Short Communication

Photo-acoustic spectroscopy detects nanostructures against resonant and absorbing substrates

¹ Sapienza University of Rome, Department SBAI, Rome, 00161, Italy
² Physics Department “A. Volta”, University of Pavia, via Bassi 6, Pavia 27100, Italy
³ Institute of Photonic and Nanotechnology (IFN) – Consiglio Nazionale delle Ricerche (CNR), LNESS Laboratory, Como, Italy
⁴ Department of Physics, Politecnico di Milano, Milano, Italy

^* Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 6 October 2025
Accepted: 20 November 2025

Abstract

Photo-acoustic spectroscopy is a powerful photo-thermal method for scattering-free, contactless and non-destructive measurements of absorption. In plasmonics and nano-photonics, it allows for characterization of light-matter interactions leading to non-radiative relaxation processes, which generate heat. Here, we address the issue of detecting absorption signal of periodically nanostructured layer of gold, sitting on a commercial, absorbing substrate. The nanostructured plasmonic layer alone exhibits resonant peaks in the near-infrared range due to a 2D array of elliptical nanoholes in Au. The substrate itself has cavity interference resonances in the same range, due to a layer of Si₃N₄ on Si wafer. Conventional optical techniques are influenced by scattering, which complicates the characterization of the nanostructures’ absorption against the properties of the substrate. We apply photo-acoustic technique with a widely tunable laser source to measure the absorption spectra of these geometries, as well as on an Au-covered substrate without nanoholes and on a bare Si substrate. A microscope enabled us to differentiate the nanohole response from its surroundings, while the tunable modulation frequency allowed for studying different absorption depths. Complementary optical simulations reveal the spatial distribution of absorption, in good agreement with experimental results. A major potential of the proposed approach lies in monitoring and discriminating nanoscale structural changes in thin absorbing layers against the strongly absorbing substrate background, which is of great importance in situations involving sensing of low quantity of absorbing material placed on commercial wafers. Moreover, besides resonant plasmonic effects, this photo-acoustic set-up and modelling can be further adapted to study light coupling with plasmonic nanohole arrays in terms of incident light angle of incidence and polarization.