Optical sensors are widely used for refractive index measurement in chemical, biomedical, and food processing industries. Due to specific field distribution of the resonances, optical sensors provide high sensitivity to ambient refractive index variations. The sensitivity of an optical sensor is highly dependent on material and structure of the sensor. Here, six major categories of optical refractive index sensors using plasmonic and photonic structures are reviewed: i) metal‐based propagating plasmonic eigenwave structures, ii) metal‐based localized plasmonic eigenmode structures, iii) dielectric‐based propagating photonic eigenwave structures, iv) dielectric‐based localized photonic eigenmode structures, v) advanced hybrid structures, and vi) 2D material integrated structures. Representative configurations working in the wavelength range of 400–2000 nm will be selected and compared in terms of bulk refractive index sensitivities, figures of merit, and working wavelengths. A technology map is established in order to define the standard and development trend for optical refractive index sensors.

MXene, a new class of two-dimensional nanomaterials, have drawn increasing attention as emerging materials for sensing applications. However, MXene-based surface plasmon resonance sensors remain largely unexplored. In this work, we theoretically show that the sensitivity of the surface plasmon resonance sensor can be significantly enhanced by combining two-dimensional ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{T}}_x$ MXene and transition metal dichalcogenides. A high sensitivity of ${198}^{\circ }$ /RIU (refractive index unit) with a sensitivity enhancement of 41.43% was achieved in aqueous solutions (refractive index ∼1.33) with the employment of monolayer ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{T}}_x$ MXene and five layers of ${\mathrm{WS}}_2$ at a 633 nm excitation wavelength. The integration of ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{T}}_x$ MXene with a conventional surface plasmon resonance sensor provides a promising approach for bio- and chemical sensing, thus opening up new opportunities for highly sensitive surface plasmon resonance sensors using two-dimensional nanomaterials.

A nonreciprocal phonon laser in a spinning microwave magnomechanical system is proposed. The system consists of a spinning microwave resonator coupled with an yttrium iron garnet sphere. The Fizeau light-dragging effect caused by the spinning of the resonator leads to a significant difference in the mechanical gain and the threshold power for driving the resonator from the opposite directions, which results in a nonreciprocal phonon laser. The nonreciprocal phonon laser is highly tunable by the spinning speed and direction of the resonator. These results provide an experimentally feasible approach for exploring various nonreciprocal effects in cavity magnomechanical systems, and may find applications in photon, magnon, and phonon manipulations in many-body coupled systems.

Two-dimensional transition metal dichalcogenides (TMDCs), as promising alternative plasmon supporting materials to graphene, exhibit potential applications in sensing. Here, we propose a TMDCs-mediated long range surface plasmon resonance (LRSPR) imaging biosensor, which shows tremendous improvements in both imaging sensitivity ( 2) and detection accuracy ( 10) as compared to conventional surface plasmon resonance (cSPR) biosensors. It is found that the imaging sensitivity of the LRSPR biosensor can be enhanced by the integration of TMDC layers, which is different from the previously reported graphene-mediated cSPR imaging sensor, whose imaging sensitivity decreases with the number of graphene layers. This imaging sensitivity enhancement effect for the TMDCs-mediated LRSPR sensor originates from the propagating nature of the LRSPR at both interfaces of sensing medium/gold and gold/cytop layer (with a matching refractive index as sensing medium). By tuning the thickness of gold film and cytop layer, it is possible to achieve optimized imaging sensitivity for LRSPR sensors with any known integrated number of TMDC layers and an analyte refractive index. The proposed TMDCs-mediated LRSPR imaging sensor could provide potential applications in chemical sensing and biosensing.

Room-temperature surface exciton polaritons (SEPs) were successfully demonstrated recently by replacement of the metal thin film in the Kretschmann-Raether configuration of surface plasmon polaritons (SPPs) with a film of the J-aggregate cyanine dye 5,6-dichloro-2-[[5,6-dichloro-1-ethyl-3-(4-sulfobutyl)benzimidazol-2-ylidene]propenyl]-1-ethyl-3-(4-sulfobutyl)benzimidazolium hydroxide (TDBC). Compared with SPPs (i.e., photons coupled to highly mobile free electrons at the metal surface), the coupling of photons to less-mobile excitons supports stronger field confinement at the TDBC surface. This extraordinary field confinement of SEPs beyond SPPs motivated us to explore the feasibility of refractive-index (RI) sensing, in particular, surface RI sensing. We find that the bulk RI sensitivity of ${118}^{\circ }$ (${140}^{\circ }$) per refractive-index unit (RIU) of the proposed SEP sensor in the RI range from 1.0 to 1.001 (from 1.33 to 1.36) for a gaseous analyte (an aqueous solution) is higher than that of a conventional gold-based SPP sensor [${73}^{\circ }/$RIU (${124}^{\circ }/$RIU)]. For surface RI sensing, the SEP sensor has both higher sensitivity and a higher figure of merit than the SPP sensor. These numerical results undoubtedly reveal SEPs to be a promising mechanism for sensing.

A surface plasmon resonance refractive index sensor based on MoS ₂ -aluminium structure is designed for high imaging sensitivity and detection accuracy. The proposed sensor exhibits better performance than graphene-based refractive index sensor in the near-infrared regime. A high imaging sensitivity of more than 970 RIU ^-1 is obtained at the wavelength of 1540 nm. The effect of aluminium thickness, number of MoS ₂ layers, and the analyte refractive index on the sensor performance was investigated. In addition, the sensor performance comparison of MoS ₂ and other two-dimensional transition metal dichalcogenide materials-based sensors in the near-infrared regime is also presented. The designed MoS ₂ -based surface plasmon resonance refractive index sensor could provide potential applications in chemical and biological sensing.