High-sensitivity and wide-range optical sensor based on three cascaded ring resonators

In-situ detection of microplastics in the aquatic environment: A systematic literature review

Microplastics are ubiquitous in the aquatic environment and have emerged as a significant environmental issue due to their potential impacts on human health and the ecosystem. Current laboratory-based microplastic detection methods suffer from various drawbacks, including a lack of standardisation, limited spatial and temporal coverage, high costs, and time-consuming procedures. Consequently, there is a need for the development of in-situ techniques to detect and monitor microplastics to effectively identify and understand their sources, pathways, and behaviours. Herein, we adopt a systematic literature review method to assess the development and application of experimental and field technologies designed for the in-situ detection and monitoring of aquatic microplastics, without the need for sample preparation. Four scientific databases were searched in March 2023, resulting in a review of 62 relevant studies. These studies were classified into seven sensor categories and their working principles were discussed. The sensor classes include optical devices, digital holography, Raman spectroscopy, other spectroscopy, hyperspectral imaging, remote sensing, and other methods. We also looked at how data from these technologies are integrated with machine learning models to develop classifiers capable of accurately characterising the physical and chemical properties of microplastics and discriminating them from other particles. This review concluded that in-situ detection of microplastics in aquatic environments is feasible and can be achieved with high accuracy, even though the methods are still in the early stages of development. Nonetheless, further research is still needed to enhance the in-situ detection of microplastics. This includes exploring the possibility of combining various detection methods and developing robust machine-learning classifiers. Additionally, there is a recommendation for in-situ implementation of the reviewed methods to assess their effectiveness in detecting microplastics and identify their limitations.

Nested micro-ring refractive index sensor based on a subwavelength grating waveguide and the Vernier effect

In this paper, a nested micro-ring refractive index sensor based on a subwavelength grating waveguide and the Vernier effect is proposed. In this scheme, the nested micro-ring structure is combined with a subwavelength grating structure to enhance the contact area between the optical field and the analyte, and the wavelength offset is doubled through the Vernier effect. The proposed sensor can effectively increase sensing sensitivity, taking into account the improvement of the free spectral range. This structure enables the device to reach a sensitivity of 8030 nm/RIU near 1550 nm wavelength in a deionized water environment, with a detection limit of 5.659×10-5 RIU and free spectral range of 41.956 nm. The device suggested in this study has a greater reduced footprint than the conventional micro-ring resonant sensor, measuring just 35µm×25µm. Due to its high integration, high sensitivity, and large free spectral range compared to conventional micro-ring resonant sensors, such structures are of great value in biosensing and environmental monitoring.

Enhancing the Performance of the Photonic Integrated Sensing System by Applying Frequency Interrogation

Lab-on-a-chip systems are currently one of the most promising areas in the development of ultra-compact sensor systems, used primarily for gas and liquid analysis to determine the concentration of impurities. Integrated photonics is an ideal basis for designing "lab-on-a-chip" systems, advantageous for its compactness, energy efficiency, and low cost in mass production. This paper presents a solution for "lab-on-a-chip" device realization, consisting of a sensor and an interrogator based on a silicon-on-insulator (SOI) integrated photonics platform. The sensor function is performed by an all-pass microring resonator (MRR), installed as a notch filter in the feedback circuit of an optoelectronic oscillator based on an electro-optic phase modulator. This structure realizes the frequency interrogation of the sensor with high accuracy and speed using a conventional single-mode laser source. The system sensitivity for the considered gases is 13,000 GHz/RIU. The results show that the use of frequency interrogation makes it possible to increase the intrinsic LoD by five orders. The proposed solution opens an opportunity for fully integrated implementation of a photonic "laboratory-on-a-chip" unit.

Microring resonator based on polarization multiplexing for simultaneous sensing of refractive index and temperature on silicon platform

Silicon photonic integrated sensors based on microring resonators are a promising candidate to achieve high-performance on-chip sensing. In this work, a novel dual-parameters sensor based on polarization multiplexing on silicon-on-insulator (SOI) platform is proposed and demonstrated experimentally, simultaneously achieving refractive index (RI) and temperature sensing with high sensitivity and large detection range (DR). The experimental results show that the RI sensitivity and temperature sensitivity of the TM-operated sensor are 489.3 nm/RIU and 20.0 pm/°C, respectively, and that of the TE-operated sensor are 102.6 nm/RIU and 43.3 pm/°C, respectively. Moreover, the DR of the fabricated sensor is 0.0296 RIU, which is 4.2 times that of the conventional TM-operated sensor based on the microring resonator. The dual-parameters sensor based on polarization multiplexing can successfully realize the simultaneous measurement of the RI and the temperature, showing potential applications of silicon photonic on-chip sensors in reality.

Advancement in Silicon Integrated Photonics Technologies for Sensing Applications in Near-Infrared and Mid-Infrared Region: A Review

Exploration and implementation of silicon (Si) photonics has surged in recent years since both photonic component performance and photonic integration complexity have considerably improved. It supports a wide range of datacom and telecom applications, as well as sensors, including light detection and ranging, gyroscopes, biosensors, and spectrometers. The advantages of low-loss Si WGs with compact size and excellent uniformity, resulting from the high quality and maturity of the Si complementary metal oxide semiconductor (CMOS) environment, are major drivers for using Si in photonics. Moreover, it has a high refractive index and a reasonably large mid-infrared (MIR) transparency window, up to roughly 7 μm wavelength, making it beneficial as a passive mid-IR optical material. Several gases and compounds with high absorption properties in the MIR spectral region are of prodigious curiosity for industrial, medicinal, and environmental applications. In comparison to current bulky systems, the implementation of Si photonics devices in this wavelength range might allow inexpensive and small optical sensing devices with greater sensitivity (S), power usage, and mobility. In this review, recent advances in Si integrated photonic sensors working in both near-infrared (NIR) and MIR wavelength ranges are discussed. We believe that this paper will be valuable for the scientific community working on Si photonic sensing devices.

Wide-range, ultra-compact, and high-sensitivity ring resonator biochemical sensor with CMOS-compatible hybrid plasmonic waveguide

A ring resonator-based biochemistry sensor with a wide range, ultra-compact footprint, and high sensitivity is proposed, which utilizes a suspended slot hybrid plasmonic (SSHP) waveguide. The waveguide consists of a suspended Si nanowire separated from a Cu metal surface by a nanoscale air gap. The hybridization of fundamental mode of a Si channel waveguide with the surface plasmon polariton (SPP) mode of Cu-Si interface achieves a strong light confinement, high waveguide sensitivity (Sw), and low optical loss, showing a great potential in integrated optical sensor. The sensitivity, the detection limit and the detection range of the SSHP waveguide-based biochemistry sensor with a miniaturized radius of 1 µm are numerically demonstrated as 458.1 nm/RIU, 3.7 × 10^-5 RIU and 0.225 RIU, respectively. These superior performances as well as the fully CMOS compatibility enable the integrated optical sensing applications.

High sensitivity temperature sensor based on a side-hole fiber

This paper proposes a temperature sensor based on a side-hole fiber (SHF). The sensor is formed by single-mode fiber (SMF)-coreless fiber (CLF)-SHF-CLF-SMF fusion splicing. The SHF adopts the dislocation fusion splicing method to ensure that one air hole is exposed. Two different interferences form a superposition, making the response more sensitive. The experiment shows that the sensitivity during heating and cooling is 1.587 nm/°C and 1.681 nm/°C, respectively, in the temperature range of 25-45°C. The sensor has high temperature sensitivity, exhibits easy processing, is smaller in size, and has important research value for temperature monitoring in daily life and industrial production.

Releasing the light field in subwavelength grating slot microring resonators for athermal and sensing applications

Silicon-on-insulator (SOI) platforms have attracted increasing interest for photonic integrated devices with an ultra-small footprint. The distinct feature is the strong light confinement in the silicon region due to a high refractive-index-contrast. In contrast, releasing the light field out of the silicon region is also of great significance for providing a useful supplement to existing light guiding mechanisms and for facilitating versatile applications. Here, subwavelength grating slot (SWGS) microring resonators, which can effectively release light out of the silicon region for athermal and sensing applications, are proposed and demonstrated. The mechanism of releasing light relies on the combination of a surface enhanced supermode in a slot waveguide and a Bloch mode in a subwavelength grating waveguide. Four types of racetrack microring resonators (strip, slot, strip-SWGS, and slot-SWGS) were fabricated for comparison. The slot-SWGS microring resonator shows the best performance for athermal and sensing applications. The demonstrations may be useful for new releasing-light-enabled devices and applications.

Design Rule of Mach-Zehnder Interferometer Sensors for Ultra-High Sensitivity

A design rule for a Mach-Zehnder interferometer (MZI) sensor is presented, allowing tunable sensitivity by appropriately choosing the MZI arm lengths according to the formula given in this paper. The present MZI sensor designed by this method can achieve an ultra-high sensitivity, which is much higher than any other traditional MZI sensors. An example is given with silicon-on-insulator (SOI) nanowires and the device sensitivity is as high as 10⁶ nm/refractive-index -unit (or even higher), by choosing the MZI arms appropriately. This makes it possible for one to realize a low-cost optical sensing system with a detection limit as high as 10⁻⁶ refractive-index-unit, even when a cheap optical spectrum analyzer with low-resolution (e.g., 1 nm) is used for the wavelength-shift measurement.

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.

Vernier effect-based tunable mid-infrared sensor using silicon-on-insulator cascaded rings

Vernier effect has been captivated as a promising approach to achieve high-performance photonic sensors. However, experimental demonstration of such sensors in mid-infrared (MIR) range, which covers abundant absorption fingerprints of molecules, is still lacking. Here, we report Vernier effect-based thermally tunable photonic sensors using cascaded ring resonators fabricated on the silicon-on-insulator (SOI) platform. The radii and the coupling gaps in two rings are investigated as key design parameters. By applying organic liquids on our device, we observe an envelope shift of 48 nm with a sensitivity of 3000 nm/RIU and an intensity drop of 6.7 dB. Besides, our device can be thermally tuned with a sensitivity of 0.091 nm/mW. Leveraging the characteristic molecular absorption in the MIR, our work offers new possibilities for complex index sensing, which has wide applications in on-chip photonic sensors.

Mid-infrared optical sensing using sub-wavelength gratings

Optical sensing has shown great potential for both quantitative and qualitative analysis of compounds. In particular sensors which are capable of detecting changes in refractive index at a surface as well as in bulk material have received much attention. Much of the recent research has focused on developing technologies that enable such sensors to be deployed in an integrated photonic device. In this work we demonstrate experimentally, using a sub-wavelength grating the detection of ethanol in aqueous solution by interrogating its large absorption band at 9.54 μm. Theoretical investigation of the operating principle of our grating sensor shows that in general, as the total field interacting with the analyte is increased, the corresponding absorption is also increased. We also theoretically demonstrate how sub-wavelength gratings can detect changes in the real part of the refractive index, similar to conventional refractive index (RI) sensors.

Optical Biosensors Based on Silicon-On-Insulator Ring Resonators: A Review

Recent developments in optical biosensors based on integrated photonic devices are reviewed with a special emphasis on silicon-on-insulator ring resonators. The review is mainly devoted to the following aspects: (1) Principles of sensing mechanism, (2) sensor design, (3) biofunctionalization procedures for specific molecule detection and (4) system integration and measurement set-ups. The inherent challenges of implementing photonics-based biosensors to meet specific requirements of applications in medicine, food analysis, and environmental monitoring are discussed.

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.

High sensitivity optical biosensor based on polymer materials and using the Vernier effect

We demonstrate the fabrication of a Vernier effect SU8/PMATRIFE polymer optical biosensor with high homogeneous sensitivity using a standard photolithography process. The sensor is based on one micro-resonator embedded on each arm of a Mach-Zehnder interferometer. Measurements are based on the refractive index variation of the optical waveguide superstrate with different concentrations of glucose solutions. The sensitivity of the sensor has been measured as 17558 nm/RIU and the limit of detection has been estimated to 1.1.10^-6 RIU.