Refractive Index Of Surrounding Medium Research Articles

High FoM quad‐band THz perfect metamaterial absorber‐based RI sensor for gas sensing applications

AbstractHere, a novel design of a microstructured quad‐band terahertz metamaterial absorber is presented consisting of a copper‐based merged circular and square split ring resonator‐integrated over a lead glass substrate intended to detect changes in the surrounding medium's refractive index for gas sensing applications through THz spectroscopy. Outstanding performance is shown by the suggested absorber, which achieves a maximum absorption rate of 99.4%, 98.7%, 99.2%, and 99.4%, at resonance frequencies of 8.055 (), 8.423 (), 9.257 (), and 9.507 () THz, respectively. The quality factors (Q‐factors) attained are 201, 129, 298, and 271 for these four absorption bands. The obtained sensitivity values at these four distinct bands are 3898, 2927, 4719, and 4102 GHz per refractive index unit with high figure of merits (FoM) of 97, 95, 152, and 117, respectively, throughout a range of refractive indices from n = 1.00 to n = 1.05 (with a step size of 0.005). The study's novelty lies in its novel design and remarkable ability to excel in gas sensing with high sensitivity and high FoM across not just one but four distinct THz bands. Furthermore, each band demonstrates significantly superior performance in sensitivity, Q‐factor, and FoM, among other parameters compared to prior research in both gas detection and RI monitoring scenarios, hence exhibiting its potential across a wide range of applications.

Multi-Bound States in the Continuum and Lineshape Tailoring of the Multi-Layers Dielectric Metasurface

Bound states in the continuum (BICs) with high quality factors (Q-factors) by confining resonances embedded in a continuous spectrum by eliminating radiation loss can boost substantially light-matter interaction for various applications in ultrathin, active, and nonlinear metadevices. Regulating and improving the modulating freedom of the BICs are important in tailoring the spectra lineshape. Here, we suggest a way to manipulate the number of BICs in the spectra by controlling the layers and the shape or arrangement of the rectangular nanoholes in each layer of the metasurface, illuminated by a p-polarized plane wave. It is convenient to achieve symmetry protected multiple at-Γ BICs and multiple off-Γ BICs associated with the decoupling of the radiation waves and improve their regulation depth only by adjusting the layers and one single nanohole arrangement and shape in one lattice in each layer, without introducing extra geometric complexity of the metasurface. Additionally, we demonstrate the multiple BICs are robust and have a good tolerance to fabrication imperfections. Beyond the multiple BICs of both types, we also explore the huge sensitivity around the multi-BICs of the metasurface on the surrounding medium refractive index. Multiple BICs with high Q-factor realized in this multi-layer metasurface promise various practical applications including ultra-high sensitivity sensors and filters.

Au nanoparticles and reduced graphene oxide based plasmonic photodetector with enhanced performance in visible spectral region

This work reports on a photodetector (PD) based on SiO2 substrate, Au nanoparticles (NPs) and reduced graphene oxide (rGO) thin layer with plasmonic enhancement of light absorption in visible spectral region. Finite difference time domain (FDTD) methodology is used for simulation. The results show that a surrounding medium with higher refractive index (RI) leads to greater absorption with plasmonic enhancement resulting in higher magnitudes of quantum efficiency (η) and responsivity (ρ). The proposed PD design operating at 654.52 nm provides the values of η and ρ as large as 0.527 and 0.278 A/W, respectively. Further, a possible practical implementation of the proposed PD design is reported while analyzing it with Au electrodes and rGO acting as a conducting layer. This analysis comprises of dark current (Idark) estimation followed by the evaluation of detectivity (D) and detectable power (Pd). SiO2-rGO (6 nm)-Au NPs (6 nm radius) structure (with surrounding medium RI = 1.33) further achieves the values of Idark, D, and Pd as 6.33 × 10−14 A, 1.36 × 1011 Jones, and 2.28 × 10−13 W, respectively. Further, the proposed PD design is able to provide superior performance (i.e., small Idark along with large values of D and ρ) compared to recently-reported (2020-23) PD designs in visible range.

Temperature-insensitive label-free SARS-CoV-2 spike protein detection based on complementary refractive index and temperature dependence of multi-mode interference and grating resonance

We demonstrated temperature-insensitive, label-free detection of SARS-CoV-2 spike protein (SSP) by harnessing the complementary refractive index and temperature dependence of multi-mode interference (MMI) created by a no-core fiber (NCF) and phase-matched resonance generated by a long-period fiber grating (LPFG). To combine MMI and grating resonance, primarily sensitive to the surrounding medium refractive index (SMRI) and ambient temperature, respectively, a fiber-optic transducer was fabricated by splicing an NCF segment with an LPFG inscribed on double-clad fiber. The transducer was functionalized with human ACE2 receptors to selectively capture SSP. The functionalized sensor head exhibited high SSP selectivity, with overall average wavelength displacements of ∼253.33 and ∼160.00 pm in PBS and saliva, respectively, for SSP with concentrations ranging from 1 to 104 ng/mL. These spectral shifts are associated with localized SMRI modulations on the sensor surface induced by specific binding between SSP and ACE2. We also examined the cross-reactivity of the sensor head for MERS-CoV spike protein to confirm its SSP specificity. Moreover, we proved the capability of temperature-independent SSP detection and ambient temperature measurement by scrutinizing the temperature effect on the sensor performance. Our functionalized fiber transducer showed great promise as a temperature-insensitive and portable platform for rapid SSP detection.

Tunable Sensitivity in Long Period Fiber Gratings During Mode Transition With Low Refractive Index Intermediate Layer

Double-clad fibers where the second cladding has a lower refractive index than the first cladding, prove to be ideal structures for potentiating and tuning the sensitivity in long-period fiber gratings (LPFGs) operating in mode transition. When a thin film is deposited on the optical fiber, the second cladding performs acts as a barrier that initially prevents the transition to guidance in the thin film of one of the modes guided in the first cladding. Finally, the transition to guidance occurs with a sensitivity increase, in analogy to the tunnel effect observed in semiconductors. This improvement has been demonstrated both as a function of the thin film thickness and the surrounding medium refractive index, with enhancement factors of 4 and 2, respectively. This idea reinforces the performance of LPFGs, adding a new degree of freedom to the mode transition and the dispersion turning point phenomena. Moreover, the control of the variation of the effective index of cladding modes could be applied in other structures, such as tilted-fiber gratings or evanescent wave sensors.

Improving the Width of Lossy Mode Resonances (LMRs) in Double-Clad Fibers

In this work, the characteristics of lossy mode resonances (LMRs) in double-clad fibers where the refractive index (RI) of the second cladding is lower than that of the first cladding are analyzed both numerically and experimentally. In the first place, the LMRs spectra obtained with a 75 nm TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin film are simulated, and it is observed that a thicker second cladding improves the width of the resonances, making them narrower. Then, two experimental cases (no second cladding, and second cladding with thickness of 1.13 µm) are assessed, showing a good agreement with the previous simulations. Finally, an experimental refractometric study is carried out in liquids (surrounding medium refractive index in the 1.34 - 1.40 range) for both fibers, calculating the full width at 1dB (FW <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1dB</sub> ), the sensitivity, and the figure of merit (FOM). The FW <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1dB</sub> is better for the LMR obtained on the fiber with second cladding while the sensitivity is slightly greater for the fiber without second cladding. In the case of the FOM, it is higher for the double-clad fiber as the narrowing of the resonances outweighs the lower sensitivity. These results show that the performance of LMR-based optical fiber sensors can be improved by employing double-clad fibers.

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.

Design and Analysis of Highly Sensitive LSPR-Based Metal–Insulator–Metal Nano-Discs as a Biosensor for Fast Detection of SARS-CoV-2

For over 2 years, the coronavirus has been the most urgent challenge to humanity, and the development of rapid and accurate detection methods is crucial to control these viruses. Here, a 3D FDTD simulation of Au/SiO2/Au metal–insulator–metal (MIM) nanostructures as a biosensor was performed. The strong coupling between the two plasmonic interfaces in the Au/SiO2/Au cavity helped us to obtain relatively higher sensitivity. The attachment of SARS-CoV-2 changed the refractive index, which was used to detect SARS-CoV-2. Due to the higher overlapping of plasmonic mode with the environment of nano-discs, a higher sensitivity of 312.8 nm/RIU was obtained. The peak wavelength of the proposed structure shifted by approximately 47 nm when the surrounding medium refractive index changed from 1.35 (no binding) to 1.5 (full binding). Consequently, the SPR peak intensity variation can be used as another sensing mechanism to detect SARS-CoV-2. Finally, the previously reported refractive index changes for various concentrations of the SARS-CoV-2 S-glycoprotein solution were used to evaluate the performance of the designed biosensor.

Study of plasmonic properties of copper monosulfide nanoparticles depending on their dielectric constant

The object of research is plasmonic properties copper of monosulfide nanoparticles. One of the most problematic areas is that there is still no unambiguous answer to which main copper monosulfide nanoparticles parameters have a decisive effect on their resonance absorption, scattering or electric field enhancement. It is necessary to study the plasmonic properties of copper monosulfide nanoparticles depending on their main parameter, namely the dielectric constant. The principle of dipole equivalence and Mee-Gans theory for the modeling of the optical nanoparticle characteristics is used. It is found that dielectric constant is a crucial parameter determining the resulting optical response of such nanoparticles. The surrounding medium refractive index affects the position and magnitude of the nanoparticles maximum plasmonic absorption. The nonspherical nanoparticles are characterized by two plasmon peaks corresponding to transverse and longitudinal localized surface plasmon resonance if the ratio between the axes is higher than 1.5. The ellipsoidal nanoparticles exhibit higher sensitivity to changes in the refractive index of the surrounding medium in comparison to the spherical ones. The obtained research results are primarily the basis for further comprehensive research of plasmonic copper monosulfide nanoparticles for their specialized applications. Second, knowledge of the influence of the nanoparticle dielectric constant on their resulting spectral characteristics allow tuning of the localized surface plasmon resonance peak position in a wide wavelength range, from 500 to 1200 nm, using the nanoparticle synthesis technique. Thus, the material under study is promising for sensor applications in a wide spectral range.

Lossy mode resonance fiber-optic sensors based on niobium pentoxide thin film

In this work, we fabricated and investigated lossy mode resonance (LMR) based fiber-optic refractometers, using a niobium pentoxide coated optical fiber as a sensitive element. In order to do that, thin Nb2O5 films were deposited on the surface of chemically thinned optical fibers by metalorganic chemical vapor deposition (MOCVD). The sensitivities of the first transverse electric (TE) and transverse magnetic (TM) LMRs to the surrounding medium refractive index (SMRI) were measured and compared. Aqueous solutions of glucose and sodium chloride were used as test liquids. The sensor sensitivity to a change in the SMRI enhanced with an increase in the dissolved substance concentration and was greater for glucose solution. The maximum response of the 1-st TE and TM LMRs was 6580 and 6120 nm per refractive index unity (RIU), respectively.

Fiber optic Lossy Mode Resonance based sensor for aggressive liquids

A fiber-optic sensor for chemically aggressive environments, such as concentrated solutions of acids and alkalis, has been manufactured. The sensor is based on a single-mode optical fiber with a chemically etched cladding, coated with a thin film of tin dioxide (SnO2) by metalorganic chemical vapor deposition (MOCVD). In the transmission spectra of a SnO2 coated taper lossy mode resonances (LMRs) were observed, the maximum of which shifted to the long-wavelength part of the spectrum, with an increase in the thickness of the deposited film. At a deposition temperature of 345 °C, it was possible to detect the separation of the transverse electric (TE) and transverse magnetic (TM) LMRs for resonances up to the third order. From the change in the position of the 1-st TE LMR, the refractive index and the molar concentration of acids and alkalis in such aggressive liquids as aqueous solutions of HCl, HNO3, H2SO4, NaOH were determined, which, according to our information, is demonstrated for the first time. The high chemical durability of the coating was confirmed by its insusceptibility to aqua regia (AR). The sensitivity of the sensor to changes in the surrounding medium refractive index (SMRI) and the molar concentration of acids was more than 5260 nm/RIU and 32 nm/M, respectively.

CHARACTERIZATION OF LONG PERIOD GRATINGS MANUFACTURED WITH FIBER OPTIC FUSION SPLICER FOR SENSOR DEVELOPMENT

A Long Period Grating (LPG) is a structure inscribed in an optical fiber that is of great interest in the field of communications and sensors. At Centro de Investigaciones Ópticas (CIOp), an LPG manufacturing process was implemented. It is based on use of electric arcs produced with an optical fiber splicer to generate a series of physical disturbances that modify optical fiber behavior as a light transmission guide. Since transmission response of LPG is sensitive to fiber environment conditions, it can be used to detect changes in certain physical and chemical parameters. In particular, it enables the implementation of optical sensors sensitive to temperature, surrounding medium refractive index and axial deformation. Characterization of locally generated LPGs is essential for future design of sensors applicable in many engineering areas. In this article, typical LPG sensitivities to the three mentioned parameters are reported.

Simultaneous measurements of a specimen quantitative-phase signal and its surrounding medium refractive index using quantitative-phase imaging.

This Letter demonstrates a method to simultaneously measure the quantitative-phase signal (QPS) of the observed specimen and the refractive index of its surrounding medium (nm) in a time-resolved manner using a micro-structured coverslip. Such coverslips, easily integrated into perfused live-cell imaging chambers, allow to use various quantitative-phase imaging techniques to achieve this dual measurement. Since QPS is crucially dependent on nm, the measurement of the latter paves the way for its manipulation in a controlled manner leading to a QPS contrast modulation for appealing applications, including visualizing the interior of cells.

Facile Tuning and Refractive Index Sensitivity of Localized Surface Plasmon Resonance Inflection Points in Hollow Silver Nanoshells

Hollow metal nanostructures are fascinating because of their unique and surrounding environment-sensitive localized surface plasmon resonances (LSPRs). Herein, we have investigated the hollow silver nanoshell (HSN) sensitivity of different inner core radii and shell thicknesses with respect to change in surrounding medium refractive index. Also, we have shown that the extinction peak of HSNs can be fine-tuned in the visible region as well as biological windows by manipulating the size parameters. Moreover, an enhancement in refractive index sensitivity of 133 to 283 nm/RIU can be achieved in the biological windows by thick hollow silver nanoshells in comparison to thin ones. To achieve a comprehensive study, the sensitivity of homogeneous LSPR extinction inflection points with change in refractive index is also investigated. A consistent improvement in bulk refractive index sensitivity of 36% and 15% is predicted with thick and thin shell layers, respectively. Finally, thick HSNs showed higher refractive index sensitivity when compared with thin ones. Moreover, we envision the idea of more than one molecule detection by tracking the curvature changes. Therefore, we provide a deeper insight into the refractive index sensitivity of homogeneous LSPR inflection points for their use in LSPR-based sensors.

Highly sensitive plasmonic biosensor based on ring shape nanoparticles for the detection of ethanol and D-glucose concentration

Today bioelectronics needs more sensitive sensors in their development. The main aim of this work is to design a highly sensitive optical sensor for the quantitative detection of ethanol and D-glucose through particular localized surface plasmon resonances (LSPRs) depending on shape and type of ring shape core@shell nanoparticles. The simulated plasmoniccore@shell nanoparticles consist of Ag or Au shell and air, Ag, or Au cores. The obtained results show the plasmonic resonance wavelength of the air@Ag nanoparticles strongly depends on their inner radiuses and using an air-core improves their resolution. Also, it is investigated that ring shape air@Agcore@shell nanoparticles are more sensitive to surrounding medium refractive index variation. It is showed that ~1.26 nm to ~6.27 nm and ~0.9 nm to ~2.6 nm in scattering plasmonic peak shifts are obtained corresponding to 1 g/dl to 10 g/dl D-glucose and 1 ml/dl to 10 ml/dl ethanol concentration changes. The sensitivity of the air@Ag nanostructure is 890.84 nm/RIU for D-glucose concentration changes from 0 to 1 g/dl. This higher sensitivity with the full width at half-maximum of 61.3 nm yields the figure of merit of 14.53. The findings of this work provide a novel perspective in the field of the optical sensors and can be applied to detect any other components.

Lossy mode resonance sensors based on nanocoated multimode-coreless-multimode fibre

In this work it is proved the ability to obtain lossy mode resonances (LMRs) in the transmission spectrum with multimode-coreless-multimode fibre optic structure coated with tin oxide on the coreless segment. The devices were characterized as a function of the surrounding medium refractive index and sensitivities of 7346.93 nm/RIU and 708.57 nm/RIU were attained for the first and the second LMR respectively. As an application proof of this technology, one of the devices was biofunctionalized and used for detecting goat anti-mouse IgG in concentrations ranging from 1 to 40 mg/L, with a limit of detection of 0.6 mg/L. This proves the ability of this simple structure to be used for biological, chemical or environmental applications.

Highly Sensitive Dual-Core PCF Based Plasmonic Refractive Index Sensor for Low Refractive Index Detection

A highly sensitive surface plasmon resonance (SPR) sensor on a dual-core photonic crystal fiber (PCF) for low refractive index (RI) detection is presented in this paper. Plasmonic material silver (Ag) is deposited outside of the fiber structure to detect changes of the surrounding medium's refractive index. To prevent oxidation a thin layer of titanium dioxide (TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) is employed on top of the silver. The sensor shows maximum wavelength sensitivity and amplitude sensitivity of 116,000 nm/RIU and 2452 RIU <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> with corresponding resolutions (R) of 8.62 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> and 5.55 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> RIU, respectively. A thorough study of the relevant literature yielded that these attained sensitivities in both interrogation methods are the highest among reported PCF-SPR sensors to date. In addition, the sensor possesses a very high figure of merit of 2320 in the sensing range of 1.29 to 1.39. Therefore, it would be a suitable candidate for pharmaceutical inspection, organic chemical sensing, and biosensing and other analytes detection.

Highly Sensitive Terahertz Metamaterial Sensor

A novel design of nearly perfect metamaterial absorber is proposed and analyzed for terahertz sensing applications. The full vectorial finite element method is used to simulate and analyze the reported design. The suggested structure is based on increasing the confinement of both electric and magnetic fields simultaneously at the resonance frequency. Therefore, an absorptivity of 0.99 is achieved at 2.249 THz with a narrow resonant peak and a $Q$ -factor of 22.05. The resonance frequency is sensitive to the surrounding medium refractive index at fixed analyte thickness. Consequently, the reported metamaterial design can be used as a refractive index (RI) sensor with the high sensitivity of 300 GHz/RIU and the figure of merit (FoM) of 2.94 through an RI range from 1.0 to 1.39 at the analyte thickness of $1.0~ \mu \text{m}$ . Furthermore, the proposed sensor has a sensitivity of 23.7 GHz/ $\mu \text{m}$ for the detection of the sensing layer thickness variation at the fixed analyte RI of 1.35. It is worth noting that most of the biomedical samples have a refractive index range from 1.3 to 1.39. Therefore, the reported sensor can be used for biomedical applications with high sensitivity.

Enhancement of Surface Plasmon Fiber Sensor Sensitivity Through the Grafting of Gold Nanoparticles

The optical fibers, coated with plasmonic active metal films, represent the simple and unpretentious sensors, potentially useful for measurements of physical or chemical quantities and wide range of analytical application. All fiber-based plasmonic sensors operate on the same physical principle based on changes in the position of the plasmon absorption peak induced by a variation of surrounding medium refractive index. However, the observed spectral differences are often weak, and thus an enhancement of sensor sensitivity is strongly required. In this paper, we propose the immobilization of gold nanoparticles with sharp edges on the thin gold layer, deposited on the multimode fiber surface for improvement of the sensor functionality. The morphological and compositional changes in the gold covered fiber surface were determined by using the atomic force microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy methods. As a result of gold nanoparticles immobilization, the pronounced plasmon energy concentration near the fiber surface occurred, thus enhancing the response of the proposed hybrid plasmonic system to the variation of ambient refractive index. The position of plasmon absorption in the case of the created plasmonic structure was shown to be more sensitive to the changes in the surrounding medium in comparison with the standard sensors based on the bare gold layer.

Bound states in the continuum in the double-period rectangular hole arrays perforated in one layer of photonic crystal slab in the visible wavelength region

Abstract We investigate optical bound states in the continuum (BICs) supported by a photonic crystal (PhC) slab penetrated with two periodic rectangular hole arrays embedded each other parallelly and vertically. It proves that the tunable off- Γ BICs can be found in the structure of both types, but the at- Γ BICs can only be obtained in the parallel structure. Quality factors (Q-factor) are obtained extremely high exceeding 105 for both types of structures. The tunable off- Γ BICs can be modulated by the structure parameters. Especially, the sensitivity of the resonant wavelength on the surrounding medium refractive index change is very large for the structure in both types, and tunable off- Γ BICs can be tuned by varying the environmental index. The results above accord well from the finite difference time domain method (FDTD) and the temporal coupled mode theory (CMT), which are beneficial to design practical resonance elements based on optical BICs in various new optical applications.