J. Eur. Opt. Society-Rapid Publ. 2026, 22, 37 Ó The Author( s), published by EDP Sciences, 2026 https:// doi. org / 10.1051 / jeos / 2026020 Available online at: https:// jeos. edpsciences. org
Journal of the European Optical Society-Rapid Publications
PLASMONICA Collection
SHORT COMMUNICATION
Tunable modulation frequency in photo-acoustic spectroscopy towards precise discrimination of resonant modes in nanostructures on absorbing substrates Nicolo ' Borriello 1, Emilija Petronijevic 1, *
, Juliette Schlachet-Lépinay 1, 2, Claudia Skubisz 1, Grigore Leahu 1, Teemu Hakkarainen 3, Concita Sibilia 1, Roberto Li Voti 1, and Alessandro Belardini 1 1 Sapienza University of Rome, Department SBAI, Rome 00161, Italy 2 École Nationale Supérieure de Techniques Avancées, 91120 Palaiseau, France 3 Optoelectronics Research Centre, Physics Unit, Tampere University, Korkeakoulunkatu 3, 33720 Tampere, Finland
Received 26 October 2025 / Accepted 25 February 2026
Abstract. Scattering-free assessment of resonantly absorbed electromagnetic field is highly important in nanophotonics 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 nearinfrared range. Moreover, in the hybrid sample, the asymmetric Au layer leads to extrinsic chiral response in the whole wavelength range.
Keywords: Photo-acoustic spectroscopy, Plasmonics, Nanostructures, Nanowires, Absorption.
1 Introduction
Nanostructured materials can be engineered to exhibit specific resonant absorption modes, optimizing the behavior of nanodevices from photodetectors [ 1 ] to molecular sensors [ 2 ]. Nanoscale structures are usually deposited on a much more robust substrate layer. This layer, though, interferes with an accurate evaluation of the sample absorption or its resonances. Photo-acoustic spectroscopy( PAS) is a non-invasive, contactless, photo-thermal technique that allows for a direct measurement of the sample’ s absorption without the scattering influence. In PAS, light, modulated at a frequency f, excites the sample in a closed cell. This periodic sample absorption leads to periodic heating, creating thermal waves that can be detected by a sensitive microphone. The electric signal is then compared with the modulation reference through a lock-in amplifier. Such extracted PAS signal depends on the sample material properties: absorption and thermal diffusivity. For optically and thermally thick samples, the absorption coefficient
* Corresponding author: emilija. petronijevic @ uniroma1. it spectrum can be calculated from the normalized PAS amplitude [ 3 ], while the PAS spectrum follows the form of the absorbance. By varying the chopper frequency, different depths along the sample’ sheightcanbeprobed. Thistool was shown to be a promising in characterization of optical and photo-thermal properties of absorbing materials on substrates, being those nanoparticles [ 4 ], thin films [ 5 ], periodic plasmonic structures [ 6 ] or ensembles of nanowires [ 7 ].
In PAS, the detected absorption signal arises from the material volume limited by the thermal diffusion length p, which scales with the modulation frequency as 1 = ffiffi f. Therefore, increased modulation frequencies allow measurements of upper zones of the samples that can be nanostructured to exhibit resonant absorption modes. Previously, we qualitatively showed that an increase in modulation frequency can be used to access the resonant absorption in nanowires( NW) against their substrates, even if both strongly absorb in the same wavelength range. In Ref. [ 8 ], we used white light lamp source to excite GaAs-based NW on Si substrate, and we monitored the difference of the NW signal with respect to the one of the substrate, i. e. normalized PAS signal. When we changed the
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