Detection Of Refractive Index Changes Research Articles

Ultra Enhancement of the Resolution of the Divergence Beam-Based SPR Sensor Using a Wavelet Filter

Surface Plasmon Resonance (SPR) sensors are highly sensitive to refractive index variations, making them ideal for biosensing and chemical detection applications. However, their performance can be constrained by noise, resolution limits, and signal distortions, particularly in divergence beam-based configurations. This research presents an ultra-enhancement of the performance of divergence beam-based SPR sensors by employing advanced wavelet filtering techniques. Wavelets, with their multi-resolution analysis capability, are applied to denoise and refine the sensor signal, significantly improving key performance metrics, including resolution, mean square error (MSE), root mean square error (RMSE), signal-to-noise ratio (SNR), sensitivity matrix (SM), and combined sensitivity factor (CSF).The wavelet filter effectively decomposes the SPR signal into distinct frequency bands, separating noise while retaining high-frequency components required for accurate sensing. This results in a significant reduction in MSE and RMSE of 30 and 92.26%, respectively, while simultaneously improving SNR by 54.08%, maintaining signal quality. The wavelet filter significantly improved the resolution of the SPR sensor by 99.95% (1.3772×10-7 RIU), allowing for more precise detection of refractive index changes and boosting diverge beam-based SPR sensor performance. In addition, a sensitivity matrix (SM) and combined sensitivity factor (CSF) were improved by 42 and 41.94%, respectively, allowing for a more thorough evaluation of the sensor's performance across multiple operational parameters. Simulation and experimentation show that wavelet filtering surpasses standard filtering approaches in terms of noise suppression and signal clarity. This technology has considerable potential for improving the accuracy and reliability of SPR sensors in high-sensitivity applications

Resonant Waveguide Grating Biosensors Based on Label-free Optical Principle

Label-free optical biosensors are powerful tools for the real-time monitoring of both molecular and cellular-scale interactions. Resonant waveguide grating biosensors are based on the detection of refractive index changes induced by molecular interactions and/or cell mass redistributions. The Epic BenchTop and Epic Cardio are two biosensors with high sensitivity and throughput that offer excellent potential for life science research. Both instruments are suitable for cell-based and biochemical assays. In this paper, I describe the principles of operation and performance of the Epic BenchTop and Epic Cardio label-free waveguide grating biosensors and discuss their applications in various research areas.

Integration Aspects of Plasmonic TiN-based Nano-Hole-Arrays on Ge Photodetectorsin a 200mm Wafer CMOS Compatible Silicon Technology

During the last decade optical sensor technologies have attracted increased attention for various applications. Plasmon-based optical sensor concepts for the detection of refractive index changes that rely on propagating surface-plasmon polaritons at metal-dielectric interfaces or on localized plasmons in metallic nanostructures prove their potential for these application due to their fast detection speed, high specificity and sensitivities [1, 2]. Combining plasmonic structures directly with optoelectronic devices could enable a high level of integration, however, it represents a significant technological challenge to develop an on-chip solution for these concepts including the integration of sensor and detector components. Previous works demonstrated first approaches mainly for the integration of refractive index sensor components on wafer level [3, 4]. In [5] and [6] a proof-of-concept of a fully integrated on-chip solution with high sensitivities was presented, which can be easily combined with microfluidics [7] for potential applications in biosensing. In this concept, a nanohole array (NHA) was structured in a 100 nm thick aluminum layer on top of a vertical PIN germanium photodetector (GePD) with an intrinsic germanium sheet of 480 nm. This sensor concept relies on extraordinary optical transmission through the NHA [8]: Light transmission is only possible for narrow wavelength ranges determined by the NHA geometry which determine the transmission peaks at the resonance wavelength of the NHA. Thus, the NHA acts as a high quality wavelength filter. Due to the change in the refractive index, a material under test (MUT) contacting directly the surface of the NHA, provokes a shift of the wavelength maximum, which can be detected by measuring the photocurrent spectra of the GePD. While responsivities and sensitivities of (0 V) = 0.075 A/W and = 1200 nm/RIU could be attained in this proof-of-concept device [6, 7], the semiconductor device layers were deposited using molecular beam epitaxy (MBE). Furthermore the vertical PIN GePD was realized by a mesa procedure to enable large areas for top illuminated operations. These techniques are unsuitable for an industrial CMOS fabrication process with high throughput. Therefore, the development of a CMOS compatible technology process with low costs and high yields is an important step towards large-scale fabrication of this sensor concept.In this work we present the progress for the realization of a surface plasmon resonance (SPR) refractive index sensor in a 200 mm wafer Silicon based technology. One main challenge is the fabrication of a large area photodetector for top illuminated sensor devices. We developed a process, which is mainly based on the IHP electronic photonic integrated circuits (ePIC) technology [9]. This ePIC technology enables the production of waveguide coupled lateral PIN GePDs with high bandwidth and high responsivities [10]. However, these PDs are unsuitable for top illuminated applications because of their small germanium areas. Due to certain process conditions with respect to chemical mechanical polishing procedures there are limits for feasible large detector areas. Furthermore, large detector areas for lateral PIN GePDs would result in very low electric fields in the intrinsic zone where carriers are generated by photon absorption. Thus, very high voltages for reversed bias are necessary for sufficient carrier drifts.For the first time we have developed a modern detector design concept which is compatible to the IHP ePIC technology. This concept allows the realization of large area detectors of 1600µm² (40µm x 40µm) with optimized optical responsivities for top illuminated applications. The detector consists of several parallel connected lateral PIN GePDs. We designed different variations and varied Ge width and distance between neighboring GePDs in order to investigate process limits. The p- and n-doped regions were defined by dopant implantation using a photo resist mask. We used a finger-like design as implantation masks to enable one contact area for each p-doped and each n-doped region (Fig. 1). This contacting approach differs from the standard GePD offered in the IHP ePIC technology. We analyzed I-V characteristics in dependence of detector design and contacting scheme (Fig. 2). In addition, process adjustments for the optimization of the germanium quality were investigated to reduce dark currents and to improve optical responsivities (Fig.3).Titanium nitride (TiN) is very promising metallic alloy with respect to thickness homogeneity and low surface roughness. Therefore we used titanium nitride which was deposited by a sputtering process to develop plasmonic active NHA layers. Various process development runs were done to evaluate the NHA performance. Ellipsometry and atomic force microscope measurements were performed to characterize the quality of the TiN layer (Fig.4). Figure 1

Ultra-high sensitivity terahertz sensor based on a five-band absorber

Terahertz sensing is one of the most promising methods for label-free and noninvasive detection of refractive index changes. However, the figure of merit (FOM) of terahertz sensors in practical applications has been low. In this paper, a metamaterial sensor based on simple stacking of gold and silicon dioxide is proposed, through whose structure not only narrow-band absorption with five absorption peaks is realized, but FOM is also improved to 1792. The excellent sensing performance and the mature manufacturing technology of this kind of structure provide a platform for the design of multi-band photodetectors and high-sensitivity sensors.

Selectively excited bimodal interferometer for highly sensitive refractive index sensing

A selectively excited bimodal interferometer was developed for highly sensitive detection of refractive index changes. Numerical simulation revealed that multimode waveguides with tall, narrow cross-sections are suitable for realizing a highly sensitive bimodal (fundamental and lateral first-order modes) interferometer, owing to deeper penetration of the lateral first-order mode through its side surfaces into the cladding medium. A device with an optimum design was determined by simulation and fabricated by electron-beam lithography using a negative photoresist as the material for the waveguide core. The detection of glucose was demonstrated, and the sensitivity and detection limit was found to be 2547 rad / RIU (RIU—refractive index unit) and 9.8 × 10 − 6 RIU, respectively.

A highly sensitive plasmonic refractive index sensor based on triangular resonator

In this paper, metal-insulator-metal based plasmonic sensor with a straight waveguide and a side coupled equilateral triangular resonator (disc-type and ring-type) has been proposed and analyzed. The detection of refractive index change has been numerically simulated and analyzed using COMSOL Multiphysics. The sensitivity achieved using our proposed structure has been found to be 2713 nm/RIU with a high figure of merit (FOM) of 35.1. It is shown that the sensor can detect a refractive index change of 3.68×10-6, which is the sensing resolution of the sensor, for dielectrics whose refractive indices are between 1 to 1.03 and also between 1.33 to 1.42. Simplicity and compact design with high refractive index sensitivity and FOM of the plasmonic device helps to design bio-sensing devices, in biomedical applications like cancer cell detection, nano-sensing devices and also optical filter applications.

Research on low-temperature blood tissues detection biosensor based on one-dimensional superconducting photonic crystal

This paper presents a one-dimensional superconducting photonic crystal (sc-PhC) refractive index biosensor with high sensitivity which consists of a periodic arrangement of superconductors and semiconductors. The biosensor has high sensitivity and accuracy in the refractive index range of 1.0 to 2.2 RIU below the critical temperature. The resonance wavelength and defect refractive index of the biosensor have a high linear correlation coefficient, and the linear correlation coefficient of all experimental groups in this paper is higher than 0.999. The sensitivity of the biosensor decreases with increasing temperature, and when the external temperature increases from 80K to 134K, the sensitivity decreases from 6.04396μm/RIU to 5.71703μm/RIU gradually. And then, the performance of biosensor in blood component detection is researched by FDTD (finite-difference time-domain) method. The result shows that the sensor has higher sensitivity and shows the same change law in the detection of refractive index changes (1.35RIU-1.47RIU) within a small range such as blood components. When the external temperature is increased from 80K to 134K, the sensitivity of the biosensor gradually decreases from 6.85789μm/RIU to 6.48073μm/RIU.

Design of Microdisk-Shaped Ge on Si Photodetector with Recess Structure for Refractive-Index Sensing.

In this paper, we introduce a disk-shaped Ge-on-Si photodetector for refractive-index difference sensing at an operating wavelength of 1550 nm. For the implementation of a small-scale sensor, a Ge layer was formed on top of a Si layer to increase the absorption coefficient at the expense of the light-detection area. Additionally, the sensor had a ring waveguide structure along the edge of the disk formed by a recess into the inner part of the disk. This increased the interaction between the dominant optical mode traveling along the edge waveguide and the refractive index of the cladding material to be sensed, and conclusively increased detection sensitivity. The simulation results show that the proposed sensor exhibited a detection sensitivity of >50 nm/RIU (Refractive Index Unit), a quality factor of approximately 3000, and a minimum detectable refractive index change of 0.95 × 10−2 RIU with a small disk radius of 3 μm. This corresponds to 1.67 times the sensitivity without a recess (>30 nm/RIU).

Speckle Noise Removal in Image-based Detection of Refractive Index Changes in Porous Silicon Microarrays

Based on porous silicon (PSi) microarray images, we propose a new method called the phagocytosis algorithm (PGY) for removing the influence of speckle noise on image gray values. In a theoretical analysis, speckle noise of different intensities is added to images, and a suitable denoising method is developed to restore the image gray level. This method can be used to reduce the influence of speckle noise on the gray values of PSi microarray images to improve the accuracy of detection and increase detection sensitivity. In experiments, the method is applied to detect refractive index changes in PSi microcavity images, and a good linear relationship between the gray level change and the refractive index change is obtained. In addition, the algorithm is applied to a PSi microarray image, and good results are obtained.

Sensitive label-free sensor with high figure of merit based on plasmonic metasurface with unit cell of double two-split nanorings

We theoretically propose a sensitive label-free sensor with high figure of merit (FoM) based on Fano resonances on a plasmonic metasurface whose unit cell consists of double two-split nanorings by finite-element method. The unit cell of the proposed Fano resonant structure comprises of two thin gold nanorings each with two splits. Two Fano modes are generated in near-infrared regime through symmetry breaking by rotating the splits in nanorings in opposite directions. The fundamental feature of the proposed sensor is its relatively simple structure, double-mode usable, a large single-side quality factor of 566, and high FoM value of 378. According to our knowledge, these values are higher than those previously reported. The sensor has high substance resolving capability that its minimum detectable refractive index change can be as small as 0.00236, making it possible to distinguish different bio-tissues.

Hybrid Ni/SiO2/Au dimer arrays for high-resolution refractive index sensing

AbstractWe introduce a novel magnetoplasmonic sensor concept for sensitive detection of refractive index changes. The sensor consists of a periodic array of Ni/SiO2/Au dimer nanodisks. Combined effects of near-field interactions between the Ni and Au disks within the individual dimers and far-field diffractive coupling between the dimers of the array produce narrow linewidth features in the magneto-optical Faraday spectrum. We associate these features with the excitation of surface lattice resonances and show that they exhibit a spectral shift when the refractive index of the surrounding environment is varied. Because the resonances are sharp, refractive index changes are accurately detected by tracking the wavelength where the Faraday signal crosses 0. Compared to random distributions of pure Ni nanodisks or Ni/SiO2/Au dimers or periodic arrays of Ni nanodisks, the sensing figure of merit of the hybrid magnetoplasmonic array is more than one order of magnitude larger.

Demonstration of versatile whispering-gallery micro-lasers for remote refractive index sensing.

We developed chip-scale remote refractive index sensors based on Rhodamine 6G (R6G)-doped polymer micro-ring lasers. The chemical, temperature, and mechanical sturdiness of the fused-silica host guaranteed a flexible deployment of dye-doped polymers for refractive index sensing. The introduction of the dye as gain medium demonstrated the feasibility of remote sensing based on the free-space optics measurement setup. Compared to the R6G-doped TZ-001, the lasing behavior of R6G-doped SU-8 polymer micro-ring laser under an aqueous environment had a narrower spectrum linewidth, producing the minimum detectable refractive index change of 4 × 10-4 RIU. The maximum bulk refractive index sensitivity (BRIS) of 75 nm/RIU was obtained for SU-8 laser-based refractive index sensors. The economical, rapid, and simple realization of polymeric micro-scale whispering-gallery-mode (WGM) laser-based refractive index sensors will further expand pathways of static and dynamic remote environmental, chemical, biological, and bio-chemical sensing.

Detection of Refractive Index Change From the Critical Wavelength of an Etched Few Mode Fiber

A novel scheme for the detection of refractive index change based on the mode interference in a special few mode fiber (FMF) is proposed in this paper. The parameters and index profile of the FMF used in the measurement are specifically designed so that only LP01 and LP02 modes are transmitted within the operation wavelength, and the propagation constant difference Δβ between these two modes reaches maximum at critical wavelength (CWL). An inline Mach–Zehnder interferometric sensor for the detection of surrounding refractive index (SRI) is constructed by splicing FMF with two sections of conventional single mode fiber. We demonstrated at the first time that the CWL will change monotonically with the change of SRI in the measurement range of 1.316–1.439. The experimental sensitivity of the CWL increases with the increase of SRI. A sensitiviy of 140.626 ± 12.560 nm/RIU in the range of 1.316–1.383 and a maximum sensitivity of 2489.796 ± 190.179 nm/RIU in the range of 1.433–1.439 were obtained, respectively, in the experiment. At present parameters of the etched FMF, the limit of detection of the refractive index is 3.413 × 10–4 RIU in the SRI range of 1.316–1.383, and 1.928 × 10–5 RIU in the SRI range of 1.433–1.439, respectively. The detection of SRI via monitoring the shift of CWL is superior to the traditional detection method via monitoring merely the shift of peak or dip wavelength of the transmission spectrum, since the latter usually suffers from limited measurement range and multivalue problem because it is difficult to identify and follow one specific peak or dip from the transmission spectrum especially if the system needs to reset in practical application. The novel detection scheme based on monitoring the shift of the exclusive CWL of FMF is promising in practical applications.

Parallel Detection of Refractive Index Changes in a Porous Silicon Microarray Based on Digital Images.

A new technique for the refractive index change with high-sensitivity measurements was proposed by the digital image of porous silicon (PSi) microarray utilization in this paper. Under the irradiation of a He-Ne laser, the surface images of the PSi array cells with the microcavity structure were obtained by the digital imaging equipment, whereas the refractive index change of each array cells was detected by calculating the average gray value of the image and the refractive index change measurement sensitivity was 10−4 order of magnitude. This technique could be utilized in the label-free and parallel detection of refraction index changes induced by a biological reaction in the microarray or the chip.

Plasmonic Sensing of Oncoproteins without Resonance Shift Using 3D Periodic Nanocavity in Nanocup Arrays

A sensor design and sensing method based on plasmonic–photonic interactions that occur when a nanocavity array is embedded in a 3D tapered nanocup plasmonic substrate are reported. This device enables highly sensitive detection of refractive index changes based on changes to the transmission peak intensity without shift in the resonance wavelength. Unlike conventional plasmonic sensors, there is a consistent and selective change in the transmission intensity at the resonance peak wavelength with no spectral shift. In addition, there are wavelength ranges that show no intensity change, which can be used as reference regions. The fabrication and characterization of the plasmonic nanocavity sensor are described and also advanced biosensing is demonstrated. Simulations are carried out to better understand the plasmon–photonic coupling mechanism. This nanocavity plasmonic sensor design has a limit of detection of 1 ng mL−1 (5 × 10−12m) for the cancer biomarker carcinoembryonic antigen (CEA), which is a significant improvement over current surface plasmon resonance systems, and a dynamic range that is clinically relevant for human CEA levels.

Detection of Pressure Waves Emitted From Plasma Jets With Fibered Optical Wave Microphone in Gas and Liquid Phases

Irradiation of atmospheric plasma jets to liquid targets induces a stream of reactive oxygen or nitrogen radicals from the liquid surface to the bottom under limited conditions, including frequency of applied voltage, gas flow rate, and so on. Pressure waves can be considered to be one of possible forces that concern with the stream inside liquid targets, because we succeeded to detect pressure waves generated by plasma jets in air with a fibered optical wave microphone. The fibered optical wave microphone works based on Fraunhofer diffraction of laser for phase objects, having superior sensitivity compared with other conventional optical methods for the detection of refractive index changes. In this paper, the fibered optical wave microphone measurement was done in air to investigate the frequency properties of the generation of pressure waves from plasma jets. In addition, the detection of pressure waves inside distilled water was attempted.

Bottom-Up Fabrication of Hybrid Plasmonic Sensors: Gold-Capped Hydrogel Microspheres Embedded in Periodic Metal Hole Arrays.

The high potential of bottom-up fabrication strategies for realizing sophisticated optical sensors combining the high sensitivity of a surface plasmon resonance with the exceptional properties of stimuli-responsive hydrogel is demonstrated. The sensor is composed of a periodic hole array in a gold film whose holes are filled with gold-capped poly(N-isoproyl-acrylamide) (polyNIPAM) microspheres. The production of this sensor relies on a pure chemical approach enabling simple, time-efficient, and cost-efficient preparation of sensor platforms covering areas of cm2. The transmission spectrum of this plasmonic sensor shows a strong interaction between propagating surface plasmon polaritons at the metal film surface and localized surface plasmon resonance of the gold cap on top of the polyNIPAM microspheres. Computer simulations support this experimental observation. These interactions lead to distinct changes in the transmission spectrum, which allow for the simultaneous, sensitive optical detection of refractive index changes in the surrounding medium and the swelling state of the embedded polyNIPAM microsphere under the gold cap. The volume of the polyNIPAM microsphere located underneath the gold cap can be changed by certain stimuli such as temperature, pH, ionic strength, and distinct molecules bound to the hydrogel matrix facilitating the detection of analytes which do not change the refractive index of the surrounding medium significantly.

Modeling of a Single-Notch Microfiber Coupler for High-Sensitivity and Low Detection-Limit Refractive Index Sensing.

A highly sensitive refractive index sensor with low detection limit based on an asymmetric optical microfiber coupler is proposed. It is composed of a silica optical microfiber and an As2Se3 optical microfiber. Due to the asymmetry of the microfiber materials, a single-notch transmission spectrum is demonstrated by the large refractive index difference between the two optical microfibers. Compared with the symmetric coupler, the bandwidth of the asymmetric structure is over one order of magnitude narrower than that of the former. Therefore, the asymmetric optical microfiber coupler based sensor can reach over one order of magnitude smaller detection limit, which is defined as the minimal detectable refractive index change caused by the surrounding analyte. With the advantage of large evanescent field, the results also show that a sensitivity of up to 3212 nm per refractive index unit with a bandwidth of 12 nm is achieved with the asymmetric optical microfiber coupler. Furthermore, a maximum sensitivity of 4549 nm per refractive index unit can be reached while the radii of the silica optical microfiber and As2Se3 optical microfiber are 0.5 μm and a 0.128 μm, respectively. This sensor component may have important potential for low detection-limit physical and biochemical sensing applications.

Optofluidic Fabry-Pérot Micro-Cavities Comprising Curved Surfaces for Homogeneous Liquid Refractometry-Design, Simulation, and Experimental Performance Assessment.

In the scope of miniaturized optical sensors for liquid refractometry, this work details the design, numerical simulation, and experimental characterization of a Fabry-Pérot resonator consisting of two deeply-etched silicon cylindrical mirrors with a micro-tube in between holding the liquid analyte under study. The curved surfaces of the tube and the cylindrical mirrors provide three-dimensional light confinement and enable achieving stability for the cavity illuminated by a Gaussian beam input. The resonant optofluidic cavity attains a high-quality factor (Q)—over 2800—which is necessary for a sensitive refractometer, not only by providing a sharp interference spectrum peak that enables accurate tracing of the peak wavelengths shifts, but also by providing steep side peaks, which enables detection of refractive index changes by power level variations when operating at a fixed wavelength. The latter method can achieve refractometry without the need for spectroscopy tools, provided certain criteria explained in the details are met. By experimentally measuring mixtures of acetone-toluene with different ratios, refractive index variations of 0.0005 < Δn < 0.0022 could be detected, with sensitivity as high as 5500 μW/RIU.

Diagnosis of immediate-type allergy using surface plasmon resonance

Surface plasmon resonance (SPR) sensors can sensitively detect refractive index (RI) changes in living cells on the surface of a sensor chip, without labeling, in real time. We and other groups have applied SPR and SPR imaging (SPRI) sensors for cell-based clinical diagnosis, such as allergies and cancer. Herein, we review techniques using SPR and SPRI sensors for the label-free detection of RI changes in living cells. We then focus on applications of SPRI techniques for the clinical diagnosis of immediate-type allergy (type I allergy) and introduce new techniques using basophil separation chip for SPRI and a human IgE receptor expressing mast cell line.