Spectral Characterization of Mid-Infrared Bloch Surface Waves Excited on a Truncated 1D Photonic Crystal.

The many fundamental roto-vibrational resonances of chemical compounds result in strong absorption lines in the mid-infrared region (λ ∼ 2-20 µm). For this reason, mid-infrared spectroscopy plays a key role in label-free sensing, in particular, for chemical recognition, but often lacks the required sensitivity to probe small numbers of molecules. In this work, we propose a vibrational sensing scheme based on Bloch surface waves (BSWs) on 1D photonic crystals to increase the sensitivity of mid-infrared sensors. We report on the design and deposition of CaF2/ZnS 1D photonic crystals. Moreover, we theoretically and experimentally demonstrate the possibility to sustain narrow σ-polarized BSW modes together with broader π-polarized modes in the range of 3-8 µm by means of a customized Fourier transform infrared spectroscopy setup. The multilayer stacks are deposited directly on CaF2 prisms, reducing the number of unnecessary interfaces when exciting in the Kretschmann-Raether configuration. Finally, we compare the performance of mid-IR sensors based on surface plasmon polaritons with the BSW-based sensor. The figures of merit found for BSWs in terms of confinement of the electromagnetic field and propagation length puts them as forefrontrunners for label-free and polarization-dependent sensing devices.