleash the potential of exploiting ChGs for developing reconfigurable integrated quantum photonic devices.

Development of Taper-in-Taper-Based Optical Fiber Sensors for Chemical and Biological Sensing

This paper presents the development of taper-in-taper fiber (TITF)–optical fiber-based sensors for chemical and biological detection. TITF structure is a fiber structure formed by fabricating a taper again on the taper region of the traditional taper fiber. The experimental results show that the TITF structure has a lower transmitted intensity than the common taper fiber structure. It is demonstrated that the structure of TITF is more conducive to evanescent waves, thereby making it more sensitive to changes in the refractive index (RI) of the external environment. To confirm this, measurements of ethanol solutions with different water contents are taken using the TITF structure to showcase the chemical sensing, thus verifying its sensitivity to RI changes in the external environment. The sensor can measure ethanol solutions from 0 to 100% with a sensitivity of 4.06 a.u./%. Thereafter, the TITF sensor is modified with ZnO-NPs to enhance its sensitivity for biological sensing (creatinine detection). The transmitted intensity of different concentrations of creatinine solution (0–2000 μM) is measured using the proposed sensor. The sensor showed good linearity and a high sensitivity of 0.11 a.u./μM for creatinine solution. Therefore, it is demonstrated that the TITF sensor with ZnO-NPs can be used for effective biological sensing.

The manufacture and reliability analysis of the all-rigid Fabry–Perot resonator for fiber-optic acoustic sensors

By the continuous development of aerospace, petroleum exploration, and other industrial fields, the fiber-optic acoustic sensor (FOAS) with high reliability is a desideration sensor, which can be used for noise monitoring in the extremely harsh environment. The FOAS based on the all-rigid Fabry–Perot resonator (FPR) relies on the new acoustic sensitive principle, where the change in the air refractive index is induced by sound waves and gets rid of the distortion caused by the mechanical characteristics of the acoustic sensor based on the movable parts. So, the FPR-based FOAS is very suitable for acoustic sensing in the harsh environment. In this paper, the reliability of this kind of FOASs is simulated and analyzed. The modal and anti-vibration simulation results of FPR with different sizes show that the FPR has a high natural frequency, and the external vibration environment does not affect the acoustic sensitivity of the FPR. The micro and small-batch all-rigid FPR can be manufactured by the optical contact. Moreover, the FPR can withstand the high temperature of 500°C that is verified by rapid heat treatment equipment. In order to improve the reliability of the FOAS, the metal packing shell is designed and fabricated. Moreover, the vibration and high-temperature tests of the packaged sensor are carried out. The two groups of tests show that the sensor can work normally under 10 g of acceleration vibration and 200°C high temperature, respectively. Therefore, the FOAS based on the FPR has high reliability and is very suitable for noise monitoring in the extreme harsh environment of various industrial fields. Furthermore, the research results of this paper will enhance the competitiveness and influence of the commercialized FOAS.

Two-Color iSCAT Imaging of Ag Nanoparticles Resolves Size and Ambient Refractive Index Changes.

The ability to discern noble metal nanoparticles (NPs) with different sizes and in ambient media with different refractive indices has important applications in imaging and sensing. Here a two-color (405 nm, 445 nm) interferometric scattering (iSCAT) detection scheme is applied to characterize the wavelength-dependent iSCAT contrast of Ag NPs with nominal diameters of 10, 20, 40, and 60 nm and to distinguish between NPs of different sizes. The iSCAT contrast also depends on the ambient refractive index and the relative iSCAT contrast on both channels revealed a spectral red-shift for 40 and 60 nm Ag NPs when the ambient refractive index was increased from n = 1.3892 to n = 1.4328. With the selected wavelength channels, the spectral resolution of the two-color imaging strategy was, however, insufficient to resolve spectral shifts induced by refractive index changes for 10 and 20 nm Ag NPs.

Physico-chemical characterization of methanolic mixtures of cholinium dihydrogen phosphate-based DES

The biocompatibility of three deep eutectic solvents based on cholinium dihydrogen phosphate for their use in lipase-catalyzed reactions was recently demonstrated, so the possible application as cosolvents with methanol in transesterification processes demands an exhaustive characterization of the physical properties. Thus, in this work binary and ternary deep eutectic solvents composed of the abovementioned salt as hydrogen bond acceptor and ethyleneglycol and/or glycerol as hydrogen bond donors were synthesized and mixed with methanol. The density, refractive index, electrical conductivity and dynamic viscosity were determined between 293.15 K and 323.15 K. The experimental data were correlated with different well-known equations and derived magnitudes like excess molar volumes, changes of refractive index on mixing and viscosity deviations were inferred in order to get an in-depth characterization of the studied mixtures with cholinium dihydrogen phosphate-based DES. The most intriguing data observed is the great peak measured for ionic conductivity at very high methanol concentration, which is difficult to explain even with the most recent theoretical models.

Imaging system aberrations through optical windows with nonuniform laser heating.

An optical window is a critical component of an imaging system. When operating in harsh environments with extreme heating, nonuniform temperature changes occur throughout the window and cause nonuniform refractive index changes and mechanical deformations due to thermal expansion, which can degrade the imaging system's performance. In this paper, we present results collected from an experimental setup developed to characterize these aberrations. This setup includes a C O 2 laser for sample heating, an infrared camera for measuring front and back surface temperatures, and a visible imaging system and a wavefront sensor for measuring degradations of a collimated beam from a point source transmitted through the heated window. Sapphire samples are laser heated with a Gaussian profile to temperatures in excess of 500K with surface temperature gradients in excess of 15K/mm. These measurements are compared with first principles models, which show quantitative agreement for window temperatures and qualitative agreement with the transmitted wavefront and imaged point source.

Tunable topological phase transition in the telecommunication wavelength

Recent progress in the Valley Hall insulator has demonstrated a nontrivial topology property due to the distinct valley index in 2D semiconductor systems. In this work, we propose a highly tunable topological phase transition based on valley photonic crystals. The topological phase transition is realized by the inversion symmetry broken due to the refractive index change of structures consisting of optical phase change material (OPCM) with thermal excitation of different sites in a honeycomb lattice structure. Besides, simulations of light propagation at sharp corners and pseudo-spin photon coupling are conducted to quantitatively examine the topological protection. Compared with other electro-optical materials based on reconfigurable topological photonics, a wider bandwidth and greater tunability of both central bandgap frequency and topological phase transition can happen in the proposed scheme. Our platform has great potential in practical applications in lasing, light sensing, and high-contrast tunable optical filters.

Investigating the effect of surface termination and thickness of MXene on the detection response of the SPR sensor using FDTD simulation

Surface plasmon resonance (SPR) is an optical-based detection technique that is of great interest in research because of its rapid detection and label-free features. Current SPR sensor development focuses on modifying the sensing surface for maximum bioreceptor and target immobilization. In this research, we proposed SPR sensor with the Kretschmann configuration modified by adding immobilization layers using a few layers of MXene (Ti3C2T2). MXene is an alternative two-dimensional material suitable for sensors because it has surface termination (T2) which active to immobilize the target. The different surface termination (T=O,OH,F) and number of MXene layers was added on the SPR sensor to obtained the optimum parameters of MXene application. Using Finite-Difference Time-Domain (FDTD) method, the SPR sensor is simulated on a different refractive index background which is assumed as the analyte. The different surface termination produces different SPR curves at the same refractive index conditions, the changes are found in the position of the SPR angle and the width of the curve. Thus, the results of surface termination variations are evaluated based on the full-width half-maximum (FWHM) of the curve which indicates the detection accuracy. Meanwhile, the optimum number of MXene layers was evaluated from the sensitivity value of the MXene-modified SPR sensor to changes in refractive index from 1.33 to 1.34. The result shows that the optimum detection performance was obtained from the application of six layers MXene with surface termination F2(Ti3C2F2). This configuration resulted a sensitivity of 150.131°/RIU and the LOD is 0.01633 RIU.

Pressure-based fast tuning of high-Q crystalline whispering gallery mode resonator

The whispering gallery mode resonator based on a fluoride crystalline material offers the advantage of an ultrahigh quality factor, however, its fast tuning remains a challenge. We propose a tuning method for a CaF2 crystalline resonator using external pressure loading. Through our theoretical calculations and simulations, we discovered that the frequency shift induced by deformation during the pressure tuning of the crystalline resonator is greater than that induced by changes in refractive index. Both shifts were of the same order of magnitude, indicating that both factors must be considered. The experimental results show tuning rates of 1114.6 and 888.2 MHz/MPa in the transverse electric and transverse magnetic modes, respectively. The tuning system can respond to a signal frequency of up to 1.2 MHz.

Designing and simulating realistic spatial frequency domain imaging systems using open-source 3D rendering software.

Spatial frequency domain imaging (SFDI) is a low-cost imaging technique that maps absorption and reduced scattering coefficients, offering improved contrast for important tissue structures such as tumours. Practical SFDI systems must cope with various imaging geometries including imaging planar samples ex vivo, imaging inside tubular lumen in vivo e.g. for endoscopy, and measuring tumours or polyps of varying morphology. There is a need for a design and simulation tool to accelerate design of new SFDI systems and simulate realistic performance under these scenarios. We present such a system implemented using open-source 3D design and ray-tracing software Blender that simulates media with realistic absorption and scattering in a wide range of geometries. By using Blender's Cycles ray-tracing engine, our system simulates effects such as varying lighting, refractive index changes, non-normal incidence, specular reflections and shadows, enabling realistic evaluation of new designs. We first demonstrate quantitative agreement between Monte-Carlo simulated absorption and reduced scattering coefficients with those simulated from our Blender system, achieving discrepancy in absorption coefficient and in reduced scattering coefficient. However, we then show that using an empirically derived look-up table the errors reduce to and respectively. Next, we simulate SFDI mapping of absorption, scattering and shape for simulated tumour spheroids, demonstrating enhanced contrast. Finally we demonstrate SFDI mapping inside a tubular lumen, which highlighted a important design insight: custom look-up tables must be generated for different longitudinal sections of the lumen. With this approach we achieved absorption error and scattering error. We anticipate our simulation system will aid in the design of novel SFDI systems for key biomedical applications.

High-Performance Refractive Index-Based Sensor Using Ellipsoid Ag–Au Nanoparticles

The nanoparticles’ localized surface plasmon resonance (LSPR) is sensitive to the surrounding medium refractive index change. Therefore, biosensing devices commonly use it as a refractive index-based sensor. In this work, an improvement strategy for the sensing performance of gold (Au) and silver (Ag) ellipsoid nanoparticles (NP-ELs) through controlling the size, aspect ratio, and core@shell structure was proposed. The investigation was started by constructing a multiple oscillator model to derive the optical permittivity function and compute the absorption of nanoparticles via discrete dipole approximation (DDA). The change in position and width of the LSPR peak concerning the refractive index of the medium was calculated to determine the sensing performance. Sensitivity and figure of merit (FOM) were used to describe sensing performance. As a result, the aspect ratio parameter of nanoparticles was found to be more dominant in increasing sensitivity and figure of merit (FOM) compared to particle size. In the core@shell structure, the absorption spectrum is also affected by the shell thickness, particle size, and aspect ratio. Therefore, those parameters can tune the sensing performance of the Ag@Au NP-ELs. The maximum sensitivity obtained for Ag@Au NP-ELs was 597.7 nm/RIU with a FOM of 34 RIU <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Therefore, the high performance refractive index-based sensor proposed in this study opens a new gate for developing biosensors in future.

A sensitive high repetition rate arrival time monitor for X-ray free electron lasers

X-ray free-electron laser sources enable time-resolved X-ray studies with unmatched temporal resolution. To fully exploit ultrashort X-ray pulses, timing tools are essential. However, new high repetition rate X-ray facilities present challenges for currently used timing tool schemes. Here we address this issue by demonstrating a sensitive timing tool scheme to enhance experimental time resolution in pump-probe experiments at very high pulse repetition rates. Our method employs a self-referenced detection scheme using a time-sheared chirped optical pulse traversing an X-ray stimulated diamond plate. By formulating an effective medium theory, we confirm subtle refractive index changes, induced by sub-milli-Joule intense X-ray pulses, that are measured in our experiment. The system utilizes a Common-Path-Interferometer to detect X-ray-induced phase shifts of the optical probe pulse transmitted through the diamond sample. Owing to the thermal stability of diamond, our approach is well-suited for MHz pulse repetition rates in superconducting linear accelerator-based free-electron lasers.

Facile and Unplugged Surface Plasmon Resonance Biosensor with NIR-Emitting Perovskite Nanocomposites for Fast Detection of SARS-CoV-2.

The emergence of the coronavirus disease 2019 (COVID-19) pandemic prompted researchers to develop portable biosensing platforms, anticipating to detect the analyte in a label-free, direct, and simple manner, for deploying on site to prevent the spread of the infectious disease. Herein, we developed a facile wavelength-based SPR sensor built with the aid of a 3D printing technology and synthesized air-stable NIR-emitting perovskite nanocomposites as the light source. The simple synthesis processes for the perovskite quantum dots enabled low-cost and large-area production and good emission stability. The integration of the two technologies enabled the proposed SPR sensor to exhibit the characteristics of lightweight, compactness, and being without a plug, just fitting the requirements of on-site detection. Experimentally, the detection limit of the proposed NIR SPR biosensor for refractive index change reached the 10-6 RIU level, comparable with that of state-of-the-art portable SPR sensors. In addition, the bio-applicability of the platform was validated by incorporating a homemade high-affinity polyclonal antibody toward the SARS-CoV-2 spike protein. The results demonstrated that the proposed system was capable of discriminating between clinical swab samples collected from COVID-19 patients and healthy subjects because the used polyclonal antibody exhibited high specificity against SARS-CoV-2. Most importantly, the whole measurement process not only took less than 15 min but also needed no complex procedures or multiple reagents. We believe that the findings disclosed in this work can open an avenue in the field of on-site detection for highly pathogenic viruses.

Label-free multiplex sensing from buffer and immunoglobulin G sensing from whole blood with photonic crystal slabs using angle-tuning of an optical interference filter.

Direct detection of biomarkers from unpurified whole blood has been a challenge for label-free detection platforms, such as photonic crystal slabs (PCS). A wide range of measurement concepts for PCS exist, but exhibit technical limitations, which render them unsuitable for label-free biosensing with unfiltered whole blood. In this work, we single out the requirements for a label-free point-of-care setup based on PCS and present a wavelength selecting concept by angle tuning of an optical interference filter, which fulfills these requirements. We investigate the limit of detection (LOD) for bulk refractive index changes and obtain a value of 3.4 E-4 refractive index units (RIU). We demonstrate label-free multiplex detection for different types of immobilization entities, including aptamers, antigens, and simple proteins. For this multiplex setup we detect thrombin at a concentration of 6.3 µg/ml, antibodies of glutathione S-transferase (GST) diluted by a factor of 250, and streptavidin at a concentration of 33 µg/ml. In a first proof of principle experiment, we demonstrate the ability to detect immunoglobulins G (IgG) from unfiltered whole blood. These experiments are conducted directly in the hospital without temperature control of the photonic crystal transducer surface or the blood sample. We set the detected concentration levels into a medical frame of reference and point out possible applications.