Ultrasensitive Silicon Photonic Refractive Index Sensor Based on Hybrid Double Slot Subwavelength Grating Microring Resonator.

Silicon photonic-based refractive index sensors are of great value in the detection of gases, biological and chemical substances. Among them, microring resonators are the most promising due to their compact size and narrow Lorentzian-shaped spectrum. The electric field in a subwavelength grating waveguide (SWG) is essentially confined in the low-refractive index dielectric, favoring enhanced analyte-photon interactions, which represents higher sensitivity. However, it is very challenging to further significantly improve the sensitivity of SWG ring resonator refractive index sensors. Here, a hybrid waveguide blocks double slot subwavelength grating microring resonator (HDSSWG-MRR) refractive index sensor operating in a water refractive index environment is proposed. By designing a new waveguide structure, a sensitivity of up to 1005 nm/RIU has been achieved, which is 182 nm/RIU higher than the currently highest sensitivity silicon photonic micro ring refractive index sensor. Meanwhile, utilizing a unique waveguide structure, a Q of 22,429 was achieved and a low limit of detection of 6.86 × 10-5 RIU was calculated.

The Highly Sensitive Refractive Index Sensing of Seawater Based on a Large Lateral Offset Mach-Zehnder Interferometer.

A novel fiber sensor for the refractive index sensing of seawater based on a Mach-Zehnder interferometer has been demonstrated. The sensor consisted of a single-mode fiber (SMF)-no-core fiber (NCF)-single-mode fiber structure (shortened to an SNS structure) with a large lateral offset spliced between the two sections of a multimode fiber (MMF). Optimization studies of the multimode fiber length, offset SNS length, and vertical axial offset distance were performed to improve the coupling efficiency of interference light and achieve the best extinction ratio. In the experiment, a large lateral offset sensor was prepared to detect the refractive index of various ratios of saltwater, which were used to simulate seawater environments. The sensor's sensitivity was up to -13,703.63 nm/RIU and -13,160 nm/RIU in the refractive index range of 1.3370 to 1.3410 based on the shift of the interference spectrum. Moreover, the sensor showed a good linear response and high stability, with an RSD of only 0.0089% for the trough of the interference in air over 1 h.

Monitoring of Curing Process of Epoxy Resin by Long-Period Fiber Gratings.

The curing of epoxy resin is a complex thermo-chemical process that is difficult to monitor using existing sensing systems. We monitored the curing process of an epoxy resin by using long-period fiber gratings. The refractive index of the epoxy resin increases during the curing process and can be measured to determine the degree of curing. We employed long-period fiber gratings that are sensitive to the refractive index of an external medium for the measurement of refractive index changes in the resin. We observed that the resonances of long-period fiber gratings increased their depth with the increased refractive index of the resin, which was well described by our simulation taking the coupling to radiation modes into account. We demonstrated that the degree of cure can be estimated from the depth of the grating resonances using a phenomenological model. At the same time, long-period fiber gratings are sensitive to temperature variations and internal strains that are induced during curing. These factors may affect the measurements of curing degree and should also be addressed.

Exploiting Thin-Film Properties and Guided-Mode Resonance for Designing Ultrahigh-Figure-of-Merit Refractive Index Sensors.

Guided-mode resonance (GMR) gratings have emerged as a promising sensing technology, with a growing number of applications in diverse fields. This study aimed to identify the optimal design parameters of a simple-to-fabricate and high-performance one-dimensional GMR grating. The structural parameters of the GMR grating were optimized, and a high-refractive-index thin film was simulated on the grating surface, resulting in efficient confinement of the electric field energy within the waveguide. Numerical simulations demonstrated that the optimized GMR grating exhibited remarkable sensitivity (252 nm/RIU) and an extremely narrow full width at half maximum (2 × 10-4 nm), resulting in an ultra-high figure of merit (839,666) at an incident angle of 50°. This performance is several orders of magnitude higher than that of conventional GMR sensors. To broaden the scope of the study and to make it more relevant to practical applications, simulations were also conducted at incident angles of 60° and 70°. This holistic approach sought to develop a comprehensive understanding of the performance of the GMR-based sensor under diverse operational conditions.

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.

Design of a High Q-Factor Label-Free Optical Biosensor Based on a Photonic Crystal Coupled Cavity Waveguide.

In recent years, there has been a significant increase in research into silicon-based on-chip sensing. In this paper, a coupled cavity waveguide (CCW) based on a slab photonic crystal structure was designed for use as a label-free biosensor. The photonic crystal consisted of holes arranged in a triangular lattice. The incorporation of defects can be used to design sensor devices, which are highly sensitive to even slight alterations in the refractive index with a small quantity of analyte. The plane wave expansion method (PWE) was used to study the dispersion and profile of the CCW modes, and the finite difference time domain (FDTD) technique was used to study the transmission spectrum, quality factor, and sensitivity. We present an analysis of adiabatically coupling light into a coupled cavity waveguide. The results of the simulation indicated that a sensitivity of 203 nm/RIU and a quality factor of 13,360 could be achieved when the refractive indices were in the range of 1.33 to 1.55.

Highly sensitive refractive index sensing with a dual-band optically transparent ITO-based perfect metamaterial absorber for biomedical applications.

In this paper, a dual-band optically transparent square-shaped perfect metamaterial absorber operating in the frequency range from 2 to 4 terahertz (THz) is proposed. The structure consists of an indium tin oxide (ITO)-based split ring resonator (SRR) structure with additional splits and rectangular inner strips to form the top layer over the lead glass substrate. Perfect absorption is attained in the frequencies of 2.089 and 3.892 THz with absorbances of 99.99% and 99.98% in TE polarization mode, respectively. Perfect absorption is also achieved in TM polarization mode at 2.23 THz. Broadband absorption is found in TM polarization mode with full width half maximum (FWHM) of 1.1742. The proposed structure has one polarization-insensitive band in TE polarization mode. Absorbance is greater than 80% and 90% in the successive absorption peaks even at 60° and 75° of incidence, respectively. The resonance frequency is sensitive to the refractive index of the medium. As a result, the proposed metamaterial structure may be implemented as a refractive index (RI) sensor with a high sensitivity of 1109 GHz/RIU and 1954 GHz/RIU in both absorption bands for a refractive index range of 1.34 to 1.40. It's interesting to note that the refractive index of most biological samples ranges from 1.3 to 1.39. The figure of merit (FOM) of the proposed sensor can reach as high as 10 and 14 for the 1st and 2nd frequency bands. As a result, the proposed sensor has a high sensitivity and can be employed in medical applications. Potential applications of the proposed absorber include imaging, biomedical sensing, etc.

An ultra-sensitive biosensor based on surface plasmon resonance and weak value amplification.

An ultra-sensitive phase plasmonic sensor combined with weak value amplification is proposed for the detection of IgG, as a model analyte. Phase detection is accomplished by self-interference between the p-polarization and the s-polarization of the light. With the principles of weak value amplification, a phase compensator is used to modulate the coupling strength and enhance the refractive index sensitivity of the system. On a simple Au-coated prism-coupled surface plasmon resonance (SPR) structure, the scheme, called WMSPR, achieves a refractive index sensitivity of 4.737 × 104 nm/RIU, which is about three times higher than that of the conventional phase-based approach. The proposed WMSPR biosensor gives great characteristics with a high resolution of 6.333 × 10-8 RIU and a low limit of detection (LOD) of 5.3 ng/mL. The results yield a great scope to promote the optimization of other SPR biosensors for high sensitivity.

Racetrack Ring Resonator-Based on Hybrid Plasmonic Waveguide for Refractive Index Sensing.

In this study, a comprehensive numerical analysis is conducted on a hybrid plasmonic waveguide (HPWG)-based racetrack ring resonator (RTRR) structure, tailored specifically for refractive index sensing applications. The sensor design optimization yields remarkable results, achieving a sensitivity of 275.7 nm/RIU. Subsequently, the boundaries of sensor performance are pushed even further by integrating a subwavelength grating (SWG) structure into the racetrack configuration, thereby augmenting the light-matter interaction. Of particular note is the pivotal role played by the length of the SWG segment in enhancing device sensitivity. It is observed that a significant sensitivity enhancement can be obtained, with values escalating from 377.1 nm/RIU to 477.7 nm/RIU as the SWG segment length increases from 5 µm to 10 µm, respectively. This investigation underscores the immense potential of HPWG in tandem with SWG for notably enhancing the sensitivity of photonic sensors. These findings not only advance the understanding of these structures but also pave the way for the development of highly efficient sensing devices with unprecedented performance capabilities.

Broadband-Tunable Vanadium Dioxide (VO2)-Based Linear Optical Cavity Sensor.

Sensors fabricated by using a silicon-on-insulator (SOI) platform provide promising solutions to issues such as size, power consumption, wavelength-specific nature of end reflectors and difficulty to detect ternary mixture. To address these limitations, we proposed and investigated a broadband-thermally tunable vanadium dioxide (VO2)-based linear optical cavity sensor model using a finite element method. The proposed structure consists of a silicon wire waveguide on a silicon-on-insulator (SOI) platform terminated with phase-change vanadium oxide (VO2) on each side to provide light confinement. A smooth transmission modulation range of 0.8 (VO2 in the insulator state) and 0.03 (VO2 in the conductive phase state) in the 125 to 230 THz spectral region was obtained due to the of Fabry-Pérot (FP) effect. For the 3.84 µm cavity length, the presented sensor resulted in a sensitivity of 20.2 THz/RIU or 179.56 nm/RIU, which is approximately two orders of magnitude higher than its counterparts in the literature. The sensitivity of the 2D model showed direct relation with the length of the optical cavity. Moreover, the change in the resonating mode line width Δν of approximately 6.94 THz/RIU or 59.96 nm/RIU was also observed when the sensor was subjected to the change of the imaginary part k of complex refractive index (RI). This property of the sensor equips it for the sensing of aternary mixture without using any chemical surface modification. The proposed sensor haspotential applications in the areas of chemical industries, environmental monitoring and biomedical sensing.