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In this paper, a whole new multistage refractive index sensor is proposed, which is based on the horizontal displacement of the photonic spin Hall effect. It is found that the angle of displacement is very sensitive to the diversification in the refractive index of the measured analyte, and the angle of horizontal displacement can vary by 12.5 degrees when the refractive index changes by 1. In addition, to further expand the function of this sensor, the multistage refractive index measurement by adjusting the Fermi level of the graphene is realized. Specifically speaking, when the Fermi level is 0.2 eV, the refractive index measurement range of this sensor is 2.2-3.2, and the sensitivity can be up to 7.5 degrees/RIU at this time. Eventually, the transformation of temperature and thicknesses of both the graphene layer and the layer to be measured in the two cases are respectively discussed, and the result is that this sensor has good temperature resistance and the stability of thickness errors in the engineering process. It can be said that the emergence of this new type of sensor will likely have broad application prospects in the measurement field, especially for some special cases of single frequency point work. [ABSTRACT FROM AUTHOR]

Transverse magneto‐optical Kerr effect (TMOKE) has attracted much attention due to its widespread application in nonreciprocal photon and sensing devices. The vast majority of studies related to TMOKE are mainly focused on p‐polarized incidence as a result of the stereotype that only p‐polarized incidence can generate TMOKE in conventional magnetic materials. Thanks to the powerful ability of metamaterials to regulate light, in recent years, the TMOKE under s‐polarized incidence is explored and observed successfully. Nevertheless, different from the universality and high amplitude of p‐polarized TMOKE, the excitation of s‐polarized TMOKE is pretty rare, and the peak values of s‐polarized TMOKE in the reported research are really weak, which limits its application potential to a large extent. Here, the iron‐based grating structure is proposed that excites surface plasmon polaritons (SPPs) and then causes the highest peak value of s‐polarized TMOKE at optical frequencies to date, which is seven orders of magnitude higher than that of the planar Fe film. Besides, its refractive index sensitivity reaches up to 685 nm RIU−1. The work broadens the design path of s‐polarized TMOKE as a promising candidate to promote the development of novel magnetoplasmonic sensors and nonreciprocal photon devices. [ABSTRACT FROM AUTHOR]

In this paper, a polarization-insensitive magnetic tunable perfect metamaterial absorber is analyzed which shows its functionality as a refractive index (RI) sensor. The unit cell structure of reported absorber consists of a cross-shaped Indium Arsenide (InAs) array over aluminum ground plane and has dimensions less than λ c / 4 with thickness of 7.5 μ m and periodicity of 9 μ m making it an ultra-thin absorber. The proposed absorber provides an average absorption of 99.99% on the implementation of magnetic field (B) at 0.4 T. The resonance frequency and corresponding absorption rates can be controlled by varying the magnetic field from B = 0. 1 –0.4 T which provides high absorption as well as shift in the frequency spectrum by 0.3 THz/T. Additionally, parametric analysis has also been done to confirm the selection of the optimum value of the designed parameters of the structure. Furthermore, simulation results reveal that the proposed structure is very sensitive to the change of RI values in the surroundings thus, the proposed structure can be employed as a RI sensor high sensitivity of 1.083 THz/RIU over the range of 1–1.6 RI. In addition, the result shows that the sensor can detect different chemical compounds such as ethanol, gasoline, paraffin, sodium chloride, carbon disulfide with an average sensitivity of 0.925 THz/RIU. Thus, the proposed absorber with obtained sensitivity can be utilized as a biochemical sensor, making it one of the best contenders for chemical industry uses in diagnosis of different hazardous chemicals as well as finding its applicability in biomedical uses. [ABSTRACT FROM AUTHOR]

Over the past decade, optical sensors have made significant advances. An optical sensor examines the environmental impact through the change of an optical signal and offers advantages such as low cost and label-free detection. In this study, a sensor consisting of a single graphene layer and a slit positioned on the substrate is proposed. The strip gap made to improve the excitation of graphene plasmons allowed to achieve 96.2% high transmission resonance mode. This demonstrates the ability of the sensor surface to detect changing environmental conditions. The results show that the sensitivity of the sensor is 6282 nm/RIU when the sensor surface is exposed to analytes with different refractive indices. The use of a single graphene sheet eliminates the need for a metal resonator and achieves a higher sensitivity compared to some experiments recently published in the literature. Thus, the disadvantage of significant ohmic losses in metal resonators is avoided. Furthermore, a thorough discussion of various factors, including the modification of the strip gap width on the graphene layer and electrical tunability, led to the achievement of optimal sensitivity. [ABSTRACT FROM AUTHOR]

Hematite (α -Fe2O3) is widely used in sensor sensitization due to its excellent optical properties. In this study, we present a sensitivity-enhanced surface plasmon resonance alcohol sensor based on Fe2O3/Au. We describe the fabrication process of the sensor and characterize its structure. We conduct performance testing on sensors coated multiple times and use solutions with the same gradient of refractive indices as the sensing medium. Within the refractive index range of 1.3335–1.3635, the sensor that was coated twice achieved the highest sensitivity, reaching 2933.2 nm/RIU. This represents a 30.26% enhancement in sensitivity compared to a sensor with a pure gold monolayer film structure. Additionally, we demonstrated the application of this sensor in alcohol concentration detection by testing the alcohol content of common beverages, showing excellent agreement with theoretical values and highlighting the sensor's potential in food testing. [ABSTRACT FROM AUTHOR]

Abstract Fabry-Pérot (FP) microcavities have attracted tremendous attention in recent years due to their favorable optical characteristics of the high quality (Q) factor and small mode volume. In this work, we presented a novel approach that utilized the soft lithography and imprinting technology to incorporate the convex micro-lens array structure into the FP (FP-lens) cavity. A strong mode-profile restriction of the micro-lens simultaneously reduced the mode volume and enhanced the Q factor, exhibiting high tolerance to non-parallelism of mirrors compared with that of the plane-plane FP (PP-FP) microcavities. In the experiment, the Q factor of the FP-lens cavity was measured to be 8.145×104, which exhibited a 5.6-fold increase than that of the PP-FP cavity. Furthermore, we experimentally measured the refractive index sensing performance of the FP-lens cavity with the sensitivity of 594.7 nm/RIU and a detection limit of 4.26×10−7 RIU. On the basis of this superior sensing performance, the FP-lens cavity has the great potential for applications in biosensors.

We demonstrated, for the first time, micro-ring resonator assisted photothermal spectroscopy measurement of a gas phase sample. The experiment used a telecoms wavelength probe laser that was coupled to a silicon nitride photonic integrated circuit using a fibre array. We excited the photothermal effect in the water vapor above the micro-ring using a 1395 nm diode laser. We measured the 1f and 2f wavelength modulation response versus excitation laser wavelength and verified the power scaling behaviour of the signal. [ABSTRACT FROM AUTHOR]

This paper presents a dual-core two-parameter optical fiber sensor based on the surface plasmonic resonance (SPR) effect. It is analyzed by the finite element method. The proposed sensor is a dual-channel structure designed with photonic crystal fiber (PCF) as the base material: one channel is coated with a gold film to measure the refractive index (RI) of the solution to be measured, and the other channel is coated with a gold film and polydimethylsiloxane to measure the temperature of the solution to be measured. The exposed microslot structure on both sides reduces the complexity of sensing measurements. The results show that the maximum RI sensitivity of the sensor is 19,900 nm/RIU, and the maximum sensitivity to temperature is 8.7 nm/℃. This work is conducive to realizing a PCF sensor with high sensitivity, large measurement range, real-time monitoring, and easy preparation. As a result, the sensor is expected to be widely used in fields such as biology and chemicals. [ABSTRACT FROM AUTHOR]

In contrast to noble metals, graphene exhibits significantly lower loss, especially useful for optical sensing applications that require ultrahigh Q factors, and offer wide range tunability via an adjustable Fermi level. However, precise graphene patterning is difficult, especially for large areas, severely limiting its applications. Here, a tunable terahertz metamaterial absorber (TMMA) with ultrahigh Q factors consisting of a continuous, pattern-free graphene is demonstrated. A graphene sheet is overlaid on an Al metal array, forming a structure that supports strong localized surface plasmon polaritons (LSPPs) with fields tightly confined in the graphene, minimizing loss. Theoretical results show that this TMMA exhibits an ultrahigh Q factor of 1730, a frequency sensitivity of 2.84 THz/RIU, and an excellent figure of merit (FoM) of 365.85 RIU−1, independent of polarization. A tunability from ~2.25 to ~3.25 THz is also achieved by tuning Ef of graphene from 0.3 to 0.7 eV. The proposed graphene-based TMMA holds many potential applications, particularly in the field of sensing. [ABSTRACT FROM AUTHOR]

Fabry-Pérot (FP) microcavities have attracted tremendous attention in recent years due to their favorable optical characteristics of the high quality (Q) factor and small mode volume. In this work, we presented a novel approach that utilized the soft lithography and imprinting technology to incorporate the convex micro-lens array structure into the FP (FP-lens) cavity. A strong mode-profile restriction of the micro-lens simultaneously reduced the mode volume and enhanced the Q factor, exhibiting high tolerance to non-parallelism of mirrors compared with that of the plane-plane FP (PP-FP) microcavities. In the experiment, the Q factor of the FP-lens cavity was measured to be 8.145×104, which exhibited a 5.6-fold increase than that of the PP-FP cavity. Furthermore, we experimentally measured the refractive index sensing performance of the FP-lens cavity with the sensitivity of 594.7 nm/RIU and a detection limit of 4.26×10−7 RIU. On the basis of this superior sensing performance, the FP-lens cavity has the great potential for applications in biosensors. [ABSTRACT FROM AUTHOR]

Abstract Optical biosensors based on plasmonic nanostructures have attracted great interest due to their ability to detect small refractive index changes with high sensitivity. In this work, a novel plasmonic coupled cavity waveguide is proposed for refractive index sensing applications. The structure consists of a metal–insulator–metal waveguide side coupled to an array of asymmetric H-shape element, designed to provide dual-band resonances. The sharp transmission dips and large field enhancements associated with dual-band resonances can enable sensitive detection of material under test. The resonator array creates a slow light effect to improve light-matter interactions. The structure was simulated using the finite integration technique as the full-wave technique, and the sensitivity and figure of merit were extracted for different ambient refractive indices. The maximum sensitivity of 1774 nm/RIU and high figure of merit of 2 × 104 RIU−1 for the basic model and 1.15 × 105 RIU−1 for the modified model were achieved, demonstrating the potential for high-performance sensing. The unique transmission characteristics also allow for combined spectral shaping and detection over a broad bandwidth. The simple, compact geometry makes the design suitable for on-chip integration. This work demonstrates a promising refractive index sensor based on coupled dual-band resonators in a plasmonic waveguide.

We propose a distributed multi-objective optimization (DMO) method for designing chiral plasmonic metasurface that satisfies multiple objectives simultaneously. We aim to improve the refractive index sensitivity of the archetypical Born-Kuhn type chiral plasmonic metasurface while ensuring that circular dichroism (CD) is as pronounced as possible at a designated resonance wavelength. By leveraging distributed technology, the proposed method significantly improves the time efficiency of the optimization process. The simulation results demonstrate approximately 33% enhancement in sensitivity by DMO, as well as greater than 100% boost in time efficiency compared to stand-alone optimization approaches. These findings highlight the potential of the proposed method to guide the design of chiral plasmonic metasurface sensors, enabling the simultaneous optimization of multiple objectives and facilitating advancements in chiral optics and sensing applications.

Au trisoctahedrons (TOHs) with sharp tips and high-index facets have exceptional properties for diverse applications, such as plasmon-enhanced spectroscopies, catalysis, sensing, and biomedicine. However, the synthesis of Au TOHs remains challenging, and most reported synthetic methods are time-consuming or involve complex steps, hindering the exploration of their potential applications. Herein, we present a facile and fast approach to prepare Au TOHs with high uniformity and good control over the final size and shape, all within less than 10 min of synthesis, for surface-enhanced Raman spectroscopy (SERS) and refractive index sensing. The size of the Au TOHs can be easily tailored over a wide range, from 39 to 268 nm, allowing a tuning of the plasmon resonance at wavelengths from visible to near-infrared regions. The exposed facets of the Au TOHs can also be varied by controlling the growth temperatures. The wide tunability of size and exposed facets of Au TOHs can greatly broaden the range of their applications. We have also encapsulated Au TOHs with zeolite imidazolate framework (ZIF-8), obtaining core–shell hybrid structures. With the ability to tune Au TOH size, we further assessed their SERS performances in function of their size by detecting 2-NaT in solution, exhibiting enhancement factors of the order of 105 with higher values when the LSPR is blue-shifted from the laser excitation wavelength. Au TOHs have been also compared for refractive index sensing applications against Au nanospheres, revealing Au TOHs as better candidates. Overall, this facile and fast method for synthesizing Au TOHs with tunable size and exposed facets simplifies the path toward the exploration of properties and applications of this highly symmetrical and high-index nanostructure. [ABSTRACT FROM AUTHOR]

Refractive index sensing is of great significance in biological detection. Especially, all-dielectric metasurfaces can greatly improve the sensitivity of biological detection. However, the Q factor and electric field enhancement of most metasurfaces are generally low, resulting in low sensitivity and figure of merit (FOM). Meanwhile, fabrication tolerances may result in inability to achieve high-performance refractive index sensing. Here, we use finite element method (FEM) simulation to design a conical hole structure on an all-dielectric metasurface. The toroidal dipole resonance based on a conical hole structure can achieve a large-area electric field enhancement and a high Q factor, thereby achieving a sensitivity of 540 nm/RIU and FOM of 154.3. Meanwhile, considering the real fabrication tolerance, the proposed structure can still achieve high-sensitivity sensing when the conical hole structure changes the ellipsoidal structure. Therefore, the high sensitivity, high FOM, and robustness of the proposed structure make it highly significant for practical applications. [ABSTRACT FROM AUTHOR]

The article presents a dual‐optimized adaptive algorithm based on continuous wavelet transform and derivative precise peak capture is developed for fiber cavity ring‐down spectroscopy. Adaptive spectral signal processing is achieved so that peak selection blindness can be effectively solved. To verify the performance of the dual‐optimized adaptive algorithm, a highly sensitive refractive index sensor using a bandgap fiber‐based linear fiber cavity is demonstrated. The developed dual‐optimized adaptive algorithm is compared with the conventional maximum extraction algorithm, and the results show that the dual‐optimized adaptive algorithm has a better performance in suppressing noise and processing superimposed peaks. A stable enhancement factor of 5 is obtained, indicating that the newly developed dual‐optimized adaptive algorithm can generate a high‐quality cavity ring‐down spectrum for applications including refractive index sensors, biosensors, strain gauges, and pressure sensors. [ABSTRACT FROM AUTHOR]

In this study, we introduce a novel hafnium dioxide (HfO2) and thin gold layer based photonic crystal fiber (HfAu-PCF) sensor for refractive index measurement and optimize in the spectrum of 1.3 µm to 1.6 µm. The proposed sensor incorporates a D-shaped geometry for enhanced light-matter interaction, and the core of the fiber is surrounded by periodic air holes to facilitate efficient coupling of the guided mode with surface plasmons. The proposed HfAu-PCF sensor is geometrically optimized using finite element method (FEM) by fine-tuning air hole diameter, core size, layer thickness dimensions of the fiber to achieve performance stability. Leveraging the unique optical properties of HfO2 and Au, the sensor demonstrates superior performance metrics, achieving confinement loss (

This article proposes an on-chip optical temperature sensor based on a cascaded microring resonator, which is composed of a reference ring and a sensing ring cascaded with different temperature sensitivities and free spectral ranges. By changing the temperature of the sensing window environment, the thermal optical effect of the waveguide causes a change in the refractive index of the waveguide, which affects the temperature sensitivity and free spectrum changes of the sensor. The output spectral response of the sensor shifts, achieving temperature sensing detection. The experimental results show that the temperature sensitivity of this microring cascaded temperature sensor is 303.6 pm/°C, which is 3.65 times the limit of a single microring temperature sensitivity of 83 pm/°C. The temperature-sensing range of the sensor is 100°C, which can well meet the temperature-monitoring requirements of ultra-large-scale integrated circuits.

Hematite (α-Fe2O3) is widely used in sensor sensitization due to its excellent optical properties. In this study, we present a sensitivity-enhanced surface plasmon resonance alcohol sensor based on Fe2O3/Au. We describe the fabrication process of the sensor and characterize its structure. We conduct performance testing on sensors coated multiple times and use solutions with the same gradient of refractive indices as the sensing medium. Within the refractive index range of 1.3335–1.3635, the sensor that was coated twice achieved the highest sensitivity, reaching 2933.2 nm/RIU. This represents a 30.26% enhancement in sensitivity compared to a sensor with a pure gold monolayer film structure. Additionally, we demonstrated the application of this sensor in alcohol concentration detection by testing the alcohol content of common beverages, showing excellent agreement with theoretical values and highlighting the sensor’s potential in food testing.

Infrared metasurfaces have exhibited exceptional optical properties that differ from naturally occurring metallic and dielectric nanostructure, enabling non-destructive and label-free sensing in a broadband region. However, implementing wavelength multiplexing sensors in broadband infrared has faced significant challenges. These challenges arise from the difficulty in efficiently exciting high Q resonances at specific wavelengths and the inability to individually tune each resonance. Herein, we present a dual resonant metasurface that utilizes a metal–dielectric–metal plasmonic grating and a dielectric–metal channel. By adjusting the vertical and horizontal structures of metasurface, we can independently modify the spectrum of the metasurface in the near-infrared and mid-infrared regions. This broadband infrared metasurface exhibits robust spectral regulation, enabling a polarization-dependent strategy for the dual-resonance. It offers a competitive advantage over traditional metallic nanostructure in refractive index sensing at the second near-infrared window and ultrasensitive vibrational spectroscopy in mid-infrared. Specifically, our proposed metasurface achieves protein concentration sensing and dynamic monitoring of protein concentration in the infrared two-zone. Additionally, it enhances the mid-infrared absorption of amide II with a high Q resonance. The metasurface which combines wavelength multiplexing and polarization dependent switch for protein recognition and trace detection, presents a novel approach for developing high-performance sensors and Integrated photonics sensors in the broadband infrared region.

Abstract A corrugated surface long period grating (LPG) was fabricated on a flat-shaped plastic optical fiber (POF) as a refractive index (RI) sensor by a simple pressing with the heat pressure and mechanical die press print method. The light propagation characteristics of an LPG imprinted on a multi-mode POF were analyzed by the method of geometrical optics. Theoretical and experimental results showed that the structural parameters of the sensor affected the RI sensing performance, and the sensor with a thinner flat thickness, a deeper groove depth of the corrugated surface LPG, and a longer LPG exhibited better RI sensing performance. When the POF with a diameter of 1 mm was pressed with the heat pressure to a flat shape with a thickness of 600 µm, an LPG with a period of 300 µm, a groove depth of 200 µm, and a length of 6 cm was fabricated on it, and the RI sensitivity of 1447%/RIU was obtained with a resolution of 5.494×10−6 RIU. In addition, the influences of the POF cladding, tilting of LPG, and bending of the sensing structure were investigated. The results demonstrated that after removing the cladding and tilting or bending the LPG, the RI sensing performance was improved. When the LPG imprinted on the flat-shaped POF was bent with a curvature radius of 6/π cm, the highest sensitivity of 6 563%/RIU was achieved with a resolution of 2.487×10−9 RIU in the RI range of 1.3330–1.4230. The proposed sensor is a low-cost solution for RI measurement with the features of easy fabrication, high sensitivity, and intensity modulation at the visible wavelengths.

Utilizing the asymmetry of the micro-nano structure to excite Fano resonance in dielectric metasurfaces presents low loss and high field enhancement factors, and has a broad range of applications in optical sensing. Nevertheless, the asymmetry of the structure often leads to polarization-sensitive characteristics, significantly reducing the quality factor and sensitivity of the metasurface, thereby hindering its further application. In this study, the metasurface composed of dielectric materials is designed to be completely insensitive to x and y polarization. The metasurface is composed of semicircular disks, which are asymmetrically and periodically placed on a quartz substrate. By controlling the radius of the two semicircular disks, the symmetry is broken, and the bound states in the continuum (BIC) transition to quasi-bound states in the continuum (q-BIC), resulting in an ultra-narrow Fano resonance peak. Simulation results show that the magnetic dipole (MD) dominates the resonance. The designed metasurface structure was fabricated and then integrated into a microfluidic system for spectral refractive index sensing experiments. In the near-infrared band, our tests demonstrate refractive index sensing performance of 405 nm/RIU and a Q factor of up to 319, with significant symmetry of the polarization angle, which agrees well with theoretical simulations. Furthermore, the spectral response of the designed structure can be flexibly controlled by adjusting the polarization angle of the incident light. We believe that this study provides a novel integrated platform for applications such as biosensing and environmental detection, and also has certain reference value for the design of polarization optical switches and metasurface absorbers for encrypted communication.

We proposed a theoretical study of silicon nitride ring resonator configuration for the application of refractive index sensing. In this paper, a slot waveguide has been used to couple the light into the slab ring structure. We have designed the slab ring and slot waveguides with silicon nitride (Si3N4) on a substrate made of silica (SiO2). Transverse electric (TE) mode has been used for the entire analysis at an operating wavelength of 1550 nm. The presented structure has been geometrically optimized to get the better transmission results. The design shows a resonance wavelength of 1538.74 nm when the cladding refractive index(ntop) $({n}_{{top}})$ is equal to 1, which is placed at the top of the slab ring structure. We have also calculated the effect on the transmission curve for different waveguide parameters. For, ntop=1.35 ${n}_{{top}}=1.35$ the structure achieves a resonance wavelength around 1563.65 nm. The geometrically optimized structure achieved a Q‐factor of 750 and a sensitivity of 70 nm/RIU. In the proposed paper, the shift in the resonance of the transmission spectrum has been utilized to detect the Refractive Index (RI) changes on the surface of the ring. Therefore, the proposed structure can be used effectively in the applications of refractive index sensing. [ABSTRACT FROM AUTHOR]

A corrugated surface long period grating (LPG) was fabricated on a flat-shaped plastic optical fiber (POF) as a refractive index (RI) sensor by a simple pressing with the heat pressure and mechanical die press print method. The light propagation characteristics of an LPG imprinted on a multi-mode POF were analyzed by the method of geometrical optics. Theoretical and experimental results showed that the structural parameters of the sensor affected the RI sensing performance, and the sensor with a thinner flat thickness, a deeper groove depth of the corrugated surface LPG, and a longer LPG exhibited better RI sensing performance. When the POF with a diameter of 1 mm was pressed with the heat pressure to a flat shape with a thickness of 600 µm, an LPG with a period of 300 µm, a groove depth of 200 µm, and a length of 6 cm was fabricated on it, and the RI sensitivity of 1447%/RIU was obtained with a resolution of 5.494×10−6 RIU. In addition, the influences of the POF cladding, tilting of LPG, and bending of the sensing structure were investigated. The results demonstrated that after removing the cladding and tilting or bending the LPG, the RI sensing performance was improved. When the LPG imprinted on the flat-shaped POF was bent with a curvature radius of 6/π cm, the highest sensitivity of 6 563%/RIU was achieved with a resolution of 2.487×10−9 RIU in the RI range of 1.3330–1.4230. The proposed sensor is a low-cost solution for RI measurement with the features of easy fabrication, high sensitivity, and intensity modulation at the visible wavelengths. [ABSTRACT FROM AUTHOR]

A chirped long-period grating inscribed in a planer optical waveguide sensor is demonstrated. The distribution of electric field within the structure has been obtained using boundary condition for the electric field equations at different dielectric boundaries. The transmission spectrum of the proposed sensor is obtained by transfer-matrix method. Further, the sensitivity for ambient refractive index and temperature has been analysed by observing the shift in resonance wavelengths. The presence of two dips in the transmission spectrum effectively doubles the sensitivity in comparison to the single resonance structure. We have achieved refractive index sensitivity of 5600 nm/RIU and temperature sensitivity of 0.8 nm/ °C, respectively. [ABSTRACT FROM AUTHOR]

Chemical and biological detection is now an indispensable task in many fields. On-chip refractive index (RI) optical sensing is a good candidate for mass-scale, low-cost sensors with high performance. While most literature works focus on enhancing the sensors' sensitivity and detection limit, other important parameters that determine the sensor's yield, reliability, and cost-effectiveness are usually overlooked. In this work, we present a comprehensive study of the different integrated photonic platforms, namely silica, silicon nitride, and silicon. Our study aims to determine the best platform for on-chip RI sensing, taking into consideration the different aspects affecting not only the sensing performance of the sensor, but also the sensor's reliability and effectiveness. The study indicates the advantages and drawbacks of each platform, serving as a guideline for RI sensing design. Modal analysis is used to determine the sensitivity of the waveguide to medium (analyte) index change, temperature fluctuations, and process variations. The study shows that a silicon platform is the best choice for high medium sensitivity and a small footprint. On the other hand, silica is the best choice for a low-loss, low-noise, and fabrication-tolerant design. The silicon nitride platform is a compromise of both. We then define a figure of merit (FOM) that includes the waveguide sensitivity to the different variations, losses, and footprint to compare the different platforms. The defined FOM shows that silicon is the best candidate for RI sensing. Finally, we compare the optical devices used for RI sensing, interferometers, and resonators. Our analysis shows that resonator-based devices can achieve much better sensing performance and detection range, due to their fine Lorentzian spectrum, with a small footprint. Interferometer based-sensors allow engineering of the sensors' performance and can also be designed to minimize phase errors, such as temperature and fabrication variations, by careful design of the interferometer waveguides. Our analysis and conclusions are also verified by experimental data from other published work. [ABSTRACT FROM AUTHOR]

In this paper, a plasmon resonance-enhanced narrow-band absorber based on the nano-resonant ring array of transparent conductive oxides (TCOs) is proposed and verified numerically. Due to the unique properties of TCOs, the structure achieves an ultra-narrowband perfect absorption by exhibiting a near-field enhancement effect. Consequently, we achieve a peak absorption rate of 99.94% at 792.2 nm. The simulation results indicate that the Full Width Half Maximum (FWHM) can be limited to within 8.8 nm. As a refractive index sensor, the device reaches a sensitivity S of 300 nm/RIU and a Figure of Merit (FOM) value of 34.1 1/RIU. By analyzing the distribution characteristics of the electromagnetic field at the 792.2 nm, we find high absorption with a narrow FWHM of the ITO nano-resonant ring (INRR) owing to plasmon resonance excited by the free carriers at the interface between the metal and the interior of the ITO. Additionally, the device exhibits polarization independence and maintains absorption rates above 90% even when the incident formed by the axis perpendicular to the film is greater than 13°. This study opens a new prospective channel for research into TCOs, which will increase the potential of compact photoelectric devices, such as optical sensing, narrowband filtering, non-radiative data transmission and biomolecular manipulation. [ABSTRACT FROM AUTHOR]

Plasmonic nanostructures as narrowband perfect absorbers have garnered significant attention due to their potential applications in biosensing and environment detection. This study emphasizes the investigation of arrayed split ring nanostructures within the configuration of metal-insulator-metal (MIM) multilayers, resulting in a maximum light absorption of 99.94% in the near-infrared (NIR) spectral range. The exceptional absorption efficiency of the device is attributed to the strong resonance of electric and magnetic fields arising from the Fabry–Pérot cavity resonance. The resonant peak can be flexibly tuned by engineering the dielectric layer thickness, the period, and the geometric parameter of split rings. Remarkably, the device exhibits promising capabilities in sensing, demonstrating a sensitivity of 326 nm/RIU in visible wavelengths and 504 nm/RIU in NIR wavelengths when exposed to bio-analytes with varying refractive indices. This designed nanostructure can serve as a promising candidate for biosensors or environmental detection. [ABSTRACT FROM AUTHOR]

In this study, we presented a simple highly sensitive sensor based on commercially available solid-core photonic crystal fiber (PCF) and surface plasmon resonance (SPR) for measuring the refractive index (RI) of analytes. The numerical simulation based on the finite element method (FEM) has been examined to compute the optical properties such as confinement loss, power spectrum, and transmission intensity of the sensor. The most sensitive and inert plasmonic materials (gold and silver) have been assumed to be coated inside the fiber with the range of analyte RI from 1.32 to 1.40. The performance of the proposed sensor has been evaluated by tracing the several optical features like wavelength sensitivity, amplitude sensitivity, resolution of the sensor, and figure of merit. As a result, the comparative study between silver and gold elements has been carried out in which the maximum sensitivity received was 1.15 μm/RIU and 1.10 μm/RIU, respectively. Whereas, on the base of power spectrum, the obtained sensitivity was 513 μm/RIU for the gold layer. Moreover, the effect of other structural parameters (air holes and plasmonic layer thickness) on the sensing performance has been taken into an account. According to the simulation analysis and results, this sensor would have a great potential in various sensing applications of biomedical and liquid refractive index. [ABSTRACT FROM AUTHOR]

Abstract In this study, we design a refractive index (RI) sensor using a novel cadmium telluride photonic crystal fiber (TPCF). Based on four-wave mixing (FWM), the changes in RI can be accurately detected, and RI sensing in the mid-infrared region (MIR) can be achieved by detecting wavelength shifts in the Stokes and anti-Stokes spectra caused by the changes in RI of the liquid to be measured. When the pump wavelength of FWM lies in the normal and abnormal dispersion regions of the TPCF, the RI response of the idler frequency wave and the signal wave are analyzed by numerical simulation methods. The simulation results show that the RI sensitivity of the sensor can be as high as 7692 nm/RIU with a linearity is up to 99.9% at the pump wavelength of 3380 nm. To our knowledge, the RI sensing sensitivity of the MIR is presented for the first time in this study by using FWM in the non-silicon PCF.

We demonstrated, for the first time, micro-ring resonator assisted photothermal spectroscopy measurement of a gas phase sample. The experiment used a telecoms wavelength probe laser that was coupled to a silicon nitride photonic integrated circuit using a fibre array. We excited the photothermal effect in the water vapor above the micro-ring using a 1395 nm diode laser. We measured the 1f and 2f wavelength modulation response versus excitation laser wavelength and verified the power scaling behaviour of the signal.

As application scenarios grow in complexity, multifunctional integrated metasurfaces have been proposed as a potential development direction. We propose a multifunctional high Q-factor metasurface-based platform composed by size-scaled metasurface polarization selective absorbers (MPSAs) capable of polarization selective absorption, refractive index sensing and chiral response. By designing a gold step-shaped unit structure that can break the in-plane and out-of-plane symmetry, bound states in the continuums (BICs) with chiral response and a surface lattice resonance (SLR) are realized. The overall absorption amplitude of the quasi-BIC can reach a maximum value of 0.87 and a Q-factor of 153 while the SLR possesses a high Q-factor exceeding 1274 and a maximum absorption amplitude of 0.93. High extinction ratio up to 13 dB between the transverse magnetic and transverse electric polarization can be achieved. For refractive index (RI) sensing, the bulk RI sensitivities of the proposed MPSA for quasi-BIC and SLR are SBIC = 4000 nm/RIU and SSLR = 3740 nm/RIU, which leads to a high figure of merit (FOM) of FOMBIC = 161 /RIU and FOMSLR = 1247 /RIU, respectively. For chiral response, the maximum circular dichroism (CD) for the proposed MPSA is 0.88 by oblique incidence still maintaining a Q-factor exceeding 120, which indicates that it has potential application for chiral sensing. The proposed multifunctional metasurfaces-based platform possesses the potential to promising future applications for polarization manipulation, chiral biosensing, chiral drug analysis and quantum optics.

Abstract Compared to single nanoparticles, strongly coupled plasmonic nanoparticles provide attractive advantages owing to their ability to exhibit multiple resonances with unique spectral features and higher local field intensity. These enhanced plasmonic properties of coupled metal nanoparticles have been used for various applications including realization of strong light‐matter interaction, photocatalysis, and sensing. In this article, the basic physics of hybrid plasmonic modes in coupled metallic nanodimers is reviewed and their potentials for refractive index sensing are assessed. In particular, the spectral line shapes of various modes of hybrid plasmons including bonding and antibonding modes in symmetric nanodimers, Fano resonances in asymmetric nanodimers, charge transfer plasmons in linked nanoparticle dimers, hybrid plasmon modes in nanoshells, gap modes in particle‐on‐mirror configurations, and hybrid magnetoplasmonic modes in heterodimers are overviewed. Beyond the dimeric nanosystems, the potentials of surface lattice resonances in periodic nanoparticle arrays for sensing applications are also showcased. Finally, based on the critical assessment of the recent research on coupled plasmonic modes, the outlook on the future prospects of hybrid plasmon‐based refractometric sensing are discussed. Given their tunable resonances and ultranarrow spectral features, coupled metal nanoparticles are expected to play key roles in developing precise plasmonic nanodevices with extreme sensitivity.

The field of plasmonic sensing has witnessed rapid advancements in recent years. This work proposes an innovative methodology that employs metal–insulator-metal (MIM) nanohole arrays integrated with ring-shaped silver nanoparticles (AgNPs) to serve as an effective plasmonic refractive index sensor. The hybrid substrates are prepared using a combination of nanosphere lithography (NSL), reactive ion etching, and an enhanced plasmonic site-selective photocatalytic reaction, ensuring large-area fabrication, high throughput, and cost-effectiveness. The resonance peaks observed in the visible-NIR spectra are attributed to the coupling of surface plasmon resonance modes between MIM structures and AgNPs. The resonance wavelength exhibits sensitively to changes in the surrounding medium within the refractive index range of 1 to 1.5, with a remarkable sensitivity of 835 nm/RIU. Our findings reveal the significant potential of this technology for advancing optical sensing applications.

In this study, we design a refractive index (RI) sensor using a novel cadmium telluride photonic crystal fiber (TPCF). Based on four-wave mixing (FWM), the changes in RI can be accurately detected, and RI sensing in the mid-infrared region (MIR) can be achieved by detecting wavelength shifts in the Stokes and anti-Stokes spectra caused by the changes in RI of the liquid to be measured. When the pump wavelength of FWM lies in the normal and abnormal dispersion regions of the TPCF, the RI response of the idler frequency wave and the signal wave are analyzed by numerical simulation methods. The simulation results show that the RI sensitivity of the sensor can be as high as 7692 nm/RIU with a linearity is up to 99.9% at the pump wavelength of 3380 nm. To our knowledge, the RI sensing sensitivity of the MIR is presented for the first time in this study by using FWM in the non-silicon PCF. [ABSTRACT FROM AUTHOR]

In this study, we propose an easy and cost-effective method to improve the refractive index (RI) sensitivity of hierarchical aluminum (Al) nanocap arrays (HAlNAs) that are suitable for RI sensing applications. By adjusting the deposited Al thickness, deposition rate, and period, HAlNAs with tunable plasmonic and sensing properties could be achieved. The optimal sensitivity could reach 631 nm/RIU, and the detection limit of adenine ethanol solution with different concentration was as low as 1 × 10–6 M. The experimental results confirmed that the RI sensitivity of the proposed sensor was primarily related to the period and deposited Al thickness. Moreover, a higher deposition rate could narrow the full width at half-maximum (FWHM) of the reflection spectra, resulting in a higher figure of merit (FOM). The simulated electric field (EF) distribution revealed that the high RI sensitivity could be attributed to the strong coupling between propagating surface plasmon polaritons (PSPPs) and localized surface plasmon resonance (LSPR). In addition, HAlNAs with larger periods could generate a greater hot spot density, resulting in higher sensing sensitivity. The results of this study provide a method to easily fabricate Al-based RI sensors that exhibit great promise for the development of highly sensitive and selective biosensors. [ABSTRACT FROM AUTHOR]

A fiber speckle sensor (FSS) based on a tapered multimode fiber (TMMF) has been developed to measure liquid analyte refractive index (RI) in this work. By the lateral and axial offset of input light into TMMF, several high-order modes are excited in TMMF, and the speckle pattern is spatially modulated, which affects an asymmetrical speckle pattern with a random intensity distribution at the output of TMMF. When the TMMF is immersed in the liquid analyte with RI variation, it influences the guided modes, as well as the mode interference, in TMMF. A digital image correlations method with zero-mean normalized cross-correlation coefficient is explored to digitize the speckle image differences, analyzing the RI variation. It is found that the lateral- and axial-offsets-induced speckle sensor can enhance the RI sensitivity from 6.41 to 19.52 RIU−1 compared to the one without offset. The developed TMMF speckle sensor shows an RI resolution of 5.84 × 10−5 over a linear response range of 1.3164 to 1.3588 at 1550 nm. The experimental results indicate the FSS provides a simple, efficient, and economic approach to RI sensing, which exhibits an enormous potential in the image-based ocean-sensing application. [ABSTRACT FROM AUTHOR]

In this article, an all-dielectric metasurface-based refractive index sensor is proposed in the near-infrared wavelength regime. The sensor employs well known magnetic dipole and electric dipole resonances for sensing. We have also computationally demonstrated that sensitivity and figure of merit of the sensor can be enhanced by increasing the interaction between the electromagnetic field and the surrounding medium using a suitable design modification. The proposed improved sensor can be used in anemia detection and also to measure its progress as proposed in this work. [ABSTRACT FROM AUTHOR]

In this work, we aim to design the digital twin of a plasmonic sensor based on hexagonally arranged ellipsoidal gold nanoparticles fixed to a glass substrate. Based on electron microscopy images of three experimentally realized nanoparticle arrangement types, we constructed numerical models in environments based on finite element method (FEM) and boundary element method (BEM), namely COMSOL Multiphysics for FEM and the MNPBEM Matlab Toolbox for BEM. Models with nonperiodic and periodic boundary conditions with different unit cells were constructed to study the plasmonic behavior of both the single ellipsoidal particles and the hexagonal nanoparticle arrangements. The effect of the geometrical parameters, namely the interparticle distance, the nanoparticle diameter and thickness, on the resulting LSPR peak positions and bulk refractive index sensitivities were studied in detail, also taking into account the effect of the SiO2 substrate (pillars) under the ellipsoidal particles. We have demonstrated that by optimizing the models, the LSPR peak positions (and sensitivities) can match the experimentally measured values within 1 nm (nm/RIU) precision. The comparison of simulation conditions and the detailed discussion of the effect of the geometrical parameters and used gold dielectric functions on the obtained sensitivities can be very beneficial for the optimization of plasmonic sensor constructions through numerical simulations. [ABSTRACT FROM AUTHOR]

Multiplexed, specific, and sensitive detection of antigens is critical for the rapid and accurate diagnosis of disease and the informed development of personalized treatment plans. Here, it is shown that polymer microsphere lasers can be used as photonic sensors to monitor and quantify direct surface binding of biomolecules via changes in the refractive index. The unique spectral signature of each individual laser can be used to find their size and effective refractive index which adds a new encoding dimension when compared to conventional fluorescent beads. Antibody‐functionalized microlasers selectively detect protein binding, as demonstrated for Immunoglobulin G and C‐reactive protein, and have the ability to resolve different stages of the multilayer surface modification. Moreover, by continuously monitoring single lasers, the possibility of real‐time monitoring of binding dynamics between antigens in solution phase and the immobilized antibodies is demonstrated. For multiplexed detection, the microlasers are employed in a flow cytometer configuration, with fast spectral detection and identification of microlasers with and without antigen binding. It is envisioned that by combining microlasers with well‐established surface modification chemistries and flow geometries, the multiplexing ability of microbead immunoassays can be strongly increased while also opening avenues for single‐cell profiling within heterogeneous cell populations. [ABSTRACT FROM AUTHOR]

With the development of aerospace, deep-sea exploration and other technologies, the demand for anti-electromagnetic, high-sensitivity and miniaturized sensors is increasingly urgent. In this paper, a model of a quasi-photonic crystal fiber (Q-PCF) refractive index (RI) sensor enhanced with surface plasmon resonance (SPR) is proposed. A stable gold film with a significant SPR effect is applied to the two identically sized and oppositely positioned air holes of the proposed sensor, and all air holes are filled with analyte. A detailed analysis of the mode characteristics, structural parameters and RI sensing performance of the sensor has been carried out using the finite element method. It has been shown that the maximum sensitivity (S) is 4977.59 nm/RIU in the RI range of 1.35–1.40, corresponding to a resolution (R) of 2.01 × 10−5 RIU and a figure of merit (FOM) of 160.36 RIU−1. The proposed Q-PCF sensor has unique fabrication advantages and outstanding sensing properties, providing a new idea for biosensing, complex environment monitoring and long-range measurement, and is of great practical value in the field of highly integrated sensing. [ABSTRACT FROM AUTHOR]