Plasmonic absorbers with optical cavity for the enhancement of photothermal/opto-calorimetric infrared spectroscopy

Plasmonic absorbers with optical cavity are fabricated using deposition of thin gold films on both sides of nanoporous anodic aluminum oxide (AAO) microcantilevers. Photothermal/opto-calorimetric responses of the microcantilevers are investigated with respect to various pore sizes and gold coating thicknesses. Photothermal/opto-calorimetric response of the AAO cantilever is significantly amplified when the thickness of the gold layer is at 40 nm due to scattering and trapping of infrared (IR) radiation in the gold nanomesh/AAO nanochannels/gold layer structure. Unlike previous photothermal IR spectroscopy of adsorbed molecules with a bimetallic AAO cantilever, the reversed IR spectra are obtained due to significantly amplified baseline photothermal signal and IR absorption of free molecules. Molecular detection sensitivity is enhanced by an order of magnitude compared with the previous photothermal IR spectroscopy of adsorbed molecules on a bimetallic AAO cantilever. The acetone vapor molecules are selectively recognized using the reversed IR spectra.