Design of Optical Resonators for Enhanced Contrast in Optical Observation Using Surface Plasmon Polaritons

Even colorless and transparent samples can be optically observed without staining by detecting refractive index differences, as in differential interference contrast microscopy or phase-contrast microscopy. Surface Plasmon Polaritons (SPPs), excited on metal surfaces by light, are also well-known for detecting such refractive index differences. By using white light as the source to excite SPPs, the light with color corresponding to the refractive index of the sample medium excites the SPPs. By observing these colors, we can optically observe the samples. However, the SPP spectra are broadened, reducing the sensitivity. This paper proposes combining an optical resonator with SPP excitation to narrow the SPP excitation spectrum. Inside the resonator, light waves are reflected and travel back and forth multiple times, which enhances the waves whose phases are matched. Since the phase progression depends on the color, the resonator exhibits color selectivity. Considering SPP excitation, resonator parameters are designed through phase simulations to achieve a single-color peak in the spectrum. As a result, SPP excitation spectra with a reduced spectral width of the peak are obtained.