Double resonance 1-D photonic crystal cavities for single-molecule mid-infrared photothermal spectroscopy: theory and design

We propose and theoretically examine a novel mid-infrared (mid-IR) photothermal spectroscopic sensing technique capable of detecting a single small molecule. Our conceptual design attains such high sensitivity by leveraging dramatically amplified photothermal effects in an optical nanocavity doubly resonant at both mid-IR pump and near-IR probe wavelengths. Unlike conventional mid-IR spectroscopy, the technique eliminates the need for cryogenically cooled mid-IR photodetectors, as optical detection is performed solely at the near-IR probe wavelength. A device design based on nested one-dimensional nanobeam photonic crystal cavities is numerically analyzed to demonstrate the technique's potential for single small gas molecule detection.