Mid-infrared ellipsometry enhanced by means of localized electromagnetic states of a one-dimensional photonic crystal

This study explores the potential of Bloch surface waves (BSWs) at the interface of a finite one-dimensional photonic crystal (1D-PC) and vacuum, exploiting spectroscopic ellipsometry in a range that encompasses the mid-infrared (4000 cm^-1 to 200 cm^-1). BSWs can be excited in both σ and π polarizations, which in the ellipsometric configuration can be detected at the same time, presenting distinct advantages for sensor applications targeting the growth of thin solid films and molecular monolayers, surface-adsorbed gas molecules, and liquid droplets. Compared to other sensing techniques exploiting mid-infrared vibrational absorption lines for chemical-specific sensitivity, like waveguides, nano-antenna arrays, metasurfaces, attenuated total reflectance (ATR) in crystals or in optical fibers, the present approach features high field enhancements, strong field confinement, and large quality factors of the resonances, all while relying on a rather simple and potentially low-cost configuration. The 1D-PCs were fabricated by depositing alternated homogeneous layers out of CaF₂ and ZnS with a geometry tailored to sustain BSW with a suitable dispersion in the 5000 cm^-1 to 1250 cm^-1 range. For the first time to our knowledge, we report the characterization of a sensor based on BSWs in the mid-infrared region using an ellipsometric approach to detect complex reflectivity. Our spectral analysis of mid-infrared 1D-PCs showed clear signatures of σ and π-polarized BSWs and guided modes. Although preliminary, our approach could lead to the development of innovative sensors, enhancing the capabilities of IR ellipsometry by utilizing BSWs in 1D-PCs.