Microcavity Sensors Research Articles

Whispering gallery mode based micro-ring resonator for magnetic field sensing

This study proposes a whispering gallery mode based micro-ring optical resonator with a simple and integrated structure for magnetic field sensing. The performance of the proposed sensor is studied theoretically with the help of the finite element method. Through an in-depth investigation into the sensitivity and reliability of the proposed sensor, the capability to sense a uniform magnetic field is established. In addition to detecting a uniform magnetic field, the sensor is also capable of estimating the spatial inhomogeneity of the applied magnetic field, provided the presence of such inhomogeneity is already known. Furthermore, it possesses the ability to determine the direction of the applied magnetic field. The results highlight the versatility of the proposed sensor in meeting various magnetic field sensing needs, and we anticipate that such a micro-cavity sensor will have promising applications in highly sensitive optical sensing.

Enhanced sensing resolution with microcavity mode oscillation generated by thermal-optic and photorefractive nonlinearity

Optical microcavity has proven its potential for unlabeled sensing. Here, we propose and demonstrate in a lithium niobate on insulator microcavity an enhanced sensing approach enabled by the nonlinear mode oscillation generated by the competition between thermal-optic and photorefractive effect, which breaks the intrinsic limitation in wavelength resolution set by the cavity's optical quality factor. It allows us to perform precise measurements of the mode shifting introduced by a nanoscale scatterer with a signal to noise ratio of 13.1 dB and paves a distinctive way to improve resonance shift resolution in widely studied microcavity sensors with a platform of great integration capability.

Enhanced Temperature Sensing by Multi‐Mode Coupling in an On‐Chip Microcavity System

AbstractThe microcavity is a promising sensor platform, but any perturbation will disturb its linewidth, cause resonance shift or splitting. However, such sensing resolution is limited by the cavity's optical quality factor and mode volume. Here, an on‐chip integrated microcavity system is proposed and demonstrated that the resolution of a self‐referenced sensor can be enhanced with multi‐mode coupling. In experiments, inter‐mode coupling strength is carefully optimized with a pulley waveguide and observed a resolution improvement of nearly 3 times in the frequency domain. While experiencing small refractive index change tuned by temperature, mode‐coupled system shows a 7.2 times sensitivity enhancement that is than the uncoupled system on the same chip and a very significant lineshape contrast ratio change as great reference for minor frequency shifts. This approach will help design microcavity sensors to improve detection sensitivity and resolution under limited manufacture precision.

Pentaerythritol tetraacrylate microcavity sensor for high performance relative humidity detection

A pentaerythritol tetraacrylate (PETA) microcavity sensor for high performance relative humidity (RH) detection is presented. The fiber tip all-polymer microcavity, with a maximum size of 99.27 µm, consists of a circular truncated cone on the fiber tip end surface and is manufactured by photopolymerization of PETA using two-photon polymerization technology, which is characterized by miniaturization and rapid manufacturing. Experimental results show that its RH sensitivity reaches 108.05 pm/%RH in the range of 14.7–84.3%RH. In particular, respiration monitoring experiments show that its response time and recovery time are 0.55 s and 2.5 s, respectively. The sensor shows the inherent advantages of compact structure, fast response, good repeatability, low hysteresis error, good stability, and high sensitivity. Therefore, it is a promising candidate for potential applications in chemical, medical and biological fields related to RH detection.

Engineering Porous Silicon-Based Microcavity for Chemical Sensing.

In this article, the authors theoretically and experimentally investigated ways to improve the efficiency of porous silicon (PS)-based optical microcavity sensors as a 1D/2D host matrix for electronic tongue/nose systems. The transfer matrix method was used to compute reflectance spectra of structures with different [nLnH] sets of low nL and high nH bilayer refractive indexes, the cavity position λc, and the number of bilayers Nbi. Sensor structures were prepared by electrochemically etching a silicon wafer. The kinetics of adsorption/desorption processes of ethanol-water-based solution was monitored in real time with a reflectivity probe-based setup. It was theoretically and experimentally demonstrated that the sensitivity of the microcavity sensor is higher for structures with refractive indexes in the lower range (and the corresponding porosity values in the upper range). The sensitivity is also improved for structures with the optical cavity mode (λc) adjusted toward longer wavelengths. The sensitivity of a distributed Bragg reflector (DBR) with cavity increases for a structure with cavity position λc in the long wavelength region. The full width at half maximum (fwhmc) of the microcavity is smaller and the quality factor of microcavity (Qc) is higher for the DBR with a larger number of structure layers Nbi. The experimental results are in good agreement with the simulated data. We believe that our results can help in developing rapid, sensitive, and reversible electronic tongue/nose sensing devices based on a PS host matrix.

Protein Concentration Sensor Based on Exceptional Points in Whispering-gallery Microdisks

Protein concentration detection is of great importance for biological research. Research on biosensors continuously always pursues higher sensitivity and interference immunity. If a system at exceptional point is subjected to a small perturbation, the two coupled modes will rapidly shift and split. The feature can be used for ultra-highly sensitive detection. Therefore, this paper proposes a whispering-gallery microdisk sensor at exceptional point that can be used for protein molecule concentration detection. The EP is introduced by a small hole in the solid microdisk. When microfluid containing protein flows through the small hole, the protein molecules will attach to the wall of the hole, resulting in a large complex frequency splitting. Thus, this system is extremely sensitive for protein concentration. Theoretical analysis shows that the sensitivity of the architecture is tens times higher than that of the conventional microcavity sensor.

Defect-assisted, spray-printed colloidal quantum dot microlasers for biosensing.

This study successfully implements spectrally distinguishable CdSe-ZnS core-shell colloidal quantum dot (CQD) microlasers by a simple, efficient spray printing technique and demonstrates its potential in biosensing. We have systematically characterized the optical properties of printed microring lasers with diameters less than 60 µm. The smallest structure that can be excited has a diameter as small as 30 µm, which is much smaller than the counterparts prepared by piezoelectric ink-jet printing. The detection sensitivity of 4.54 nm/min/refractive index unit is verified in glucose sensing using a printed CQD microlaser. Biosensing of diverse glucose and bovine serum albumin solutions using printed microlasers with the assistance of defects demonstrates a new, to the best of our knowledge, prototype for the development of high-performance, low-cost on-chip microcavity sensors.

Sensitivity Dependence of Single Nanoparticle Mass Detection Using Mechanical Oscillations in Optical Microcavities

Single nanoparticle detection is demanding in fields such as early-stage diagnostics, environmental monitoring and biochemical researches. Microcavities are becoming excellent platforms for ultrasensitive detection due to the extremely strong enhancement of light-matter interactions. However, the analytes to be detected will introduce optical losses, which eventually spoils the detection limit of the optical sensor. The strong light confinement enables optical induced mechanical oscillations that is also sensitive to analyte attachments and can thus be applied as an alternative sensing signal in microcavity sensors. In this work, the mass sensitivities of the mechanical oscillations of a microcavity are theoretically discussed. The sensitivity dependence on different modes, analyte sensing position and microcavity structure are explored via finite-element-method simulations, which are consistent with theoretical predictions. Our results provide an effective guideline for the sensitivity optimization during microcavity design aiming for single nanoparticle mass detection.

Label-Free LSPR-Vertical Microcavity Biosensor for On-Site SARS-CoV-2 Detection.

Cost-effective, rapid, and sensitive detection of SARS-CoV-2, in high-throughput, is crucial in controlling the COVID-19 epidemic. In this study, we proposed a vertical microcavity and localized surface plasmon resonance hybrid biosensor for SARS-CoV-2 detection in artificial saliva and assessed its efficacy. The proposed biosensor monitors the valley shifts in the reflectance spectrum, as induced by changes in the refractive index within the proximity of the sensor surface. A low-cost and fast method was developed to form nanoporous gold (NPG) with different surface morphologies on the vertical microcavity wafer, followed by immobilization with the SARS-CoV-2 antibody for capturing the virus. Modeling and simulation were conducted to optimize the microcavity structure and the NPG parameters. Simulation results revealed that NPG-deposited sensors performed better in resonance quality and in sensitivity compared to gold-deposited and pure microcavity sensors. The experiment confirmed the effect of NPG surface morphology on the biosensor sensitivity as demonstrated by simulation. Pre-clinical validation revealed that 40% porosity led to the highest sensitivity for SARS-CoV-2 pseudovirus at 319 copies/mL in artificial saliva. The proposed automatic biosensing system delivered the results of 100 samples within 30 min, demonstrating its potential for on-site coronavirus detection with sufficient sensitivity.

Single Virus Detection on Silicon Photonic Crystal Random Cavities.

On-chip silicon microcavity sensors are advantageous for the detection of virus and biomolecules due to their compactness and the enhanced light-matter interaction with the analyte. While their theoretical sensitivity is at the single-molecule level, the fabrication of high quality (Q) factor silicon cavities and their integration with optical couplers remain as major hurdles in applications such as single virus detection. Here, label-free single virus detection using silicon photonic crystal random cavities is proposed and demonstrated. The sensor chips consist of free-standing silicon photonic crystal waveguides and do not require pre-fabricated defect cavities or optical couplers. Residual fabrication disorder results in Anderson-localized cavity modes which are excited by a free space beam. The Q≈105 is sufficient for observing discrete step-changes in resonance wavelength for the binding of single adenoviruses (≈50nm radius). The authors' findings point to future applications of CMOS-compatible silicon sensor chips supporting Anderson-localized modes that have detection capabilities at the level of single nanoparticles and molecules.

Self-referred microcavity-based fused-fiber fabry-perot refractometer

In this paper, a fiber-optic Fabry-Perot microcavity sensor is presented based on splicing a catastrophic fused fiber (CFF) to a single-mode fiber (SMF) for the simultaneous measurement of the refractive index (RI) and temperature. CFF formed by the catastrophic fuse effect consists of a periodic void in the fiber core that creates a teardrop shape or elliptical air microcavity after splicing. By simulation, the elliptical microcavity sensor is selected to measure external RI changes by Fast Fourier Transform (FFT) analysis. The RI sensitivity was obtained to be −2.5 ± 0.0074/RIU with a resolution of about 4 × 10−5 RIU. Also, the experimental results showed that temperature is obtained independently from the FFT parameter without being affected by the RI. The sensor temperature was determined by tracking the wavelength shift of the interference spectrum. Consequently, the temperature sensitivity of 7 pm/°C in the range of 20 °C to 95 °C was achieved. Therefore, the RI and the temperature were simultaneously measured by analyzing the spatial frequency and wavelength shift. The proposed dual-microcavity FP sensor can compensate for any power fluctuation in the input light source.

Microcavity Sensor Enhanced by Spontaneous Chiral Symmetry Breaking

An optical microcavity provides a prominent platform for single-nanoparticle detection with ultrahigh sensitivity. Recently, microcavity sensors working at the exceptional point have caught great attention due to their ability to enhance sensitivity, but suffer from simultaneously amplified noise. Here, we propose employing a nonlinear microcavity to enhance the sensitivity of single-nanoparticle detection based on spontaneous chiral symmetry breaking. It is found that sensors operating at the symmetry-breaking threshold experience a gigantic enhancement in sensitivity, which is caused by the square-root response to perturbation in a lossless microcavity. Through the analysis for a realistic microcavity, a 30-fold sensitivity enhancement is demonstrated by working at the threshold, and the enhancement is also confirmed by numerical simulation. Furthermore, the noise performance is analyzed to be superior in thermorefractive noise and quantum noise performance in comparison to a conventional microcavity-sensing scheme. Merging the spontaneous chiral symmetry-breaking effect with practical sensing applications, the results pave a universal way for high-performance microcavity sensing with enhanced sensitivity and reduced noise.

Polymer Optical Microcavity Sensor for Volatile Organic Compounds with Distinct Selectivity toward Aromatic Hydrocarbons

A whispering-gallery mode (WGM) optical resonance sensor for volatile organic compounds (VOCs) is developed from polystyrene (PS) microspheres doped with fluorescent β-cyano-appended oligo(p-phenylenevinylene) (β-COPV). The β-COPV-doped PS microspheres (MSCOPV) are formed by the miniemulsion method in a binary solvent. MSCOPV expand upon permeation of VOCs into the PS matrix and exhibit a spectral shift of the WGM resonance peak. The permeation efficiency is highly dependent on the chemical affinity between the analyte and the polymer matrix, with exceptionally high selectivity toward aromatic hydrocarbons such as benzene, toluene, and xylenes (BTXs). The high selectivity and sensitivity of MSCOPV are in clear contrast to those of conventional WGM sensors that just detect VOCs nonpreferentially through adsorption onto the surface.

A sensitivity-enhanced temperature sensor with end-coated PDMS in few mode fiber based on vernier effect

In this paper, a few mode fiber temperature sensor based on vernier effect is proposed. The sensor consists of two cascaded microcavities and realizes temperature sensing based on vernier effect. The single mode fiber(SMF) and few mode fiber(FMF) are fixed by a capillary glass tube to form an air reference microcavity. The end of the FMF is dipped with an appropriate amount of polydimethylsiloxane(PDMS) to form a sensing microcavity with a certain thickness, which is an air microcavity-FMF–PDMS structure. Due to the different refractive index of each medium, the light is reflected on the four critical surfaces, and the four reflected light beams interfere, the vernier effect is formed by adjusting the microcavity length to a suitable length. When the external temperature changes, the microcavity length and refractive index of PDMS change, which leads to the change of interference spectrum. Therefore, the high sensitivity measurement of temperature can be realized by the change of reflection spectrum. The temperature characteristics of the sensor are studied experimentally. The results show that the temperature sensitivity is significantly improved compared with the traditional single microcavity sensor. In the temperature range of 35°C∼45°C, the temperature sensitivity can reach 4.7nm/°C, and the linearity is very good. The sensor has the advantages of compact structure, simple fabrication and high sensitivity, so it has wide application prospects in the field of temperature sensing.

Bubble microcavity strain and gravity sensor with temperature and bending insensitivity using an ultra-thin core optical fiber

Recent progress in optical fiber sensors based on Fabry-Perot interferometers (FPIs) has achieved much attention. In this paper, we designed and fabricated an FPI sensor by building a bubble in an ultra-thin core optical fiber. The size of the bubble decreased gradually with further arc discharges, which was inverse to the bubble fabrication in standard single mode fibers and multimode fibers. The bubble microcavity in the ultra-thin core optical fiber demonstrated a strain sensitivity of 2.08 pm/με in the range of 0–5000 με and a gravity sensitivity of 2.908 nm/N in the range of 0–4.89 N with high linear responses. Meanwhile, the bubble microcavity sensor was insensitive to temperature and bending, which was benefit to avoid the cross influences. The FPI sensor based on a bubble microcavity in an ultra-thin core optical fiber possesses the properties of easy to fabricate, small size, robustness, and high sensitivity.

Experimental Demonstration of Multimode Microresonator Sensing by Machine Learning

A multimode microcavity sensor based on a self-interference microring resonator is demonstrated experimentally. The proposed multimode sensing method is implemented by recording wide band transmission spectra that consist of multiple resonant modes. It is different from previous dissipative sensing scheme, which aims at measuring the transmission depth changes of a single resonant mode in a microcavity. Here, by combining the dissipative sensing mechanism and the machine learning algorithm, the multimode sensing information extracted from a broadband spectrum can be efficiently fused to estimate the target parameter. The multimode sensing method is immune to laser frequency noises and robust against system imperfection, thus our work presents a great step towards practical applications of microcavity sensors outside the research laboratory. The voltage applied across the microheater on the chip was adjusted to bring its influence on transmittance through the thermo-optic effects. As a proof-of-principle experiment, the voltage was detected by the multimode sensing approach. The experimental results demonstrate that the limit of detection of the multimode sensing by the general regression neural network is reduced to 6.7% of that of single-mode sensing within a large measuring range.

Gas identification in high-Q microbubble resonators.

A new, to the best of our knowledge, experimental mechanism is reported to realize the identification of gas by a microcavity sensor. Instead of measuring the change in the environment refractive index or absorption, the gas is detected indirectly and indentified by using the thermo-optics effect of a high-quality-factor microbubble resonator. When passing gas through the microbubble, the pressure induces a geometric deformation and thus an observable frequency shift, and the thermal bistability response varies due to the higher heat dissipation by gas molecules. With the two output parameters, we can unambiguously distinguish gas with different molecular weights, e.g., He, N2, and CO2. Our demonstration opens a new avenue of microcavity sensing by using indirect interaction between light and matter, realizing a multiple-parameter sensing approach for gas or solvent identification.

Measurement of water content in heavy oil with cavity resonator

The study of optical microcavity sensors in current micro optical electromechanical systems (MOEMS) devices is an important research direction in this field. In this study, a surface plasmon optical waveguide structure with MIM (Metal Insulator Metal), semi-circular resonator, and T-shaped resonator was examined. The effects of the coupling distance, the geometrical size of the T-shaped cavity and its asymmetry, and the radius of the semi-circular cavity on the Fano resonance spectrum were analyzed numerically. The sensitivity of the refractive index sensor was up to1066.67nm/RIU. The detection of the water content of the heavy oil found that the Fano resonance line shifted to the short-wave direction as the volume percentage of the contained water increased. The volume percentage resolution of the sensor's heavy oil water content could be as low as2.05×10-9. The proposed structure can overcome the shortcomings of common refractive index sensors and therefore has great application prospects in chemical and biological sensing.

Side-coupled liquid sensor and its array with magneto-optical photonic crystal.

In this paper, a side-coupled liquid sensor and its array based on magneto-optical photonic crystal are proposed. The sensitivity and quality factor of a single magneto-optical photonic crystal sensor are 0.492 mm/RIU and 181,760, respectively. After arraying, the sensitivity of the single microcavity sensor can reach from 0.1 to 0.35 mm/RIU, and the corresponding quality factor can reach from 5×104 to 7×104. Moreover, the structure has good robustness. This work will provide reference for the design of high-performance photonic crystal microcavity sensor arrays.

Highly Sensitive Photoacoustic Microcavity Gas Sensor for Leak Detection.

A highly sensitive photoacoustic (PA) microcavity gas sensor for leak detection is proposed. The miniature and low-cost gas sensor mainly consisted of a micro-electro-mechanical system (MEMS) microphone and a stainless-steel capillary with two small holes opened on the side wall. Different from traditional PA sensors, the designed low-power sensor had no gas valves and pumps. Gas could diffuse into the stainless-steel PA microcavity from two holes. The volume of the cavity in the sensor was only 7.9 μL. We use a 1650.96 nm distributed feedback (DFB) laser and the second-harmonic wavelength modulation spectroscopy (2f-WMS) method to measure PA signals. The measurement result of diffused methane (CH4) gas shows a response time of 5.8 s and a recovery time of 5.2 s. The detection limit was achieved at 1.7 ppm with a 1-s lock-in integral time. In addition, the calculated normalized noise equivalent absorption (NNEA) coefficient was 1.2 × 10−8 W·cm−1·Hz−1/2. The designed PA microcavity sensor can be used for the early warning of gas leakage.