Mid-Infrared Plasmonic Excitation in Indium Tin Oxide Microhole Arrays

Transparent conducting oxides (TCOs) are emerging as possible alternative constituent materials to replace noble metals such as silver and gold for low-loss plasmonic applications in the near-infrared (NIR) and mid-infrared (MIR) regimes. In particular, TCO-based nanostructures are extensively investigated for biospectroscopy exploiting their surface-enhanced infrared absorption (SEIRA). The latter enhances the absorption from vibrational and rotational modes of nearby biomolecules, making TCO nanostructures a promising candidate for IR sensing applications. Nevertheless, in order to produce inexpensive devices for lab-on-a-chip diagnostics, it would be favorable to achieve surface-enhanced infrared absorption with very simple microstructures not requiring nanosize control. In this work, we attempt to demonstrate a SEIRA effect with the least challenging fabrication, μm-scale instead of nm-scale, by tailoring both device design and charge density of the indium tin oxide (ITO) film. We show that microperiodic hole arrays in a ITO film are able to produce SEIRA via grating coupling. Such a study opens the way for innovative and disrupting biosensing devices.