SURFACE PLASMON/PHONON RESONANCE-BASED OPTICAL SENSORS

Abstract

Surface plasmon resonance (SPR) has attracted great attention and substantially contributed to sensing applications in recent years. The salient features of SPR-based sensors, such as quick and label-free detection, high sensitivity, small sample size, simple structure, reliable results at a reasonable cost, and smooth operation, make them an excellent tool for sensing applications in the field of chemical sensing, environment monitoring, medical diagnosis, and biosensing. SPR phenomenon occurs at the metal-dielectric interface and can be realized using special phase-matching schemes such as prism and grating coupling techniques. High optical losses and very large negative permittivity of the metals at longer wavelengths limit the use of the SPR beyond the NIR-spectral ranges. Polar dielectric materials serve as an alternative to metals, providing low optical loss and a functional spectral range from mid-IR to THz. The surface phonon resonance (SPhR) phenomenon takes place at the polar dielectric material-dielectric interface in a particular wavelength range known as the reststrahlen band. Similar to SPR, SPhR also offers potential applications such as surface-enhanced infrared absorption, coherent thermal emission, and refractive index sensing. In this thesis, we numerically presented and analyzed the SPR and SPhR-based refractive index sensors. The reflection dip shift observed in the SPR and SPhR response curves, due to a small change in the refractive index of the analyte, serves as the basic premise underlying refractive index sensing. A higher resolution of the sensor is correlated with a smaller width of the reflection dip, allowing for more accurate detection of changes in the refractive index. First, we designed a highly sensitive SPhR-based refractive index sensor in the mid-IR wavelength range. The structure of this sensor contains an Au grating on the polar dielectric material SiC substrate, which supports the excitation of the surface phonon polaritons. The numerical simulations of the sensor are carried out using RCWA to investigate the performance of the sensor in terms of sensitivity, quality factor, and detection accuracy. The optimized value of the grating depth provides the maximized performance of 7066.67 nm/RIU for sensitivity, 225.1 RIU-1 for the quality factor, and 6.75 for detection accuracy. The study, including other polar dielectric materials in the proposed structure, shows that the performance of the sensor can be further improved using the polar dielectric materials of high wavelength reststrahlen band.