Optimized sensitivity of Silicon-on-Insulator (SOI) strip waveguide resonator sensor

Theoretical and Experimental Analysis of Single-Arm Bimodal Plasmo-Photonic Refractive Index Sensors

In this paper, we study both theoretically and experimentally the sensitivity of bimodal interferometric sensors where interference occurs between two plasmonic modes with different properties propagating in the same physical waveguide. In contrast to the well-known Mach-Zehnder interferometric (MZI) sensor, we show for the first time that the sensitivity of the bimodal sensor is independent of the sensing area length. This is validated by applying the theory to an integrated plasmo-photonic bimodal sensor that comprises an aluminum (Al) plasmonic stripe waveguide co-integrated between two accessible SU-8 photonic waveguides. A series of such bimodal sensors utilizing plasmonic stripes of different lengths were numerically simulated, demonstrating bulk refractive index (RI) sensitivities around 5700 nm/RIU for all sensor variants, confirming the theoretical results. The theoretical and numerical results were also validated experimentally through chip-level RI sensing experiments on three fabricated SU-8/Al bimodal sensors with plasmonic sensing lengths of 50, 75, and 100 μm. The obtained experimental RI sensitivities were found to be very close and equal to 4464, 4386, and 4362 nm/RIU, respectively, confirming that the sensing length has no effect on the bimodal sensor sensitivity. The above outcome alleviates the design and optical loss constraints, paving the way for more compact and powerful sensors that can achieve high sensitivity values at ultra-short sensing lengths.

Interferometer-based chemical sensor on chip with enhanced responsivity and low-cost interrogation

We report experimental results of an interferometric chemical sensor integrated on a silicon chip. The sensor measures refractive index variations of the liquid that contacts exposed spiraled silicon waveguides on one branch of a Mach-Zehnder interferometer. The system requires neither laser tuning nor spectral analysis, but a laser at a fixed wavelength, and a demodulation architecture that includes an internal phase modulator and a real-time processing algorithm based on multitone mixing. Two devices are compared in terms of sensitivity and noise, one at 1550 nm wavelength and TE polarization, and an optimized device at 1310 nm and TM polarization, which shows 3 times higher sensitivity and a limit of detection of 2.24·10-7 RIU.

Sidewall grating slot waveguide microring resonator biochemical sensor

Integrated microring resonator structures based on silicon-on-insulator (SOI) platforms are promising candidates for high-performance on-chip sensing. In this work, a novel sidewall grating slot microring resonator (SG-SMRR) with a compact size (5 µm center radius) based on the SOI platform is proposed and demonstrated experimentally. The experiment results show that the refractive index (RI) sensitivity and the limit of detection value are 620 nm/RIU and 1.4 × 10-4 RIU, respectively. The concentration sensitivity and minimum concentration detection limit are 1120 pm/% and 0.05%, respectively. Moreover, the sidewall grating structure makes this sensor free of free spectral range (FSR) limitation. The detection range is significantly enlarged to 84.5 nm in lab measurement, four times that of the FSR of conventional SMRRs. The measured Q-factor is 3.1 × 103, and the straight slot waveguide transmission loss is 24.2 dB/cm under sensing conditions. These results combined with the small form factor associated with a silicon photonics sensor open up applications where high sensitivity and large measurement range are essential.

Double slot micro ring resonators with inner wall angular gratings as ultra-sensitive biochemical sensors

We simulate and demonstrate experimentally an inner-wall grating double slot micro ring resonator (IG-DSMRR) with a center slot ring radius of only 6.72 µm based on the silicon-on-insulator platform. This novel photonic-integrated sensor for optical label-free biochemical analysis boosts the measured refractive index (RI) sensitivity in glucose solutions to 563 nm/RIU with the limit of detection value being 3.7 × 10^-6 RIU (refractive index units). The concentration sensitivity for sodium chloride solutions can reach 981 pm/%, with a minimum concentration detection limit of 0.02%. Using the combination of DSMRR and IG, the detection range is enlarged significantly to 72.62 nm, three times the free spectral range of conventional slot micro ring resonators. The measured Q-factor is 1.6 × 10⁴, and the straight strip and double slot waveguide transmission losses are 0.9 dB/cm and 20.2 dB/cm, respectively. This IG-DSMRR combines the advantages of a micro ring resonator, slot waveguide, and angular grating and is highly desirable for biochemical sensing in liquids and gases offering an ultra-high sensitivity and ultra-large measurement range. This is the first report of a fabricated and measured double-slot micro ring resonator with an inner sidewall grating structure.

A Review on Photonic Sensing Technologies: Status and Outlook

In contemporary science and technology, photonic sensors are essential. They may be made to be extremely resistant to some physical parameters while also being extremely sensitive to other physical variables. Most photonic sensors may be incorporated on chips and operate with CMOS technology, making them suitable for use as extremely sensitive, compact, and affordable sensors. Photonic sensors can detect electromagnetic (EM) wave changes and convert them into an electric signal due to the photoelectric effect. Depending on the requirements, scientists have found ways to develop photonic sensors based on several interesting platforms. In this work, we extensively review the most generally utilized photonic sensors for detecting vital environmental parameters and personal health care. These sensing systems include optical waveguides, optical fibers, plasmonics, metasurfaces, and photonic crystals. Various aspects of light are used to investigate the transmission or reflection spectra of photonic sensors. In general, resonant cavity or grating-based sensor configurations that work on wavelength interrogation methods are preferred, so these sensor types are mostly presented. We believe that this paper will provide insight into the novel types of available photonic sensors.

Optimization of bulk sensitivity for strip, slot, and subwavelength grating-based waveguides for dual-polarization operation

We propose a dual-polarization optimization method for the bulk sensitivity of silicon-on-insulator (SOI) waveguides by defining a multi-objective function that accounts for the substrate leakage losses. The proposed optimization method was used to design micro-ring resonator bulk sensors with strip, slot, subwavelength grating, and subwavelength grating slot waveguides. The subwavelength grating slot waveguide has a bulk sensitivity of 520 nm/RIU and 325 nm/RIU for the TE and TM modes, respectively, both of which are higher than the bulk sensitivities of strip, slot, and subwavelength grating waveguides. Moreover, our Monte Carlo analysis shows that the subwavelength grating slot waveguide has the highest immunity to fabrication errors.

SNROW-based highly sensitive label-free surface biosensor for hepatitis B detection

Despite the availability of effective hepatitis B vaccinations, the hepatitis B virus remains a serious global health concern. It is expected that early detection could aid in initiating therapy before the infection progresses to liver damage. A silicon nanowire rectangular optical waveguide has been demonstrated theoretically to detect the surface antigen of hepatitis B "HBsAg" based on label-free surface sensing using finite-element method-based COMSOL Multiphysics. Different procedural segments of the biomarker detection have been mimicked on the surface of a waveguide as adlayers to investigate the device theoretically. Initially, the parameters of the waveguide have been optimized to provide a large interaction of light and bio-analyte, i.e., to provide high sensitivity. The analyses are first performed at the waveguide level based on the light-analyte interaction. Furthermore, performances of the sensor have been obtained by incorporating this waveguide structure in the sensing arm of the Mach-Zehnder interferometer. The device structure shows ultra-high surface sensitivities such as phase surface sensitivity of 7.03×2πrad/nm and MZI surface sensitivity of 3421.89 µW/nm with an excellent detection limit of 2.92×10^-3pg/mm² for HBsAg detection. The proposed device can measure the HBsAg concentration as low as 0.00973 ng/mL, which is significantly low to detect the infection in an early stage.

Hollow-Core Fiber-Based Biosensor: A Platform for Lab-in-Fiber Optical Biosensors for DNA Detection

In this paper, a novel platform for lab-in-fiber-based biosensors is studied. Hollow-core tube lattice fibers (HC-TLFs) are proposed as a label-free biosensor for the detection of DNA molecules. The particular light-guiding mechanism makes them a highly sensitive tool. Their transmission spectrum is featured by alternations of high and low transmittance at wavelength regions whose values depend on the thickness of the microstructured web composing the cladding around the hollow core. In order to achieve DNA detection by using these fibers, an internal chemical functionalization process of the fiber has been performed in five steps in order to link specific peptide nucleic acid (PNA) probes, then the functionalized fiber was used for a three-step assay. When a solution containing a particular DNA sequence is made to flow through the HC of the TLF in an ‘optofluidic’ format, a bio-layer is formed on the cladding surfaces causing a red-shift of the fiber transmission spectrum. By comparing the fiber transmission spectra before and after the flowing it is possible to identify the eventual formation of the layer and, therefore, the presence or not of a particular DNA sequence in the solution.

Surface Plasmon Resonance (SPR) Spectroscopy and Photonic Integrated Circuit (PIC) Biosensors: A Comparative Review

Label-free direct-optical biosensors such as surface-plasmon resonance (SPR) spectroscopy has become a gold standard in biochemical analytics in centralized laboratories. Biosensors based on photonic integrated circuits (PIC) are based on the same physical sensing mechanism: evanescent field sensing. PIC-based biosensors can play an important role in healthcare, especially for point-of-care diagnostics, if challenges for a transfer from research laboratory to industrial applications can be overcome. Research is at this threshold, which presents a great opportunity for innovative on-site analyses in the health and environmental sectors. A deeper understanding of the innovative PIC technology is possible by comparing it with the well-established SPR spectroscopy. In this work, we shortly introduce both technologies and reveal similarities and differences. Further, we review some latest advances and compare both technologies in terms of surface functionalization and sensor performance.

Mode Sensitivity Exploration of Silica-Titania Waveguide for Refractive Index Sensing Applications

In this paper, a novel and cost-effective photonic platform based on silica–titania material is discussed. The silica–titania thin films were grown utilizing the sol–gel dip-coating method and characterized with the help of the prism-insertion technique. Afterwards, the mode sensitivity analysis of the silica–titania ridge waveguide is investigated via the finite element method. Silica–titania waveguide systems are highly attractive due to their ease of development, low fabrication cost, low propagation losses and operation in both visible and near-infrared wavelength ranges. Finally, a ring resonator (RR) sensor device was modelled for refractive index sensing applications, offering a sensitivity of 230 nm/RIU, a figure of merit (FOM) of 418.2 RIU⁻¹, and Q-factor of 2247.5 at the improved geometric parameters. We believe that the abovementioned integrated photonics platform is highly suitable for high-performance and economically reasonable optical sensing devices.

Performance improvement of a silicon nitride ring resonator biosensor operated in the TM mode at 1310 nm

Silicon-based ring resonators have been demonstrated to be a key element to build lab-on-chip devices due to their ability to perform as label-free photonic sensors. In this work, we demonstrate photonic biosensing using silicon nitride ring resonators operated in the TM mode around 1310 nm wavelengths. Our results show that operating the devices using the TM mode results in an increased sensitivity in comparison with the typically used TE mode, while working at 1310 nm wavelengths compared to 1550 nm contributes to an increased quality factor. As a result, a reduction in the intrinsic limit of detection is achieved, indicating the suitability of TM modes in the 1310 nm regime for biosensing using integrated photonics.

A Numerical Investigation of a Plasmonic Sensor Based on a Metal-Insulator-Metal Waveguide for Simultaneous Detection of Biological Analytes and Ambient Temperature

A multipurpose plasmonic sensor design based on a metal-insulator-metal (MIM) waveguide is numerically investigated in this paper. The proposed design can be instantaneously employed for biosensing and temperature sensing applications. The sensor consists of two simple resonant cavities having a square and circular shape, with the side coupled to an MIM bus waveguide. For biosensing operation, the analytes can be injected into the square cavity while a thermo-optic polymer is deposited in the circular cavity, which provides a shift in resonance wavelength according to the variation in ambient temperature. Both sensing processes work independently. Each cavity provides a resonance dip at a distinct position in the transmission spectrum of the sensor, which does not obscure the analysis process. Such a simple configuration embedded in the single-chip can potentially provide a sensitivity of 700 nm/RIU and -0.35 nm/°C for biosensing and temperature sensing, respectively. Furthermore, the figure of merit (FOM) for the biosensing module and temperature sensing module is around 21.9 and 0.008, respectively. FOM is the ratio between the sensitivity of the device and width of the resonance dip. We suppose that the suggested sensor design can be valuable in twofold ways: (i) in the scenarios where the testing of the biological analytes should be conducted in a controlled temperature environment and (ii) for reducing the influence on ambient temperature fluctuations on refractometric measurements in real-time mode.

Post-fabrication tuning of microring resonators using 3D-printed microfluidics

We demonstrate a method of tuning the resonant frequencies of silicon microring resonators using a 3D-printed microfluidic chip overlaid directly on the photonic circuit with zero energy consumption following the initial tuning. Aqueous solutions with different concentrations of NaCl are used in experimentation. A shift of a full free spectral range is observed at a concentration of 10% NaCl. On a 60 µm microring resonator, this equals a resonant wavelength shift of 1.514 nm when the index of the cladding changes by 0.017 refractive index units (RIUs), or at a rate of 89.05 nm/RIU.

Double-peak one-dimensional photonic crystal cavity in parallel configuration for temperature self-compensation in sensing

We designed and demonstrated a double-peak one-dimensional photonic crystal (1D PhC) cavity device by integrating two 1D PhCs cavities in a parallel configuration. The device design is proposed so that it can be used for bio-sensing purposes and has a self-compensation ability to reduce the measurement error caused by the change of the surrounding temperature. By combining two light resonances, two resonance peaks are obtained. The peak's separation, which gives the initial value for a sensing system, can be controlled by varying the cavity length difference (Δc) between the first and second 1D PhCs in parallel. Then, by making one arm of the device as the reference arm and the other arm as the sensing arm, the temperature self-compensation device can be realized. The design and simulation of this device are done by using Lumerical software, which are Lumerical MODE, Lumerical finite-difference time-domain, and Lumerical Interconnect. Electron-beam-lithography and deep reactive-ion-etching processes were used for device fabrication. The experimental results show the controllable peaks' separation, which solves the double-peak requirement for a temperature self-compensated bio-sensor design.

Hyperchromatic structural color for perceptually enhanced sensing by the naked eye

Colorimetric sensors offer the prospect for on-demand sensing diagnostics in simple and low-cost form factors, enabling rapid spatiotemporal inspection by digital cameras or the naked eye. However, realizing strong dynamic color variations in response to small changes in sample properties has remained a considerable challenge, which is often pursued through the use of highly responsive materials under broadband illumination. In this work, we demonstrate a general colorimetric sensing technique that overcomes the performance limitations of existing chromatic and luminance-based sensing techniques. Our approach combines structural color optical filters as sensing elements alongside a multichromatic laser illuminant. We experimentally demonstrate our approach in the context of label-free biosensing and achieve ultrasensitive and perceptually enhanced chromatic color changes in response to refractive index changes and small molecule surface attachment. Using structurally enabled chromaticity variations, the human eye is able to resolve ∼0.1-nm spectral shifts with low-quality factor (e.g., Q ∼ 15) structural filters. This enables spatially resolved biosensing in large area (approximately centimeters squared) lithography-free sensing films with a naked eye limit of detection of ∼3 pg/mm², lower than industry standard sensors based on surface plasmon resonance that require spectral or angular interrogation. This work highlights the key roles played by both the choice of illuminant and design of structural color filter, and it offers a promising pathway for colorimetric devices to meet the strong demand for high-performance, rapid, and portable (or point-of-care) diagnostic sensors in applications spanning from biomedicine to environmental/structural monitoring.

Ultra-sensitive refractive index sensor using CMOS plasmonic transducers on silicon photonic interferometric platform

Optical refractive-index sensors exploiting selective co-integration of plasmonics with silicon photonics has emerged as an attractive technology for biosensing applications that can unleash unprecedented performance breakthroughs that reaps the benefits of both technologies. However, towards this direction, a major challenge remains their integration using exclusively CMOS-compatible materials. In this context, herein, we demonstrate, for the first time to our knowledge, a CMOS-compatible plasmo-photonic Mach-Zehnder-interferometer (MZI) based on aluminum and Si3N4 waveguides, exhibiting record-high bulk sensitivity of 4764 nm/RIU with clear potential to scale up the bulk sensitivity values by properly engineering the design parameters of the MZI. The proposed sensor is composed of Si3N4 waveguides butt-coupled with an aluminum stripe in one branch to realize the sensing transducer. The reference arm is built by Si3N4 waveguides, incorporating a thermo-optic phase shifter followed by an MZI-based variable optical attenuation stage to maximize extinction ratio up to 38 dB, hence optimizing the overall sensing performance. The proposed sensor exhibits the highest bulk sensitivity among all plasmo-photonic counterparts, while complying with CMOS manufacturing standards, enabling volume manufacturing.

Enhanced Sensitivity of Microring Resonator-Based Sensors Using the Finite Difference Time Domain Method to Detect Glucose Levels for Diabetes Monitoring

The properties of light and its interaction with biological analytes have made it possible to design sophisticated and reliable optical-based biomedical sensors. In this paper, we report the simulation, design, and fabrication of microring resonator (MRR)-based sensors for the detection of diabetic glucose levels. Electron Beam Lithography (EBL) with 1:1 hydrogen silsesquioxane (HSQ) negative tone resist were used to fabricate MRR on a Silicon-on-Insulator (SOI) platform. Scanning Electron Microscopy (SEM) was then used to characterize the morphology of the MRR device. The full-width at half-maximum (FWHM) and quality factors of MRR were obtained by using a tunable laser source (TLS) and optical spectrum analyzer (OSA). In this paper, the three-dimensional Finite Difference Time Domain (3D FDTD) approach has been used to simulate the proposed design. The simulation results show an accurate approximation with the experimental results. Next, the sensitivity of MRR-based sensors to detect glucose levels is obtained. The sensitivity value for glucose level detection in the range 0% to 18% is 69.44 nm/RIU. This proved that our MRR design has a great potential as a sensor to detect diabetic glucose levels.

Subwavelength Grating Double Slot Waveguide Racetrack Ring Resonator for Refractive Index Sensing Application

In this paper, a racetrack ring resonator design based on a subwavelength grating double slot waveguide is presented. The proposed waveguide scheme is capable of confining the transverse electric field in the slots and the gaps between the grating segments. This configuration facilitates a large light-matter interaction which elevates the sensitivity of the device approximately 2.5 times higher than the one that can be obtained via a standard slot waveguide resonator. The best sensitivity of the design is obtained at 1000 nm/RIU by utilizing a subwavelength grating double slot waveguide of period 300 nm. The numerical study is conducted via 2D and 3D finite element methods. We believe that the proposed sensor design can play an important role in the realization of highly sensitive lab-on-chip sensors.

Suspended slotted photonic crystal cavities for high-sensitivity refractive index sensing

We present the design, fabrication, and characterization of a suspended slotted photonic crystal (SSPhC) cavity sensor based on the silicon-on-insulator platform. The sensing performance can be dramatically enhanced by the optimized SSPhC cavity as most of the light energy can be distributed in the low index region (∼57%). By measuring the spectrum response of the cavity sensor immersed in NaCl solutions with different mass concentrations, an ultra-high sensitivity around 656 nm/RIU has been experimentally demonstrated. Furthermore, the whole size of the cavity sensor (including the grating couplers) is 320 × 40 µm², making the high-sensitivity device attractive for the realization of large-scale multi-channel on-chip sensors.

Progress of infrared guided-wave nanophotonic sensors and devices

Nanophotonics, manipulating light-matter interactions at the nanoscale, is an appealing technology for diversified biochemical and physical sensing applications. Guided-wave nanophotonics paves the way to miniaturize the sensors and realize on-chip integration of various photonic components, so as to realize chip-scale sensing systems for the future realization of the Internet of Things which requires the deployment of numerous sensor nodes. Starting from the popular CMOS-compatible silicon nanophotonics in the infrared, many infrared guided-wave nanophotonic sensors have been developed, showing the advantages of high sensitivity, low limit of detection, low crosstalk, strong detection multiplexing capability, immunity to electromagnetic interference, small footprint and low cost. In this review, we provide an overview of the recent progress of research on infrared guided-wave nanophotonic sensors. The sensor configurations, sensing mechanisms, sensing performances, performance improvement strategies, and system integrations are described. Future development directions are also proposed to overcome current technological obstacles toward industrialization.

Single-mode porous silicon waveguide interferometers with unity confinement factors for ultra-sensitive surface adlayer sensing

Guided wave-optics has emerged as a promising platform for label free biosensing. However, device sensitivity toward surface-bound small molecules is directly limited by the evanescent interaction and low confinement factor with the active sensing region. Here, we report a mesoporous silicon waveguide design and inverse fabrication technique that resolves the evanescent field interaction limitation while achieving maximal transverse confinement factors and preserving single-mode operation. The waveguide sensors are characterized in a Fabry-Perot interferometer configuration and the ultra-high sensitivity to small molecule adlayers is demonstrated. We also identify dispersion to be a promising degree of freedom for exceeding the sensitivity limits predicted by the conventional non-dispersive effective medium theory.

Multi-slot photonic crystal cavities for high-sensitivity refractive index sensing

We present the design, fabrication, and characterization of a multi-slot photonic crystal (PhC) cavity sensor on the silicon-on-insulator platform. By optimizing the structure of the PhC cavity, most of the light can be distributed in the lower index region; thus, the sensitivity can be dramatically improved. By exposing the cavities to different mass concentrations of NaCl solutions, we obtained that the wavelength shift per refractive index unit (RIU) for the sensor is 586 nm/RIU, which is one of the highest sensitivities achieved in a non-suspended cavity. Furthermore, the size of the sensing region of the reported sensor is only 22.8 μm × 1.5 μm, making the high-sensitivity PhC cavity sensor attractive for the realization of on-chip sensor arrays.

A Tellurium Oxide Microcavity Resonator Sensor Integrated On-Chip with a Silicon Waveguide

We report on thermal and evanescent field sensing from a tellurium oxide optical microcavity resonator on a silicon photonics platform. The on-chip resonator structure is fabricated using silicon-photonics-compatible processing steps and consists of a silicon-on-insulator waveguide next to a circular trench that is coated in a tellurium oxide film. We characterize the device’s sensitivity by both changing the temperature and coating water over the chip and measuring the corresponding shift in the cavity resonance wavelength for different tellurium oxide film thicknesses. We obtain a thermal sensitivity of up to 47 pm/°C and a limit of detection of 2.2 × 10⁻³ RIU for a device with an evanescent field sensitivity of 10.6 nm/RIU. These results demonstrate a promising approach to integrating tellurium oxide and other novel microcavity materials into silicon microphotonic circuits for new sensing applications.

Silicon Photonic Biosensors Using Label-Free Detection

Thanks to advanced semiconductor microfabrication technology, chip-scale integration and miniaturization of lab-on-a-chip components, silicon-based optical biosensors have made significant progress for the purpose of point-of-care diagnosis. In this review, we provide an overview of the state-of-the-art in evanescent field biosensing technologies including interferometer, microcavity, photonic crystal, and Bragg grating waveguide-based sensors. Their sensing mechanisms and sensor performances, as well as real biomarkers for label-free detection, are exhibited and compared. We also review the development of chip-level integration for lab-on-a-chip photonic sensing platforms, which consist of the optical sensing device, flow delivery system, optical input and readout equipment. At last, some advanced system-level complementary metal-oxide semiconductor (CMOS) chip packaging examples are presented, indicating the commercialization potential for the low cost, high yield, portable biosensing platform leveraging CMOS processes.

TDM-controlled ring resonator arrays for fast, fixed-wavelength optical biosensing

A novel control concept for serial ring resonator arrays based on a time-division multiplex (TDM) approach is presented. It allows fast sampling rates in terms of biological kinetics. The novelty consists of using both thermal tuning of the effective refractive index and thermo-optical multiplexing for the silicon-on-insulator (SOI) ring resonator arrays, without the need for a tunable laser source. Using a fixed wavelength, fast read-out rates of 100 Hz are demonstrated for each ring.

Silicon-on-insulator microring resonator sensor based on an amplitude comparison sensing function

A novel, highly sensitive integrated sensor based on a silicon-on-insulator microring resonator is proposed and experimentally demonstrated. To achieve a fast-response and cost-effective sensing system, the new structure establishes a linear amplitude comparison sensing function (ACSF) by monitoring the optical powers from both the through port and drop port of an add-drop microring resonator simultaneously, where the contrast of the two ports eliminates the effect of unexpected power fluctuation of the input laser on sensor performance. A highly enhanced linear relationship between the resonant wavelength shift and the ACSF value is achieved with an R-squared value over 0.99. A proof-of-concept experiment for temperature sensing demonstrates an almost constant ACSF with only ±0.9% discrepancy, while the laser power is varied between 0 dBm and -7  dBm.