Ambient Refractive Index Research Articles

A three-band narrow-band terahertz perfect absorber for patch antennas and other sensors

We have studied a multi-mode resonance and actively tunable high-sensitivity terahertz perfect absorber using a microstructured Dirac semimetal. The absorber is composed of gold-dielectric-Dirac semimetal three-layer structure, which has good optical sensing characteristics. The absorber produces three perfect absorption modes (>95%) in the range of 2–6 THz, with a minimum bandwidth (FWHM) of 0.18 THz. We studied the adaptability of the device response mode to the actual complex electromagnetic environment by simulating the deflection incident angle. In addition, we also analyzed the influence of various parameter variables on the absorption performance, which provides a clearer idea and more possibilities for the wider application of the device. In terms of sensing detection, the absorber has a maximum refractive index sensitivity S of 610 GHz/RIU and a maximum figure of merit FOM of 3.4 in the ambient refractive index range of 1 to 1.8. Finally, we also realized the tunable absorption frequency of the absorber in 60–80 meV. We believe that based on the good optical properties of the absorber, the device has potential application value in communication technology, space exploration and so on.

Relative intensity noise management and thermal/shot noise control for high speed ultra high bandwidth fiber reach transmission performance

Abstract This work has clarified the relative intensity noise management and thermal/shot noise control for high speed ultra high bandwidth fiber reach transmission performance. Total link losses variations are clarified against ambient temperature and relative refractive index difference variations at 1,550 nm wavelength, 20 km fiber link length, and 45 % fluoride dopant ratio inside the fiber material cable. Besides the total link losses variations are demonstrated versus fiber link length and relative refractive index difference variations at 1,550 nm wavelength, room temperature, and 45 % fluoride dopant ratio inside the fiber material cable. Required optical signal per noise ratio (OSNR), electronic signal to noise ratio (SNR), and optimum gain variations are measured and analyzed versus the fiber link length and relative refractive index difference variations at 1,550 nm wavelength, room temperature, and 45 % fluoride dopant ratio inside the fiber material cable.

Tunable perfect meta-absorber with high sensitivity for refractive index sensing application

An infrared (IR) tunable perfect meta-absorber (TPM) is presented, which is configured of metal-insulator-metal (MIM) structure. The unit cell of the TPM device is composed of two opposite T-shaped and two rod-shaped Au resonators on the reflective mirror layer. The optimized feature size of TPM can achieve perfect absorption and the resonant wavelength can be tuned from blue-shift first and then red-shift by increasing the gap size between two T-shaped resonators continuously. The tuning process of red shift is linear. By changing the distance between two rod-shaped resonators, the TPM device exhibits red shift linearly. Moreover, TPM device shows polarization-dependent characteristic that the main absorption resonance can be attenuated from 100% to 0% by changing polarization angle from 0° to 90° of incident IR light. The resonance of TPM device has an excellent linear relationship with the ambient refractive index, and the sensitivity and maximum figure of merit (FOM) are calculated as 1249 nm/RIU and 21 RIU−1, respectively. These results prove that the design of the proposed TPM device is very suitable for the use of IR sensing, polarization switching, and refractive index sensing applications.

Hourglass-Shaped Fiber-Optic Mach-Zehnder interferometer for pressure sensing

A fiber-optic pressure sensor based on an hourglass-shaped structure (HSS) is proposed and demonstrated. A Mach-Zehnder interferometer (MZI) is constructed in the inside cavity of the HSS. The HSS consists of a silica frame and two photoresist diaphragms covered on side holes of the frame. Pressure response of the MZI is analyzed by theoretical calculation, the mechanical deformation of the HSS is simulated by using the finite element method, and the spectral characteristics of the sensor are simulated by using the finite-difference beam propagation method. Experimental results indicate that the pressure sensitivity of the sensor with a size of 0.2 × 0.2 × 0.5 mm3 reaches –1.27 nm/kPa with a linear response range of 0–35 kPa. In addition, repeatability, time stability, the cross-responses to ambient refractive index (RI) and temperature, and response time of the sensor are evaluated experimentally. With advantages of high sensitivity, compact size, and insensitivity to ambient RI, the proposed sensor shows promising potential in optofluidic and biomedical sensing fields.

Periodic stub implementation with plasmonic waveguide as a slow-wave coupled cavity for optical refractive index sensing

Optical biosensors based on plasmonic nanostructures have attracted great interest due to their ability to detect small refractive index changes with high sensitivity. In this work, a novel plasmonic coupled cavity waveguide is proposed for refractive index sensing applications. The structure consists of a metal–insulator–metal waveguide side coupled to an array of asymmetric H-shape element, designed to provide dual-band resonances. The sharp transmission dips and large field enhancements associated with dual-band resonances can enable sensitive detection of material under test. The resonator array creates a slow light effect to improve light-matter interactions. The structure was simulated using the finite integration technique as the full-wave technique, and the sensitivity and figure of merit were extracted for different ambient refractive indices. The maximum sensitivity of 1774 nm/RIU and high figure of merit of 2 × 104 RIU−1 for the basic model and 1.15 × 105 RIU−1 for the modified model were achieved, demonstrating the potential for high-performance sensing. The unique transmission characteristics also allow for combined spectral shaping and detection over a broad bandwidth. The simple, compact geometry makes the design suitable for on-chip integration. This work demonstrates a promising refractive index sensor based on coupled dual-band resonators in a plasmonic waveguide.

Specific detection of CA242 by antibody-modified chiral symmetric double “N” metasurface biosensor

In this work, a biosensor based on Fano resonance metasurface is proposed for the specific detection of CA242 which is a typical marker of pancreatic cancer. The biosensor consists of a chiral symmetric plasma double “N” structure, which utilises coherent coupling of bright and dark modes to generate Fano resonance, achieving suppression of radiation loss, concentrating and storing energy more efficiently in the structure, and contributing to increased sensitivity to changes in ambient refractive index, resulting in a sensitivity of the sensor of up to 842.8 nm /RIU. After a series of antibody functionalization modifications, the metasurface has become an immune biosensor that can specifically detect the tumor marker CA242 of pancreatic cancer. The detection of mixed and single antigen solutions with different concentrations has verified the high sensitivity, high specificity, and high linear relationship of the biosensor to CA242, and the detection limit is as low as 0.0692 ng/mL. It is superior to other common methods and breaks the traditional disadvantages of lower detection accuracy and greater damage in tumour detection methods. The detection of the wavelength shift of localized surface plasmon resonance in plasma metasurface has been successfully applied to the highly sensitive detection of tumor markers. This study demonstrates the sensitivity and maneuverability of the chiral symmetric double “N” plasmonic metasurface biosensor, suggesting the potential application of metamaterials in biosensing based on environmental refractive index changes.

All-dielectric metasurface with multiple Fano resonances supporting high-performance refractive index sensing

All-dielectric metasurface has important application value in many fields, especially in refractive index sensing. In this paper, an all-dielectric metasurface composed of a silicon triangular-hole nanodisk array is designed and studied numerically. Through introducing asymmetry in the proposed metasurface and transforming the bound states in the continuum (BIC) into the quasi-BIC, three sharp Fano resonances with almost 100% modulation depth can be achieved, and the highest quality factor (Q-factor) can reach 49,915. Further, the three Fano resonances can be regulated and controlled by adjusting the structural parameters. Moreover, the sensing performance of the structure is researched by changing the ambient refractive index. The maximum sensitivity obtained is 248 nm/RIU, and the best figure of merit is 3815RIU−1. The proposed structure provides a scheme for the design of optical devices, especially refractive index sensors.

Multi-tapered polarization-maintaining fiber-optic sensor for refractive index and temperature measurements

In this paper, a fiber-optic refractive index and temperature sensor based on Mach-Zehnder interferometer (MZI) is designed and fabricated. The sensor structure consists of a section of polarization-maintaining fiber (PMF) with multi-tapers and sandwiched by two sections of multi-mode fiber (MMF). Due to the changes of ambient refractive index and temperature, the effective refractive index of the core and cladding modes of the multi-tapered PMF will be changed, which will cause the interference spectral shift of core mode and cladding mode. The series experiments show that the structure has better repeatability, the maximum refractive index sensitivity is 250.861 nm/RIU, the maximum temperature sensitivity is −78 pm/°C. As the sensitivity matrix is used to realize the two-parameter distinguishing measurement, the resolutions of 1.2 × 10−5 RIU and 0.38 °C for refractive index and temperature are achieved. The structure has superior mechanical properties, and has broad application prospects in petrochemical industry, underwater exploration and other fields.

Design of self-coupled plasmonic hyperbolic metamaterials refractive index sensor based on intensity shift

Achieving efficient, accurate, label-free, and real-time biodetection is urgently required; hence, we propose a miniaturized, easily integrated, high-sensitivity plasmonic metamaterial light intensity refractive index sensor. The main structure of the sensor is layered hyperbolic metamaterial grating comprises eight pairs of Au/Al2O3 thin film, and the highly sensitive bulk plasmon polaritons can be effectively excited inside by the self-coupled effect without external prism or nanograting. The periodic fishnet arrays built in the layered HMM structure can not only be used as nanograting to achieve efficient coupling between incident light and layered HMM, but also increase the volume of the sensing, and the measured substance can get full interaction with the enhancement field to obtain high sensitivity. By detecting the change of reflected optical intensity with the ambient refractive index, the sensor exhibits intensity sensitivity of 36 RIU−1 (refractive index unit) and figure of merit of 403; moreover, the full width at half maximum of resonant peak is low at 5 nm. The sensing performances indicate that the sensor we designed has a significant potential to achieve portable, highly sensitive sensing platforms for precise detection.

A four-narrowband terahertz tunable absorber with perfect absorption and high sensitivity

Here, a simple four-band terahertz tunable narrowband absorber is proposed, which can achieve multimodal perfect absorption within 1–9 THz. We analyzed the physical mechanisms behind local surface plasmon resonance and impedance matching based on theoretical principles. By controlling the structural parameters, we use the eigen-mode and external field excitation to obtain the absorption peak and electromagnetic field distribution of the narrowband sensor under different structural parameters. Additionally, we studied the effect of the angle of incidence electromagnetic waves on the function of the absorber, found that the absorber has excellent characteristics in both low and high frequency states. We carry out a substance detection on four perfect absorption modes. The ambient refractive index sensitivity is 881.35 GHz/RIU, 439.65 GHz/RIU, 2664.46 GHz/RIU, 4501.19 GHz/RIU. The four-peak narrowband high-performance absorber designed by us has a huge development prospect in most important areas such as resource detection and biomedical diagnosis.

A multifrequency narrow-band perfect absorber based on graphene metamaterial

This article proposes a multifrequency narrow-band perfect absorber based on a single-layer graphene-patterned metamaterial, its absorption spectrum in the near-infrared (NIR) band is calculated by the finite-difference time-domain (FDTD) method. The absorption at 2120.5 nm, 2143 nm, 2216.5 nm, and 2325.5 nm, exhibit a high absorption rate of 99.993 %, 99.771 %, 93.004 %, and 99.179 %, respectively. The absorber's absorption spectra are studied with respect to various factors such as polarized light, graphene pattern, graphene Fermi energy and relaxation time, the refractive index of the dielectric layer, ambient refractive index, and angle of incidence. The study reveals that using graphene designs with constantly changing slopes (such as circular or elliptical) at the pattern boundaries generates more absorption peaks than other designs. Furthermore, a refractive index sensor can be developed based on the effect of the ambient refractive index on the absorber's range of absorption. The absorption peaks display quality figure of merit (FOM) values of 99.53, 85.16, 111.23, and 72.13, with sensitivities of 405, 425, 450, and 455 nm/RIU, respectively. The absorber has a high absorption sequence over a wide range of 0° to 65°, and it is insensitive to the angle of incidence of light from 0° to 65°. Due to its numerous wavelengths, high absorbance, polarization insensitivity, tuneability, and high FOM, the suggested absorber may find widespread application in NIR thermal radiation, optical detectors, and NIR sensors.

Spectral analysis of oxidation on localized surface plasmon resonance of copper nanoparticles thin film

Copper nanoparticles (CuNPs) possess localized surface plasmon resonance (LSPR) effect. Cu thin films composed of individual CuNPs exhibit stronger LSPR than the individual CuNPs due to the LSPR coupling among CuNPs. However, CuNPs are easy to be oxidized, which results in the rapid LSPR damping of the CuNPs thin films. Simulation of the variations of the coupled LSPR of two adjacent CuNPs with the thickness of oxide shells formed during oxidation is of great importance for understanding the mechanisms of the strong LSPR of CuNPs thin films and its rapid attenuation. In this paper, Discrete-dipole approximation method is used to simulate the extinction spectra of two adjacent spherical CuNPs as a function of the shell thickness (t), the ambient refractive index (n), the diameter (D) of the CuNPs, and the inter-nanoparticle spacing (L). The calculation is validated by experimental results. According to our model, for a definite CuNPs thin films, the oxide shell thickness of CuNPs can be calculated only if the extinction spectra and the morphology are provided. Further, it is found when the oxide shell thickness is small (t/R< 0.3), increasing n and decreasing L/D have an obvious synergistic effect on enhancing the coupled LSPR, but this synergistic effect weakens with the deepening of oxidation, and disappeared when t/R > 0.5. This study provides a calculation method for coupled core-shell nanoparticles and throws light on the role of oxidation on the rapid damped LSPR of CuNPs thin films.

Dynamically Adjusting Borophene-Based Plasmon-Induced Transparency in a Polymer-Separated Hybrid System for Broadband-Tunable Sensing.

Borophene, an emerging two-dimensional (2D) material platform, is capable of supporting highly confined plasmonic modes in the visible and near-infrared wavebands. This provides a novel building block for light manipulation at the deep subwavelength scale, thus making it well-suited for designing ultracompact optical devices. Here, we theoretically explore a borophene-based plasmonic hybrid system comprising a continuous borophene monolayer (CBM) and sodium nanostrip gratings (SNGs), separated by a polymer spacer layer. In such a structure, a dynamically tunable plasmon-induced transparency (PIT) effect can be achieved by strongly coupling dark and bright plasmonic modes, while actively controlling borophene. Here, the bright mode is generated through the localized plasmon resonance of SNGs when directly excited by TM-polarized incident light. Meanwhile, the dark mode corresponds to a propagating borophene surface plasmon (BSP) mode in the CBM waveguide, which cannot be directly excited, but requires phase matching with the assistance of SNGs. The thickness of the polymer layer has a significant impact on the coupling strength of the two modes. Owing to the BSP mode, highly sensitive to variations in the ambient refractive index (RI), this borophene-based hybrid system exhibits a good RI-sensing performance (643.8 nm/RIU) associated with a wide range of dynamically adjustable wavebands (1420-2150 nm) by tuning the electron density of borophene. This work offers a novel concept for designing active plasmonic sensors dependent on electrically gating borophene, which has promising applications in next-generation point-of-care (PoC) biomedical diagnostic techniques.

Backside Absorbing Layer Microscopy: Monolayer Counting in 2D Crystal Flakes

The properties of two‐dimensional (2D) material stacks critically depend on the number of monolayers (m) in the stack. It is therefore important to quantify this number, which is a local quantity since 2D stacks are essentially heterogeneous. Optical interferential techniques based on contrast‐enhancing surfaces may be sensitive enough to visualize m variations but the experimental determination of m requires heavy and unstable comparisons with multiparameter numerical models. Focusing on the recent backside absorbing layer microscopy, the most sensitive to date among interferential techniques, a self‐calibrating method is demonstrated allowing instantaneous monolayer counting all over the sample surface which does not require the knowledge of the instrumental parameters, the sample or ambient refractive indices or the detailed structure of the contrast‐enhancing layer. This method is introduced step by step using examples of hexagonal boron nitride (hBN) stacks with increasing complexity. Exact monolayer counting up to 36 hBN monolayers is obtained using basic image analysis.

Numerical Investigation of a High-Quality Factor Refractometric Nano-Sensor Comprising All-Dielectric Metamaterial Structures

This paper proposes an optical sensor based on nanoscale metamaterial structures. The design of the sensor has been explored with respect to biosensing applications through numerical modeling and analysis. The sensor comprises silica substrate and diamond nanostructures, both of which represent dielectrics. The sensing principle is based on the detection of ambient refractive index change. As the analyte properties change, the refractive index changes, as well. The refractive index change has been detected by striking electromagnetic waves onto the structure and noting the spectral response. Ultraviolet waves have been utilized for recording spectral responses and evaluating sensor performance. The sensor displays multiple sharp resonance peaks in the reflected beam. By altering the refractive index of the analyte present around the sensor, the peaks can be seen choosing different wavelengths. The resonance peaks have been investigated to observe electric and magnetic field dipoles in the sensor structure. The spectrum peaks have also been studied to understand fabrication tolerances. The sensor displays a linear response, along with a large Quality (Q) factor. The maximum value of the achieved Quality (Q) factor for the proposed sensor is 1229 while operating across the refractive index range of 1.4–1.45. The claim has been supported by comparison with contemporary works on similar platforms. A range of other sensing parameters have also been calculated and benchmarked. Metamaterial-based optical sensors can provide smaller device sizes, faster response times and label-free detection.

Perfect absorber with high sensitivity based on hexagonal star graphene surface

Graphene absorbers have attracted wide attention due to their high absorption rate and narrow bandwidth in the terahertz field. In this study, we proposed a dual-band terahertz tunable perfect absorber designed with a hexagonal star ring and a circular ring monolayer graphene metasurface, which exhibits tunable, polarization-insensitive, and high sensitivity characteristics. The graphene absorber was simulated by using the finite element method and validated by using the impedance matching method. Simulation results show that this structure has two perfect absorption peaks at 4.8 and 8.04 THz. The variation of the dielectric layer material was analyzed, and the parameters of the structure and the intrinsic graphene parameters were adjusted to control the absorption efficiency of the dual-band absorption peaks. Simulation results show that the high symmetry of the structure brings incident and polarization-insensitive properties, maintaining absorption higher than 98% over a wide range of incident and azimuth angles. The sensing performance of the device was investigated by varying the ambient refractive index, and the highest sensitivity parameter S of the absorber reached to 2.375 THz/RIU. These results demonstrate that this high-sensitivity sensor has great potential for terahertz imaging and microstructure detection.

Extraordinary upconversion enhancement in mono-dispersed plasmonic resonant nanocavity coupling with lanthanide doped upconversion nanoparticles

In this work, we reported a monodispersed MDM (metal–dielectric-metal) type upconversion composite material with ultrahigh emission enhancement for the first time. Different from the planar configuration of traditional MDM systems, the proposed materials here consisting of silver nanoparticles as the metallic core, upconversion nanoparticles(UCNPs) as the intermediate dielectric layer, and silver layer as the metallic shell. The structural characteristics were investigated to explore the spectral band match between the plasmonics and UCNPs. The sandwich structure can create high density hot carriers at the nearfield region around the UCNPs, leading to the remarkable excitation enhancement. According to the proper structure design, the upconversion intensity can be enhanced by over five orders of magnitude. The outer metallic layer thickness, the diameter of attached UCNPs and the ambient refractive index were found responsible for the intensity enhancement. Such upconversion materials combined the merit of monodispersity and the high emission efficiency, which can improve their performance in bioimaging and photovoltaic device.

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.

Near-infrared microfiber Bragg grating for sensitive measurement of tension and bending.

Fiber-optic devices working in the visible and near-infrared windows are attracting attention due to the rapid development of biomedicine that involves optics. In this work, we have successfully realized the fabrication of near-infrared microfiber Bragg grating (NIR-µFBG), which was operated at the wavelength of 785 nm, by harnessing the fourth harmonic order of Bragg resonance. The NIR-µFBG provided the maximum sensitivity of axial tension and bending to 211 nm/N and 0.18 nm/deg, respectively. By conferring the considerably lower cross-sensitivity, such as response to temperature or ambient refractive index, the NIR-µFBG can be potentially implemented as the highly sensitive tensile force and curve sensor.

A Novel Large-Diameter Optical Fiber Glucose Biosensor Based on Glucose Oxidase– Graphene Oxide–Gold Nanoparticles

A novel glucose large-diameter optical fiber LSPR sensor was constructed based on chemical cross-linking and electrostatic methods of glucose oxidase-graphene oxide-gold nanoparticles (GOD-GO-AuNPs). The GOD-catalyzed oxidation of glucose reacts with oxygen to produce gluconic acid and H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , leading to a change in its ambient refractive index, which in turn leads to a significant shift in the LSPR peak of GO-AuNPs on the fiber. The structure and morphology of GOD-GO-AuNPs were studied by FTIR, UV-vis spectroscopy, SEM, and TEM, respectively. GO improves the stability of the composite particles, reduces the biotoxicity of the metal particles, and the immobilization of glucose oxidase on the surface of the composite particles improved the enzyme activity. A fiber-optic sensor was prepared based on the optimal conditions, and the sensitivity of the sensor was 11.134 nm/(mg/mL) in the range of 0 to 1.0 mL with a response time of 55 s. It showed good anti-interference, selectivity and can be used for real detection. This method combines the advantages of GO, AuNPs, and large diameter fiber-optical which show promise for simple, stable, and cost-effective applications in the field of glucose detection.