Surface-Enhanced Infrared Absorption Spectroscopy Using Charge Transfer Plasmons

Plasmonic antennas have been widely used to enhance the infrared absorption of molecular moieties, owing to their ability to localize mid-infrared light into nanometer spaces at desired wavelengths. However, at mid-infrared wavelengths, these metallic plasmonic antennas do not benefit from the wavelength-scaling that allows for the smaller sizes of antennas operating at shorter wavelengths. Here, we show that charge transfer plasmons (CTPs) enable a smaller antenna footprint with resonances in the mid-infrared that are tunable with nanometer-scale variation in the width of a conductive bridge. The CTP resonance is readily tuned to match the vibrational modes and realize surface enhanced infrared absorption (SEIRA) of a self-assembled monolayer of hexadecanethiol molecules. Moreover, with their small footprint and the width tuning ability, the CTP antennas are arranged in a grating array to further increase the plasmonic hot-spot density as well as the SEIRA signal. Our results show that CTPs provide a new approach for the mid-infrared antenna design for SEIRA with nanometer-scale resonance tuning ability.