Resonance Wavelength Shift Research Articles

All fiber-optic temperature sensor based on cladding etched no-core fiber coated with nanostructured copper oxide-polyvinyl alcohol thin film

The demonstration of an interferometric temperature sensor based on a no-core fiber (NCF) with a thin layer of copper oxide-polyvinyl alcohol (CuO-PVA) coating is presented in this paper. The sensing principle relies on the multimode interference (MMI) effect that occurs in a pure silica multimode fiber (MMF) sandwiched between two single-mode fibers (SMFs), where the MMF section is an NCF. To accomplish high sensitivity, the cladding of the NCF was chemically etched at a rate of ∼2.84 μm min−1 until it reached a diameter of 18 μm. Then a thin film of CuO-PVA was coated onto the etched region to act as the sensitive cladding layer, whose refractive index (RI) varies with the environmental temperature variation. The effective interaction between the CuO-PVA and the strong evanescent field of the etched NCF results in temperature sensing with a maximum resonant wavelength shift of 21.5 nm. The maximum sensor sensitivity obtained was 0.101 nm/°C with a linear coefficient of 99.1 % in the range of 25 °C to 235 °C. To the best of our knowledge, this is the first demonstration of a temperature sensor based on an etched NCF coated with CuO-PVA thin film. This work presents an important step to enhance the performance of an all optical temperature sensor in terms of sensitivity and repeatability.

Enhancing the nanoplasmonic signal by a nanoparticle sandwiching strategy to detect viruses

Nanoparticles that can assemble and bind selectively on surfaces in intricate geometries can trigger multiple plasmonic modalities and enable wide applications in agriculture such as pesticide monitoring, in medical imaging such as targeted cancer detection, in bioengineering such as biotarget detection and biosensing, and in healthcare such as selection of drugs and their binding kinetics. However, these particles mainly rely on binding of the target to a surface to create a plasmonic resonance and subsequent shifts by binding of biotargets, which limit the flexibility to control overall sensitivity. Here, we present an unconventional way that sandwiches a virus (i.e., Hepatitis B virus: HBV) topographically between two or more nanoparticles on the top and the bottom to create a double-step shifting effect amplifying the total resonance wavelength shift on the surface by 1.53 - 1.77 times that significantly enhances the sensitivity. We successfully applied this approach to an intact HBV sensing application, which accurately quantified the viral load. This method establishes a new nanoparticle-based sandwiched nanoplasmonic approach to detect and quantify viral load using two-step sensing with broad applications in biosensing.

Controllable gap in microsphere resonator integrated with a deformable ferrofluid droplet for magnetic field sensing

Abstract A magnetic field sensor based on silica microsphere resonator and ferrofluid is proposed and experimentally demonstrated in this paper. A silica fiber taper with a waist diameter of 2 μm is employed to couple the evanescent light into the microsphere with a diameter of 31.21 μm. Whispering gallery modes (WGMs) are excited inside the microsphere resonator and circulate inside the equatorial plane. The essence of the sensing concept is to control the coupling gap between fiber taper and microsphere using the magnetically induced deformation of ferrofluid droplet. The shape-varying ferrofluid droplet is operated under different magnetic fields to observe the resonance shift and Q-factors of WGMs. As the magnetic field rises gradually, a longer resonant wavelength shifts with a decreasing Q-factor and a high maximum sensitivity of 120.9 pm/mT in the vertical magnetic field. On the contrary, resonance under the parallel magnetic field shifts to shorter wavelength with an increasing Q-factor and a low maximum sensitivity of 16.4 pm/mT. All experimental results are in good agreement with the theoretical model, which provides a disparate approach to integrate photonics with ferrofluid for signal processing and sensing applications.

Enhanced Surface Plasmon Resonance Wavelength Shifts by Molecular Electronic Absorption in Far- and Deep-Ultraviolet Regions

In this study, surface plasmon resonance (SPR) wavelength shifts due to molecular electronic absorptions in the far-ultraviolet (FUV, < 200 nm) and deep-ultraviolet (DUV, < 300 nm) regions were investigated by attenuated total reflectance (ATR) spectroscopy. Due to the strong absorption in the DUV region, N,N-dimethylformamide (DMF) significantly increased the SPR wavelength shift of Al film. On the other hand, no such shift enhancement was observed in the visible region for Au film because DMF does not have absorbance compared to non-absorbing materials such as water and alcohols. The enhanced SPR wavelength shift, caused by the overlap between SPR and molecular resonance wavelengths in FUV-DUV region, is expected to result in high sensitivity for resonant materials.

Dynamic Responses of Cylindrical Whispering Gallery Mode Revealed by Axial Liquid Disturbance

Cylindrical whispering gallery mode (WGM) microresonators have achieved much attention in the sensing field due to high sensitivity, compatibility and integration. The excitation of spiral modes in such a resonator is unavoidable, causing the axial field distribution, and the injected analytes will undoubtedly interact with the axial mode field, resulting in unclear spectral responses. Here, these responses of the cylindrical WGM are investigated experimentally and theoretically for the first time. Using an optical fiber as the resonator, the experiment is performed by introducing the liquid into a part of the evanescent field along the axis. The perturbation is attributed to the modification of the resonator's profile, leading to an increase of the effective slope. We find the experimental demonstrations of the short-wavelength oscillations of spectral line shapes and the blue shifts of resonant wavelengths. Theoretical studies are consistent with the experimental phenomena. The research in the responses to the axial perturbation provides valuable insights into the effect of spiral modes on cylindrical WGMs and a potentially powerful tool for real-time biochemical analysis.

Biophotonic sensor for rapid detection of brain lesions using 1D photonic crystal

In the present study, we study theoretically the detection of brain lesions by using the defected one-dimensional photonic crystal. The transfer matrix method is used to analyze the optical transmission characteristics of one-dimensional photonic crystals. The device performance is verified by measuring the red shift in resonant wavelength when the refractive index of brain lesions layer changes from 1.3333 to 1. 4833. Through the shifting spectrum, the sensitivity of the designed sensor for oligodendroglioma cells reaches to 3080.808 nm/RIU. The high figure of merit [FOM = 6.1 × 107 (1/RIU)] is achieved which evaluates the sensing performance comprehensively.

Coupled Mode Characteristics From the Perturbation of 3D Printed Long-Period Fiber Grating Devices

Characteristics of electric field from a coupled mode inside an optical fiber under perturbation by three-dimensional (3D) printed long-period fiber grating (LPFG) device have been observed in this work by the experiment and simulation. The various periodic index differences referring to the weights of perturbation by 3D printed LPFG device are applied on the single-mode fiber. The experimental results show that the resonant wavelength shift is a linear function of the grating period with the maximum coefficient of determination R2 of 0.9995. Some of resonant wavelengths are chosen to run simulations to investigate the electric field distribution. The scattering direction of the electric field states the magnitude of leaking optical power when the light transmits through the grating region applied to the single-mode fiber. Both the experimental and simulation results demonstrate that our proposed scheme can usefully be applied to selective tunable filters, intruder sensors, etc.

Optical sensor based on two-dimensional photonic crystals for measuring glucose in urine

The design and simulation of two bio-optical sensors for the noninvasive measurement of glucose levels in urine of diabetic patients are presented. While the sensors are of the same shape and structure, their quality factors (QFs), sensitivities, and sizes are different. The structure developed is based on two-dimensional (2D)-hexagonal-latticed photonic crystals containing silicon rods immersed in urine. By creating line and point defects in the structure, two resonant modes are localized in the photonic bandgap (the photonic bandgap is a frequency range corresponding to mirror action for incident light of frequency within that range). Sensor 1 design was based on localized resonant mode 1, while sensor 2 uses localized resonant mode 2. The mechanism of these sensors relies on the change of the refractive index of urine as a result of the change in glucose concentration, which in turn causes shifts in resonant modes of the sensors. For tuning in the wavelength of 1.55 μm, the scaling method is utilized. The major difference between the proposed sensors and previously reported bio-optical sensors is the determination of glucose level based on the shift in resonant wavelengths. Previous works relied on the change of the amplitude of emerging waves for the same purpose. Consequently, the sensors presented here exhibit a higher QF and operate within the wavelength window of 1.55 μm. QF calculations and sensitivity simulations for sensor 1 yielded approximate values of 3800 and 500 nm/refractive index unit (RIU), respectively. For sensor 2, QF and sensitivity were 5540 and 500 nm / RIU, respectively. The sizes of sensor 1 and sensor 2 are 14.93 × 8.8 μm2 and 15.7 × 9.47 μm2, respectively. The 2D finite difference time domain and plane wave expansion methods were relied upon in the simulations.

Characterization method of a mid-infrared graphene-on-silicon microring with a monochromatic laser

Mid-infrared microring resonators have been extensively studied for biochemical sensing applications. To characterize such resonators, a tunable laser or spectrometer has usually been utilized in previous studies. However, it is challenging to integrate these bulky instruments on a chip, ineluctably limiting practical applications. Here, we study a method for characterizing a microring resonator by using a monochromatic laser to conquer this limitation. Specifically, we theoretically probe the quality factor, extinction ratio, and resonant wavelength shift of a graphene-on-silicon microring resonator at a wavelength of 2.7 µm. Our work paves the way for studying monochromatically chip-integrated cavities and their applications.

Simulation of reconfigurable double-input optical gates based on a microring flower-like structure. part I. basic gates.

In this paper, a simulation and general analysis of nonlinear microring resonators (MRRs) as all-optical switches are discussed in a flower-like structure. These MRRs are modulated through an optical pump beam, which receives temporal shift of MRRs' resonance wavelengths. These shifts are a result of the refractive index changing due to carrier injection. A green laser is used as the optical pump to shift the resonant wavelength of each MRR. This proposed structure can operate as all-optical logic gates. Of those innovations are reconfigurable structure to achieve various gates, integrating capability, and a high extinction ratio between zero and one logical levels. This configuration would be useful to design optical integrated circuits.

Graphene/Au/SiC Optical Waveguide-Based Surface Plasmon Resonance Sensor for Chemical and Biomedical Sensing in Aqueous Medium

In chemical and biomedical sensing, identification and quantification of analytes in aqueous medium such as water, blood or other carriers are very important. It requires the core sensors to have the properties of low cost, high sensitivity, on-chip integration capability, etc. Various sensing methods have been developed such as prism coupler and grating coupler, etc. and among them, waveguide-based surface plasmon resonance (SPR) sensors, with sensitivity of the optical waveguide significantly improved by SPR, have been identified as a powerful tool for detection and quantitative characterization of chemical and biological species or biomolecular reactions in aqueous medium. For sensing in aqueous medium, the optical waveguide sensors are desirable to operate in the visible light wavelength to overcome the large absorption coefficient of water in the near infrared range. This high absorption coefficient causes optical energy in the near infrared incident light be absorbed by water rather than by the sensing medium. Conventional silicon (Si) is not the choice of material for such sensors due to the strong absorption below the wavelength of 1.1 μm due to its narrow bandgap (Eg=1.12 eV, 1.1 µm wavelength).As a wide bandgap semiconductor, silicon carbide (SiC) is more desirable for such sensors with advantages of: (1) bandgap energy of 3.2 eV in 4H and 6H polytypes and 2.2 eV in 3C polytype with transparent wavelength in visible and near-infrared range, (2) excellent material properties especially the chemical inertness and biocompatibility, and (3) device fabrication compatibility with standard Si device fabrication for manufacturing and on-chip integration. Therefore, SiC is a desirable material candidate for SPR sensors for chemical and biomedical sensing in aqueous medium, and approaches to enhance its sensitivity have been explored. 2-D materials such as graphene, TMDCs (MoS2, WS2, MoSe2, WSe2), etc. and their applications in sensing technology has aroused wide attention because of their distinctive electrical and optical properties. SPR sensors with 2-D and Si to enhance the sensitivity have been studied by publications. In this paper, we investigated graphene/Au/SiC waveguide-based SPR sensors (Fig. 1) for aqueous medium sensing. The resonance frequency shift with different numbers of graphene layer were studied (Fig. 2). Comparison of resonance wavelength shift from conventional and proposed sensor were performed (Fig. 3). At the incident light of 633 nm, the confinement factor above 0.9 can be achieved at the refractive index of n=1.3~1.5. With the change of refractive index, the shift of resonant wavelength showed linear characteristics, and a sensitivity of 2580 nm/RIU (refractive index unit) was obtained in a refractive index range of n=1.34~1.36 (Fig. 4). Figure 1

Mid-infrared gas sensor based on high-Q/V point-defect photonic crystal nanocavities

Point-defect nanocavities based on silicon planar photonic crystal (PhC) have been optimized and studied for sensing the refractive index of gases in the mid-infrared wavelength region. The point-defect has been introduced at the center of the triangular lattice of the photonic crystal that is made up of circular air holes, making it suitable for probing the properties of the gas found within the cavity. By optimizing the radius and position of the air holes closest to the defect region precisely, on the order of a few nanometers, the ratio of the quality factor to mode volume (Q/V) for the point-defect PhC nanocavities can be increased considerably. Moreover, a perturbation method has been implemented in order to study the resonant wavelength shift of the optimized point-defect nanocavity modes caused by a small change in the refractive index of the gas. The results obtained show that sensitivity of 270 nm/RIU (Refractive Index Unit) and a detection limit of $$10^{-4}$$ RIU can be achieved for the optimized point-defect PhC nanocavities. These nanocavities have been designed to oscillate at a single mode with a high Q/V thus enabling to sense the refractive index of gases with high sensitivity.

Graphene oxide functionalized micro-tapered long-period fiber grating for sensitive heavy metal sensing

We report a Ni2+ heavy metal sensor based on a graphene oxide (GO) functionalized micro-tapered long-period fiber grating (MTLPG) where light–matter interaction is enhanced. With high-quality GO coating on fiber with strong adhesion and controllable thickness, the GO-coated MTLPG demonstrated a resonant wavelength shift and intensity change, corresponding to a sensitivity of up to 5.12 × 10−4 nm ppb−1 and 3.07 × 10–4 dB ppb−1, respectively. Moreover, the limits of detection were 2.5 ppb and 0.27 ppb, respectively, operating in a wider concentration range of 1 ppb to 1 × 107 ppb. The proposed optical platform can be developed for superior chemical sensing applications.

Thermal shift of whispering gallery modes in tellurite glass microspheres

The tremendous progress made in recent years in the production of tellurite glasses has led to the development of different optical elements and devices on their basis such as microresonators and microlasers. The investigation of tellurite glass microresonators is of great importance for many potential applications. We studied theoretically and experimentally the nonlinear thermo-optical effects in tellurite glass microresonators under continuous-wave laser pumping. We calculated the temperature distributions for both, steady-state and transient non-stationary regimes and simulated shifts of the resonant wavelengths of whispering gallery modes. We produced Tm-doped tellurite glass microspheres and measured the resonant wavelength shifts as functions of the heat power. Our experimental results are in a good agreement with the theoretical predictions.

Packaged Microbubble Resonator for Versatile Optical Sensing

Microbubble whispering-gallery resonator with the intrinsic microfluidic channel is a promising platform for biochemical sensing due to their ultrahigh quality ( Q ) factor and ultralow detection limit. In this work, we realize a packaged microbubble resonator (PMBR) of ultrahigh- Q factor ( $\sim\!3.52\times 10^6$ ) in a moisture curable polymer for practical applications. The long-term stability and thermal response of a PMBR are measured. Further, we demonstrate the dynamic hydrogel gelation transition from hydrophilic to hydrophobic state by detecting the resonant wavelength shift and mode broadening in a filled PMBR simultaneously. The refractive index changes of hydrogel during the transition is explicitly modelled. Our work shows the versatility of PMBR for practical optical sensing and may open a new route to microfluidic dynamic reaction.

Real-time monitoring of hydrogel phase transition in an ultrahigh Q microbubble resonator

The ability to sense dynamic biochemical reactions and material processes is particularly crucial for a wide range of applications, such as early-stage disease diagnosis and biomedicine development. Optical microcavities-based label-free biosensors are renowned for ultrahigh sensitivities, and the detection limit has reached a single nanoparticle/molecule level. In particular, a microbubble resonator combined with an ultrahigh quality factor ( Q ) and inherent microfluidic channel is an intriguing platform for optical biosensing in an aqueous environment. In this work, an ultrahigh Q microbubble resonator-based sensor is used to characterize dynamic phase transition of a thermosensitive hydrogel. Experimentally, by monitoring resonance wavelength shift and linewidth broadening, we (for the first time to our knowledge) reveal that the refractive index is increased and light scattering is enhanced simultaneously during the hydrogel hydrophobic transition process. The platform demonstrated here paves the way to microfluidical biochemical dynamic detection and can be further adapted to investigating single-molecule kinetics.

Corrosion-Induced Mass Loss Measurement under Strain Conditions through Gr/AgNW-Based, Fe-C Coated LPFG Sensors.

In this study, graphene/silver nanowire (Gr/AgNW)-based, Fe-C coated long period fiber gratings (LPFG) sensors were tested up to 72 hours in 3.5 w.t% NaCl solution for corrosion-induced mass loss measurement under four strain levels: 0, 500, 1000 and 1500 µε. The crack and interfacial bonding behaviors of laminate Fe-C and Gr/AgNW layer structures were characterized using Scanning Electron Microscopy (SEM) and electrical resistance measurement. Both optical transmission spectra and electrical impedance spectroscopy (EIS) data were simultaneously measured from each sensor. Under increasing strains, transverse cracks appeared first and were followed by longitudinal cracks on the laminate layer structures. The spacing of transverse cracks and the length of longitudinal cracks were determined by the bond strength at the weak Fe-C and Gr/AgNW interface. During corrosion tests, the shift in resonant wavelength of the Fe-C coated LPFG sensors resulted from the effects of the Fe-C layer thinning and the NaCl solution penetration through cracks on the evanescent field surrounding the LPFG sensors. Compared with the zero-strained sensor, the strain-induced cracks on the laminate layer structures initially increased and then decreased the shift in resonant wavelength in two main stages of the Fe-C corrosion process. In each corrosion stage, the Fe-C mass loss was linearly related to the shift in resonant wavelength under zero strain and with the applied strain taken into account in general cases. The general correlation equation was validated at 700 and 1200 µε to a maximum error of 2.5% in comparison with 46.5% from the zero-strain correlation equation.

High-Sensitivity Detection of IgG Operating near the Dispersion Turning Point in Tapered Two-Mode Fibers.

The conventional methods for monitoring IgG levels suffer from some apparent problems such as long assay time, multistep processing, and high overall cost. An effective and suitable optical platform for label-free biosensing was investigated by the implementation of antibody/antigen immunoassays. The ultrasensitive detection of IgG levels could be achieved by exploiting the dispersion turning point (DTP) existing in the tapered two-mode fibers (TTMFs) because the sensitivity will reach ±∞ on either side of the DTP. Tracking the resonant wavelength shift, it was found that the fabricated TTMF device exhibited limits of detection (LOD) down to concentrations of 10 fg/mL of IgG in PBS solution. Such immunosensors based on DTP have great significance on trace detection of IgG due to simple detection scheme, quick response time, and miniaturization.

Controllable coupling between an ultra-high-Q microtoroid cavity and a graphene monolayer for optical filtering and switching applications.

Whispering-gallery-mode optical microresonators have found impactful applications in various areas due to their remarkable properties such as ultra-high quality factor (Q-factor), small mode volume, and strong evanescent field. Among these applications, controllable tuning of the optical Q-factor is vital for on-chip optical modulation and various opto-electronic devices. Here, we report an experimental demonstration with a hybrid structure formed by an ultra-high-Q microtoroid cavity and a graphene monolayer. Thanks to the strong interaction of the evanescent wave with the graphene, the structure allows the Q-factor to be controllably varied in the range of 3.9 × 105 ∼ 6.2 × 107 by engineering optical absorption via changing the gap distance in between. At the same time, a resonant wavelength shift of 32 pm was also observed. Besides, the scheme enables us to approach the critical coupling with a coupling depth of 99.6%. As potential applications in integrated opto-electronic devices, we further use the system to realize a tunable optical filter with tunable bandwidth from 116.5 MHz to 2.2 GHz as well as an optical switch with a maximal extinction ratio of 31 dB and response time of 21 ms.

Fabrication of multi-mode tip fiber sensor based on surface plasmon resonance (SPR)

In this research, a fiber optic sensor based on surface plasmon resonance (SPR) was prepared. The sensor of SPR was configured using a gold thin layer at the end of a cleaved optical fiber by a sputtering technique. The source of white light was utilized to produce a series of wavelengths and excites surface plasmon resonance at the fiber tip. The SPR sensor was immersed into media with different refractive indices in the range of 1 to 1.58, including their similar plasmon resonance wavelength shifts that were saved by the optical spectrum analyzer. Experimental results that were obtained show there is a redshift when increasing the refractive index of solutions and sensitivity reach 298nm/ RIU, and resolution 4.31x?10?^(-4).