Resonant modes of reflecting gratings engineered for multimodal sensing

Abstract

Reflecting gratings with narrow grooves exhibit multiple electromagnetic modes. Using a simple setup, surface plasmon (SP), cavity mode (CM) resonance shift, surface-enhanced fluorescence (SEF), and surface-enhanced Raman scattering signals can be measured, thus forming a multimodal sensing or imaging system. The nature of these modes is first analyzed using dispersion curves as a function of the wavelength, thickness, and period and then confirmed experimentally. For a thin (20 nm) enough grating, the resonant modes are shown to be mainly attributed to SP excitation. Increasing the grating thickness allows the excitation of CMs, and more importantly, coupling between the two resonant modes can take place under certain conditions, leading to a change in the sign of the radius of curvature of the CM branch near the SP wavelength. Field distribution calculations show an agreement with the dispersion curve analysis expressing the nature of the three mode field. Additionally, the SP wavelength was shown to separate between the cavity and diffraction mode branches. The resonant modes can be controlled by tuning the grating parameters and are shown to be spread over a wide spectral range. Experimental verification (sensing in the visible and infrared ranges and SEF experiments) of the observed phenomena is performed on a 154 nm thick silver grating with 1050 nm period fabricated using electron beam lithography. Multimodal systems are important to provide as much as possible information on the measured samples, such as the concentration of analytes and characterization of cells and tissue.