Surface-Enhanced Infrared Absorption for the Periodic Array of Indium Tin Oxide and Gold Microdiscs: Effect of in-Plane Light Diffraction

Surface-enhanced infrared absorption (SEIRA) is an important phenomenon to achieve nondestructive, simplified, and in situ high-sensitivity infrared (IR) sensors. Conventionally, metal structures with nanogaps are employed to realize the high sensitivity owing to the extremely strong field enhancement in the hot spot. Although a library of surface modifiers has been developed, the manipulation of nanogaps and immobilization of target molecules in the hot spot are still complicated. In addition, target molecules immobilized at the positions other than the hot spot have relatively low sensitivity. A periodic array with pitch comparable to the wavelength of interest is an alternative structure in which the coupling of the plasmonic mode to in-plane light diffraction provides the hybrid mode accompanied by an enhanced electric field. Although the field enhancement by the hybrid mode depends on matching between localized surface plasmon resonance (LSPR) and diffraction, the contribution of the matching to SEIRA enhancement has never been clarified. In this work, we fabricated periodic arrays of indium tin oxide (ITO) and Au microdiscs (pitch: 3 μm, diameter: 2 μm) to analyze the contribution of the hybrid mode through varied LSPR and diffraction conditions. As a result, the ITO and Au arrays demonstrate a similar plasmonic–photonic hybrid mode in the mid-IR region despite the different excitation frequency of LSPR. To estimate the effect of the hybrid mode on SEIRA enhancement, the incident angular profiles of IR spectra of the polymer layer on the ITO and Au arrays were measured. The SEIRA enhancement factors for ITO and Au arrays are comparable in the IR measurement region (2200–1400 cm–1). Our results verify that the plasmonic–photonic hybrid mode is very efficient for SEIRA enhancement, and the periodic array of microdiscs is very suitable for this application.