Dual-band in situ molecular spectroscopy using single-sized Al-disk perfect absorbers

Abstract

We propose antenna-enhanced infrared vibrational spectroscopy by adopting single-sized Al disks on Al₂O₃-Al films fabricated by colloidal-mask lithography. The precisely designed plasmonic resonator with dual-band perfect absorption (DPA) shows strongly-enhanced nearfield intensity and polarization independence, at both resonances, providing a powerful antenna platform for the multi-band vibrational sensing. As a proof of concept, we experimentally apply the plasmonic DPAs in bond-selective dual-band infrared sensing of an ultrathin polydimethylsiloxane (PDMS) film, simultaneously amplifying two representative vibrational bands (asymmetric C–H stretching of CH₃ at 2962 cm⁻¹ and CH₃ deformation of Si–CH₃ at 1263 cm⁻¹) by surface-enhanced infrared absorption spectroscopy (SEIRA). The plasmonic DPA was successfully adopted for the in situ monitoring of reaction kinetics, by recording the spectral changes in C–H stretching and Si–CH₃ deformation modes of a 10 nm PDMS elastomer, which are selectively enhanced by the two antenna resonances, during its gelation process. Our systematic study of the SEIRA spectra has demonstrated mode splitting and a clear avoided-crossing in the dispersion curve as a function of resonance frequency of DPA, manifesting itself as a promising basis for future polaritonic devices utilizing the hybridization between the molecular vibrational states and the enhanced light field.