J. Eur. Opt. Society-Rapid Publ. 22, 37( 2026) 373
Fig
. 3. Schematic of the excitation that enables extrinsic chirality in Sample C-Au( a) Sample C-Au is excited with LCP or RCP, at ± 20 ° oblique incidence, and the difference between the NW signal and the substrate is measured at 1225 Hz.( b) Difference of the PAS signal for LCP and RCP excitation for the incidence angles �20 °, 0 ° and 20 °.( c) Experimental normalized absorption difference as a function of the incidence angle, at 720 nm and 800 nm.( d) Distribution of the absorption density in a single NW on the substrate, excited with LCP and RCP, at 720 nm and at ± 20 ° of incidence. LCP leads to stronger absorption in the Au sidewalls than RCP for + 20 °, which inverts at �20 °, in agreement with the PAS experiment.
we were able to characterize absorption and extrinsic chirality in 20 – 50 nm of absorbing material [ 27 ]. However, with substrates that are thermally conductive and absorbing, the heat sinks into the substrate, leading to the lower nanostructure signal hidden by the substrate response. This can be partially solved by increasing f; however, the PAS amplitude decreases with the f increase, thus deteriorating signal to noise ratio. Finally, the increase of the incident power at higher modulation frequencies can be used to improve PAS sensitivity while discriminating the nanostructure response from the background substrate. All the PAS signals presented here are normalized to the incident power at that frequency. Moreover, we anticipate that investigating both PAS amplitude and phase while continuously tuning the frequency, and, in parallel, spectrally investigating the resonances, could reveal the depth sensitivity, and lead to novel applications in heat transport at the nanoscale.
4 Conclusion
We have studied semiconductor and semiconductor-plasmonic NW ensembles by means of photo-acoustic technique with modulation frequency in kHz range, and the laser light source in the near-infrared range. In semiconductor NW sample, the increase of the chopper frequency from 54 Hz to 1225 Hz leads to the more precise discrimination of the resonant leaky-waveguide mode peaking at 780 nm, in great agreement with numerical predictions. At 1225 Hz, the same sample, asymmetrically covered by Au, has hybridized plasmonic-photonic mode, broadened and red-shifted to 850 nm. Furthermore, with this modulation frequency we were able to detect weak extrinsic chiral behavior arising from the asymmetric geometry of Au on the NW sidewalls. We believe that tunable modulation frequency will become an important parameter for both revealing of the depth sensitivity, and for accessing novel photo-thermal phenomena at the nanoscale. Furthermore, enriching the set-up with photo-elastic modulators could allow for improved sensitivity of chiral sensing at much higher modulation frequencies, thus discriminating the properties of thin surface layers of samples [ 18 ].
Acknowledgments
The authors thank the ENSEMBLE 3 Project carried within the Teaming for Excellence Horizon 2020 program of the European Commission( GA No. 857543). A. B. and E. P. acknowledges LA- SAFEM Sapienza Università di Roma Infrastructure Project 2017 n. MA31715C8215A268 and PRIN 2022 PNRR Project IN- SPIRE n. P2022LETN5 founded by the European Union – NextGenerationEU. C. Skubisz acknowledges PNRR MUR project PE0000023-NQSTI. All authors acknowledge M. Magi for technical support.