Enhancement Of Refractive Index Research Articles

Negative refractive index enhancement with zero absorption in a concentric chiral metal-atomic nanoshell

We investigate the electromagnetic chirality and negative refraction in a concentric nanoshell of a chiral metal sphere and a chiral atomic shell. The medium of the atomic shell with a four-level system is driven by a laser field and an incoherent pump field in a diamond configuration. We show that the electric and magnetic absorption spectra connecting through the chiral coefficients of the respective dipole moments of the two media, produce five and three lines spectral profiles. We explain that the spectral lines separated by dips are the manifestation of the classical (quantum) coherence effect of the wave field excitation in the medium of the metal sphere (atomic shell), and the interaction of the respective dipole moments at the interface of the two media. Furthermore, we show negative refraction with zero absorption without requiring permittivity (ϵ) and permeability (μ) simultaneously negative, where for all values of the incident wavelength, Re [μ] ≈ 1, representing a strong chiral electromagnetic behavior. Consequently, the negative refractive index enhances sufficiently beyond n = −1 for a wide range of parameters depending on the coupling parameters, chiral coefficients, and the radii ratio of the concentric metal-atomic nanoshell.

Vortex beam induced spatial modulation of quantum-optical effects in a coherent atomic medium

We have studied two-dimensional absorption, gain, and corresponding refractive index profiles in a ladder-type three-level atomic system interacting with three coherent fields. One is a weak probe field considered as a plane wave, while the other two are the control fields taken as two Laguerre–Gaussian doughnut beams. Position dependence of two vortex beams induces the spatially modulated coherence at the condition of resonance, which enables us to obtain space-dependent absorption, transparency, gain without inversion (GWI), and refractive index enhancement in the present scheme. The azimuthal modulation of coherence effects is attributed to the presence of optical angular momentum of the vortex beams. Under the influence of an additional travelling wave field with the presence of two vortex beams, the present model leads us to obtain an ultra-large enhancement of refractive index at the resonant detuning of the fields. This phenomenon makes the atom-field system to play the equivalent role of a high-refractive-index prism. The role of near dipole–dipole (NDD) interaction on the modulation of position-dependent coherence effects has also been investigated. It has been shown that, without any inclusion of the travelling wave field in the system, the phenomenon of resonant enhancement of refractive index may also occur in the presence of NDD effect. The new way of generating spatially controlled GWI and nonlinear refractive index enhancement is specific to the present model. This work seems to be useful for finding its applications in spatially modulated coherence controlled electromagnetically induced transparency-based quantum devices like quantum optical memory, switches, and quantum logic gates, where the refractive index switching phenomenon is a prerequisite.

Superlens-Assisted laser nanostructuring of Long Period optical fiber Gratings (LPGs) for enhanced refractive index sensing

We introduce an innovative approach to enhance refractive index sensing utilizing Long Period Optical Fiber Gratings (LPGs) processed through microsphere-assisted superlens laser nanostructuring technology. This method involves the self-assembly of a silica microsphere monolayer on the outer surface of LPGs, followed by pulsed laser irradiation to create nanoholes (with a diameter of 300–500 nm) on the surface, for forming the nanohole-structured LPGs (NS-LPGs). The sensing capabilities of NS-LPGs, featuring two different nanohole densities (5 % and 7.9 %), were experimentally evaluated in sucrose and glycerin solutions. The results revealed a notable improvement in sensitivity, with increases of 16.08 % and 19.57 %, respectively, compared to conventional LPGs. This suggests that higher nanohole density contributes to greater enhancement in refractive index sensing. Furthermore, the permanent nanohole structures showed improved durability and lifespan in various environments compared to surface-coating-based LPGs. Further enhancements can be achieved by refining the nanostructuring density and controlling the dimensions of the nanoholes.

Enhancing surface sensing performance of cascaded high contrast gratings using bound states in the continuum.

We proposed the cascaded high contrast grating (CHCG) structure to enhance surface sensing capabilities through bound states in the continuum (BICs). Utilizing the finite element method (FEM) and rigorous coupled-wave analysis (RCWA), we studied the dispersion relations, far-field contribution CHCGs, and near-field distributions of BICs corresponding to resonance peaks at different wavelengths. Results demonstrate the ability to precisely control symmetry-protected BIC (SP-BIC) and Friedrich-Wintgen BIC (FW-BIC) resonance peaks by altering incident angles and structural parameters, enhancing structure robustness and tunability. Significantly, modes 1 and 2 have demonstrated substantial enhancement in surface refractive index sensing, achieving highest sensitivities at 51 nm/RIU and the figure of merit reaching 490.8 RIU-1, indicating notable advancement in detecting subtle surface changes. In contrast, mode 3 has shown robust performance in bulk refractive index sensing, achieving a sensitivity of 602 nm/RIU and a figure of merit of 5189.65 RIU-1. These findings underscore the significant potential of the structure as a high-performance integrated sensor, particularly for precise environmental and biological monitoring in surface refractive index sensing.

Effects of Mn doping on the structural, linear and nonlinear optical properties of Gd2O3 nanoparticles

We report on the effects of Mn doping on the structural, linear, and nonlinear optical properties of Gd2-xMnxO3 nanoparticles (x = 0.00, 0.03, 0.05, and 0.10). The Gd2-xMnxO3 powders were prepared using sol-gel process. The x-ray diffraction technique shows that the Mn3+ doping results in a small decrease in lattice parameter and an increase in crystallite size. UV–visible measurements were carried out in the wavelength range of 200–800 nm. The direct optical band gap estimated using Tauc model decreases with increasing Mn content from 5.152 to 5.008 eV, while Urbach energy increases from 0.753 to 1.012 eV. Moreover, the insertion of Mn results in a significant enhancement in the optical parameters (refractive index, extinction coefficient, absorption coefficient, dielectric constants (ε', ε"), optical and electrical conductivity. The dispersion energy (Ed) increases from 24.13 to 30.1 eV, while the single oscillator energy (E0) declines lightly from 10.28 to 9.998 eV with the increase of Mn content. With Mn doping an enhancement in the static linear refractive index (n0) and high-frequency dielectric constant (ε∞) were also observed. Analysis of linear and nonlinear optical parameters shows that the presence of manganese enhanced the linear optical susceptibility (χ(1)), third-order non-linear susceptibility (χ(3)), and non-linear refractive index (n2). These optical results show the potential applications of Gd2-xMnxO3 nanoparticles (x = 0.00, 0.03, 0.05 and 0.10) in various optical devices.

Nonlinear refractive index enhancement of Nd3+ ions loaded borate glasses in the near–infrared region by silver nanoparticles

The study presents an evaluation and in–depth discussion of the efficacy of silver (Ag) nanoparticles (NPs) concentration on the third–order nonlinear optical (NLO) proeprties of borate glasses. The optical absorption data revealed Nd3+ ions and Ag NPs to be present in the studied glasses. Measurements carried out on the tested glass through transmission electron microscopy revealed spherical–shaped NPs with a median size of ∼16.53 nm (analyzed using the ImageJ software). Open and closed aperture Z–scan configurations were employed to ascertain the attributes of NLO absorption and refraction, respectively. Open–aperture Z–scan data revelaed two–photon absorption nonlinearity in the studied glasses, while the closed aperture Z–scan data revealed self–focusing nonlinear refraction. With the increase in silver NPs concentration, the nonlinear refraction coefficient was significantly increased compared to the nonlinear absorption parameter. The results instantiate large concentrations of Ag NPs embedded in borate glasses bearing Nd3+ ions are suitable for all–optical switching applications that excel in their functioning in near–infrared spectral range.

Study on PID performance degradation based on passivation materials such as alumina/silicon nitride SiN crystalline silicon solar cells

Solar energy is a pure and reproducible energy. China has paid more and more consideration to the investigation and employment of solar energy. The investigation focuses on the phenomenon of PID capability degradation of inactivation mediums for instance alumina/silicon nitride in transparent silicon high-efficiency solar cells. Through the laboratorial investigation on the effect of individual in-activation membrane processes on the PID damping behavior, it is found that the deposition approach of silicon oxide and silicon nitride inactivation membranes on the surface of transparent silicon cells directly affects the PID damping. Excellent anti-PID capability; single-layer silicon oxide membrane with the same thickness has better anti-PID capability than silicon nitride membrane. Double-layer silicon oxide/silicon nitride superimposed membrane with the enhancement of refractive index, the anti-PID damping capability gradually enhancements, and better anti-PID capability than monolayer silicon nitride. Investigation on the PID capability damping of inactivation mediums is of great significance to further the capability of solar cells and help to further the effective employment of solar energy.

Ultrasensitive Glucose Biosensor Using Micro-Nano Interface of Tilted Fiber Grating Coupled With Biofunctionalized Au Nanoparticles

Monitoring glucose levels is one of the main diagnostic methods for preventing and treating diabetes. Conventionally monitoring of blood glucose through a finger-stick procedure brings pain. It is essential to develop biosensors for low concentration glucose detection in non-invasive samples. In this paper, a tilted fiber grating based localized surface plasmon resonance (LSPR) biosensor for ultrasensitive glucose detection has been demonstrated. The glucose biosensor was fabricated by immobilization of Au nanoparticles (AuNPs) onto an excessively tilted fiber grating (Ex-TFG) and subsequent biofunctionalization by coating polydopamine and concanavalin A onto the AuNPs/Ex-TFG surface. The biosensor has the response regions covering low, middle and high glucose concentrations. The resonance peak indicates a redshift of 0.24 nm, 0.06 nm and 0.48 nm in the concentration ranges of 1.0 nM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 1.0\,\, \boldsymbol {\mu }\text{M}$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.0\,\, \boldsymbol {\mu }\text{M}- 50.0\,\, \boldsymbol {\mu }\text{M}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$50.0\,\, \boldsymbol {\mu }\text{M}-5.0$ </tex-math></inline-formula> mM, respectively. The limit of detection (LOD) of the biosensor in pure phosphate buffered saline is 2.5 nM. The high sensitivity and specific selectivity for D-glucose detection are attributed to strong coupling between the evanescent fields from transverse magnetic (TM) cladding modes of the Ex-TFG and the localized electric fields from LSPR of the AuNPs and their biofunctionalization, and resulting in remarkable enhancement of localized effective refractive index with slightly increased D-glucose concentration. Especially, the glucose biosensor based on the Ex-TFG coupled with the biofunctionalized AuNPs, has been demonstrated to be of practical utility to detect the glucose in artificial urine, which has potential applications in non-invasive glucose monitoring for early diagnosis and better management of diabetes.

Highly sensitive metal ion sensing by graphene oxide functionalized micro-tapered long-period fiber grating.

An accurate as well as highly sensitive label-free chemical sensing platform for the detection of various metal ions was demonstrated. The chemical sensor was derived from the micro-tapered long-period fiber grating (MLPG) by depositing graphene oxide (GO) by chemical-bonding and optical-tweezer effects. The enhancement in refractive index (RI) sensitivity as well as reusability was obtained by evaluating the deposition thickness in the range of approximately 97.7 to 158.9 nm. Based on the analysis of adsorption principles, the enhanced RI sensitivity leads to a limit of detection as low as 3.2 ppb. The highest sensitivities for the cases studied using sodium and manganese ions in a wide concentration range of 1 ppb to 1 × 106 ppb are respectively 2.2 × 10-3 dB per ppb and 3.2 × 10-3 dB per ppb. Mixture samples were also studied to evaluate the properties of sensing the doped ions. This demonstration of GO modified MLPG is bound to find potential applications that require sensing of mixed samples and illustrates significant importance in developing cost-effective, label-free, reusable, and real-time chemical sensors.

Femtosecond nearly resonant self-focusing in gold nanorod colloids.

We evaluate the threshold power for self-focusing in gold nanorod colloids of varying concentration by a power limiting method in the femtosecond filamentation regime. The pulses are tuned near the longitudinal plasmon peak of the nanorods, leading to saturation of linear absorption and reshaping of the particles. We evaluated the last two effects by optical transmission measurements and spectroscopic analysis and estimated that considerable particle deformation does not occur before the collapse of the beam. We performed numerical simulations based on the experimental results, and evaluated only a subtle, monotonically increasing enhancement of the nonlinear refractive index of the host material (water) as the nanoparticles concentration increases. The role of higher-order contributions is discussed. Our work provides an alternative characterization approach of ultrafast nonlinearities in absorbing media. It further emphasizes that self-focusing of intense femtosecond pulses in gold nanocomposites is hampered by the ultrafast modulation of the susceptibility of the metal.

Low-order optical nonlinearities of PbS quantum dot liquids and films

We demonstrate the variations of the nonlinear optical parameters in the suspensions and thin films containing PbS quantum dots of different sizes (9 and 2.5 nm). We show that these colloidal quantum dots possess the thermal-lens induced negative nonlinear refraction and reverse saturable absorption in the case of the 532-nm, 10-ns probe pulses. The larger nonlinear absorption coefficients of 9-nm quantum dots compared with 2.5-nm species (1 × 10−7 and 8 × 10−9 cm W−1, respectively) in the case of 1064 nm, 10 ns probe pulses were determined. Similar relations between the nonlinear optical parameters of 9- and 2.5-nm species were maintained in the case of twice shorter wavelength (532 nm) of the probe pulses. The almost ten-fold growth of the nonlinear optical parameters in the visible range compared with the near infrared region was attributed to the stronger influence of the excitonic states on the propagation effects in the former case. We show that the thin films containing these quantum dots demonstrate saturable absorption, while the suspensions show mostly the reverse saturable absorption at 532 nm. A significant enhancement of the nonlinear refractive index of films compared with suspensions (−2 × 10−8 cm2 W−1 and -1 × 10−11 cm2 W−1), as well as a large negative nonlinear absorption coefficient (−3.2 × 10−5 cm W−1) in the former case are demonstrated. We discuss the signs of nonlinear refraction and nonlinear absorption of the quantum dots, as well as the role of the sizes of these species in variations of the nonlinear parameters.

Enhancing the Refractive Index of Polymers with a Plant‐Based Pigment

AbstractPolymers are essential components of many nanostructured materials. However, the refractive indices of common polymers fall in a relatively narrow range between 1.4 and 1.6. Here, it is demonstrated that loading commercially‐available polymers with large concentrations of a plant‐based pigment can effectively enhance their refractive index. For polystyrene (PS) loaded with 67 w/w% β‐carotene (BC), a peak value of 2.2 near the absorption edge at 531 nm is achieved, while maintaining values above 1.75 across longer wavelengths of the visible spectrum. Despite high pigment loadings, this blend maintains the thermoforming ability of PS, and BC remains molecularly dispersed. Similar results are demonstrated for the plant‐derived polymer ethyl cellulose (EC). Since the refractive index enhancement is intimately connected to the introduction of strong absorption, it is best suited to applications where light travels short distances through the material, such as reflectors and nanophotonic systems. Enhanced reflectance from films is experimentally demonstrated, as large as sevenfold for EC at selected wavelengths. Theoretical calculations highlight that this simple strategy can significantly increase light scattering by nanoparticles and enhance the performance of Bragg reflectors.

Enhancement of refractive index sensing for an infrared plasmonic metamaterial absorber with a nanogap.

An infrared plasmonic metamaterial absorber with a nanogap was numerically and experimentally investigated as a refractive index sensor. We experimentally demonstrated large enhancements of both sensitivity (approximately 1091 nm/refractive index unit) and figure of merit (FOM*; approximately 273) owing to the nanogap formation in the metamaterial absorber to achieve perfect absorption (99%). The refractive index sensing platform was fabricated by producible nanoimprint lithography and isotropic dry etching processes to have a large area and low cost while providing a practical solution for high-performance plasmonic biosensors.

Enhanced Optical Sensitivity of Polyvinyl Alcohol–Reduced Graphene Oxide Electrospun Nanofiber Coated Etched Fiber Bragg Grating Sensor for Detection of Myoglobin a Cardiac Biomarker

Etched fiber Bragg grating (eFBG) sensors have shown to be highly sensitive with the capability of giving accurate real‐time response to a variety of measurands such as pressure, gas, biomolecules, to name a few. These sensors have not shown their mettle as competitive products mainly due to nonreproducibility in results and inefficiency in upscaling for large‐scale production; the main reason being nonuniform and complicated coating procedures. Herein, the enhancement in refractive index (RI) sensitivity (≈4 times) obtained with electrospinning of polyvinyl alcohol–reduced graphene oxide (PVA–rGO) nanofibers onto eFBG sensor using a customized target and a unique sandwich arrangement is demonstrated. The enhancement in RI sensitivity has led to a lower detection limit and increased sensitivity and linear range for a case study using myoglobin (Mb), an early‐stage cardiac biomarker with high reproducible results (standard error ≤±2.3%). rGO embedded PVA nanofiber electrospun onto an eFBG sensor (PVA–rGO sensor) is the first of its kind and has significant importance in developing cost‐effective, label‐free, multianalyte, portable, real‐time, point‐of‐care (POC) kits at ambient conditions.

Gain without inversion and enhancement of refractive index via intervalley quantum coherence transfer in hybrid WS2 -metallic nanoantenna systems

We theoretically study formation of gain without inversion and resonant enhancement of refractive index with suppressed absorption in a system consisting of a monolayer of transition metal dichalcogenide (${\mathrm{WS}}_{2}$) and a metallic nanoantenna. We show that these processes happen when the hybrid system interacts with a right-circularly polarized laser beam, allowing quantum coherence transfer to happen from the K to ${\mathrm{K}}^{\ensuremath{'}}$ valley. Our results show that the quantum coherence transfer can convert the ${\mathrm{K}}^{\ensuremath{'}}$ valley into a conjugated system wherein the real and imaginary parts of its polarization become similar to the imaginary and real parts of the K valley polarization, respectively. This allows the maximum refractive index in the ${\mathrm{K}}^{\ensuremath{'}}$ valley to occur as the absorption becomes zero. We characterize the conjugated valley system by inspecting the evolution of its Bloch vector as the incident laser frequency varies. The theoretical approach includes density matrix formalism while considering a pure dephasing rate that represents the fast decay of exciton valley polarization in transition metal dichalcogenide monolayers.

Large optical nonlinearity enhancement under electronic strong coupling

Nonlinear optical responses provide a powerful way to understand the microscopic interactions between laser fields and matter. They are critical for plenty of applications, such as in lasers, integrated photonic circuits, biosensing and medical tools. However, most materials exhibit weak optical nonlinearities or long response times when they interact with intense optical fields. Here, we strongly couple the exciton of cyanine dye J-aggregates to an optical mode of a Fabry-Perot (FP) cavity, and achieve an enhancement of the complex nonlinear refractive index by two orders of magnitude compared with that of the uncoupled condition. Moreover, the coupled system shows an ultrafast response of ~120 fs that we extract from optical cross-correlation measurements. The ultrafast and large enhancement of the optical nonlinar coefficients in this work paves the way for exploring strong coupling effects on various third-order nonlinear optical phenomena and for technological applications.

Distortion-free conductivity-dependent temporal cloak based on tunnelling chiral medium

Complex conductivity-dependent birefringent undistorted pulses propagation and development of time cloak are investigated. The left and right circularly polarized (LCP/RCP) beams refractive indices, absorptions, dispersion and intensity distribution are controlled and modified with complex conductivity and tunnelling effect in a chiral medium. Sixteen types of waves and corresponding absorption coefficients and refractive indices are investigated. The corresponding dispersions of LCP/RCP beams fluctuated with complex conductivity and tunnelling frequency significantly modified the time cloak. The manipulation of dispersion with significant enhancement of refractive index has direct relation with time gap or hole for temporal cloaking devices. A time gap of $$8\,\mu s-10\,\mu s$$ has been calculated between the LCP/RCP beams with forward current. Furthermore, the time gap of $$5 \mu s$$ is reported in each LCP/RCP beams with backward current. The pulses propagation in conductive medium is the superposition of sixteen types of waves in the medium and any external event $$E_E (t)$$ when interacting with the transmitting pulse $$E^{R,L}(\sigma ,t)$$ in this time hole interval, the result is $$E_E(t)E^{R,L}(\sigma ,t)=0$$ and the information will be undetectable to the outside hackers. These theoretical results may be more helpful to the sensitivity of information detection and noise-free secure communications.

Influence of bismuth nitrate doping towards the characteristics of L-Alanine nonlinear optical crystals

In the present study, an amino acid derivatives, L-alanine (LA) and Bismuth nitrate-doped L-alanine (BNLA) crystals were grown by the slow evaporation technique. The crystal structure and quality of the grown crystal were examined by single-crystal X-ray diffraction (XRD) and powder XRD. The X-ray diffraction analysis of LA and BNLA crystals confirmed the orthorhombic crystal system without any internal structural grain boundaries. The optical properties of the LA and BNLA crystals have been determined using UV-Visible spectroscopy and Photoluminescence (PL). The second harmonic generation (SHG) efficiency of the crystal was measured using Kurtz powder technique. The dielectric constant measurement was performed at room temperature for different frequencies. Photoconductivity tests show that the LA and BNLA crystals have negative photoconductivity value. On testing of the retentivity or remanent magnetization, the BNLA crystal has increased values as compared to the pure LA crystals, in addition to the observation of changes in coercivity. Further, the observation of maximum intensity peak for the BNLA crystal at 346.9 nm pointed out that the crystal has blue fluorescence emission. The third-order nonlinear optical properties of the grown crystal were studied using Z-scan technique. The enhancement in nonlinear refractive index and third-order NLO susceptibility value shows that the grown BNLA crystals may be of good quality and less in defect than that of pure LA crystals.

Enhancement of the volume refractive index sensing by ROTE and its application on cancer and normal cells discrimination

A volume refractive index (RI) sensor based on the resonant optical tunneling effect (ROTE) is presented in this study. Compared with the classic volume RI sensing structure, the Fabry-Pérot (FP) etalon, the ROTE structure has a higher figure of merit (FOM) than that of the FP cavity with the same cavity length. The sensitivity of the ROTE structure is almost 20 times as high as that of the FP structure, although the Q value of the ROTE structure is lower than that of the FP etalon. Given the excellent volume RI sensing performance of the ROTE structure, three groups of normal human cells and corresponding cancer cells were tested for discrimination in this paper. According to experimental results, the reflection resonant dips of each type of human cancer cell showed a red shift with respect to corresponding normal cells at the same density. The RI sensor based on the ROTE has broad application prospects in biomedical, drug screening, and early cancer diagnoses.

Recent Advances in Plasmonic Perovskite Solar Cells.

Perovskite solar cells (PSCs) have emerged recently as promising candidates for next generation photovoltaics and have reached power conversion efficiencies of 25.2%. Among the various methods to advance solar cell technologies, the implementation of nanoparticles with plasmonic effects is an alternative way for photon and charge carrier management. Surface plasmons at the interfaces or surfaces of sophisticated metal nanostructures are able to interact with electromagnetic radiation. The properties of surface plasmons can be tuned specifically by controlling the shape, size, and dielectric environment of the metal nanostructures. Thus, incorporating metallic nanostructures in solar cells is reported as a possible strategy to explore the enhancement of energy conversion efficiency mainly in semi‐transparent solar cells. One particularly interesting option is PSCs with plasmonic structures enable thinner photovoltaic absorber layers without compromising their thickness while maintaining a high light harvest. In this Review, the effects of plasmonic nanostructures in electron transport material, perovskite absorbers, the hole transport material, as well as enhancement of effective refractive index of the medium and the resulting solar cell performance are presented. Aside from providing general considerations and a review of plasmonic nanostructures, the current efforts to introduce these plasmonic structures into semi‐transparent solar cells are outlined.