Plasmonic-enhanced infrared photoexpansion nano-spectroscopy using tunable quantum cascade lasers

Mid-IR photoexpansion nano-spectroscopy measures spectra of samples on nanoscale by detecting local thermal expansion associated with light absorption using a standard atomic force microscope (AFM). Cantilever deflection is directly proportional to sample absorption. This method results in a simple experimental setup with no optical detectors. We have recently demonstrated that the sensitivity of photoexpansion nano-spectroscopy can be dramatically enhanced by moving the laser pulses repetition frequency in resonance with the mechanical frequency of the AFM cantilever. We were able to produce spectra from ~100 nm thin films using low energy (4 nJ) pulses from a tunable quantum cascade laser (QCL). The spatial resolution, which is determined by thermal diffusion length, has been demonstrated to be better than 50 nm. Sample heating is limited to ~10 mK. Here we present a novel approach to increase both the sensitivity and spatial resolution of photoexpansion nano-spectroscopy. We utilize the plasmonic local-intensity enhancement below a gold-coated AFM tip. We successfully produced high-quality vibrational absorption spectra from samples as thin as 10 nm positioned on top of gold-coated silicon substrates. In addition to higher photoexpansion signal, our technique features higher spatial resolution, which is no longer limited by thermal diffusion length but is instead determined by the dimensions of the high-intensity field region below the metal tip, which can be 10 nm or smaller.