Molecular Fingerprint Detection with a Mid-infrared Guided-Mode Resonance

Guided-mode resonance (GMR) is a compelling scheme for optical sensing of various physical quantities, which holds great promise in non-invasive detection of chemical and biological species. Current GMR sensors are mostly based on detection of environmental refractive index change, which presents a challenge for obtaining sensing selectivity as required in practical applications. In this work, we report on a prototype demonstration of using GMR for detecting molecular fingerprint absorption. In a waveguide grating made of a ZnSe layer sandwiched between a Cu film and an Au grating, a mid-infrared GMR is excited and tuned over a wide frequency range of 617.4 cm −1 . Coupling of the GMR with a polydimethylsiloxane (PDMS) analyte film reveals the signature absorptions of CH 2 group, whose vibrational signal is enhanced by a factor of 2.37 at a frequency detuning of 30 cm −1 between the GMR and the molecular vibration. Our theoretical analysis reveals that both near-field enhancement of GMR and its coupling strength with molecules are responsible for the molecular detection performance. These results suggest that angle-tuned GMR presents an alternative design paradigm for surface-enhanced infrared absorption spectroscopy (SEIRAS).