J. Eur. Opt. Society-Rapid Publ. 22, 37( 2026) 371
Fig
. 1.( a) Photo-acoustic spectroscopy, allows for tunable polarization, power, modulation frequency, incidence angle; inset: schematic of the two samples. Both have diameter of 165 nm and height of 4.6 lm; Sample C-Au has two out of six sidewalls covered by 20 nm Au.( b) Simulation of a single NW of Sample C, in the air and standing on the Si substrate, and the 3D distribution of the absorption density at the resonance of 780 nm. The Si substrate widens the leaky waveguide mode resonance.( c) Simulation of a single NW covered by Au, and the 3D distribution of the absorption density at 850 nm. Here, the substrate strongly influences absorption as its parts contain absorbing material.
plots PAS signal amplitudes, normalized to the incident laser power, for Samples C, C-Au and the substrate. At 54 Hz, both nanostructured samples absorb more than the substrate, with no particular resonant features. In our previous work [ 8 ], we observed the resonant absorption modesinsimilarsamplesat225Hz. Inthiswork, thelaser allows for wider power tunability, so we choose higher frequencies of 408 Hz and 625 Hz. At both frequencies, starting from wavelength of 820 nm, we observe a distinct absorption drop for Sample C; moreover, in that range, the difference between this sample and the substrate decreases. This does not happen for Sample C-Au, as it absorbs in the whole investigated spectral range; instead, for wavelengths larger than 840 nm, the absorption is increased with respect to both substrate and Sample C. In this sample, apart from the Au layer on the NW, Au layer on the substrate also contributes to the absorption signal.
Next, to study the shapes of the resonant modes, we define the normalized PAS signal by subtracting the substrate signal from the NW signal at each frequency, and normalizing it to its maximum, Figure 2b. For both samples, the low modulation frequency of 54 Hz results in poor discrimination of the NW resonant behavior with respect to the substrates. For Sample C, the increase of modulation frequency leads to sharpening of the resonant modes. At 1225 Hz, the substrate behavior can be neglected, as the normalized signal vanishes after the GaAs bandgap; this result is in great agreement with Figure 1b. For Sample
C-Au, Au-substrate strongly influences the resonances at all frequencies, making the normalized spectra without prominent absorption features. In Figure 1c, we plot the difference in the PAS amplitude between the NWs and the substrate at 1225 Hz. For Sample C-Au, this frequency allows to resolve the NW absorption, predicted in Figure 1c, although the agreement with simulations is worse than in the case without Au. This is likely due to the lower uniformity of the geometric parameters regarding shadows and parasitic layers, resulting from the tilted evaporation of Au.
Polarization control of a photo-thermal set-up allow for monitoring the difference in the absorption when the nanostructure is excited with left versus right circular polarization( LCP versus RCP, respectively), i. e. circular dichroism( CD). Photo-thermal techniques that characterize CD can probe chirality from intrinsically chiral ensembles of vertically standing nanohelices [ 17 ], to single nanostructures [ 18 ]. However, asymmetric Au layer does not make the sample 2D nor 3D intrinsically chiral, hence in a perfectly aligned set-up, CD must be zero at normal incidence [ 19 – 22 ]. Oblique incidence, instead, leads to the phenomena of extrinsic chirality for specific sample orientations. While being an artifact in intrinsic chirality measurements, extrinsic chirality can be obtained in simpler geometries which include asymmetric layers without helical or gammadion shapes. Indeed, in similar NWs, extrinsic chirality has been first demonstrated with single wavelength PAS [ 23 ], and then proposed for the tuning of GaAs emission [ 24 ] andsecond harmonic generation [ 25 ]. Previously, we measured