Tunable modulation frequency in photo-acoustic spectroscopy towards precise discrimination of resonant modes in nanostructures on absorbing substrates

Scattering-free assessment of resonantly absorbed electromagnetic field is highly important in nano-photonics and plasmonics: nanostructures and metamaterials can be tailored to optimize absorption for applications spanning from solar cells to efficient gas sensing. Photo-acoustic technique converts absorption-induced periodic heating in an acoustic signal, giving powerful means of characterizing absorption in scattering-free and non-destructive way. In this work, we use a widely tunable laser and tunable modulation frequency to perform photo-acoustic experiment on nanowire ensembles. We study absorption in two samples: semiconductor and semiconductor-plasmonic nanowires, both vertically standing on absorbing substrates. Increasing the modulation frequency from 54 Hz to 1225 Hz leads to a more precise discrimination of the resonant absorption modes of the nanowires, against the absorbing substrate. We show this behavior for GaAs-based nanowires in the near-infrared range. Moreover, in the hybrid sample, the asymmetric Au layer leads to extrinsic chiral response in the whole wavelength range.