Plasmonic nanoantennas, typically made of noble metals, have emerged as promising
platforms for integrating optical and mechanical functionalities at the nanoscale, as they can
sustain GHz mechanical vibrations upon ultrafast optical excitation [1]. However, their
mechanical quality factor (Q) is typically limited by intrinsic losses and thermal effects, as well
as by acoustic energy leakage into the substrate. Here, we demonstrate that inserting a
mesoporous film between the nanoantenna and the substrate offers a scalable route to
engineering high-f*Q nanoresonators, paving the way for tunable and low-loss
nanomechanical systems relevant to sensing and nanophotonics.
We investigate gold nanorods (GNRs) fabricated by e-beam lithography on mesoporous silica
films with controlled porosity (0%, ~20%, ~40%) synthesized via a sol-gel process. Their
mechanical response is characterized using two-color ultrafast pump-probe spectroscopy. A
consistent enhancement of Q, up to 25%, is observed for GNRs on porous substrates compared
to non-porous ones, indicating that mesoporous films act as effective acoustic insulators. This
effect is further supported by time-domain FEM simulations, which reveal longer phonon
lifetimes with increasing porosity, achieved without the need for full suspension or complex
nanofabrication.
Interestingly, we also reveal that both f and Q are tunable via environmental humidity due to
water adsorption in the pores, which modifies the local density and acoustic impedance. This
results in a consistent downshift of the resonance frequency with increasing humidity. While
low-porosity substrates show reduced Q at high humidity (consistent with enhanced damping),
high-porosity substrates display a slight Q improvement, likely due to the passivation of
surface scattering centers by infiltrated water.
These results demonstrate that mesoporous substrates can act as passive acoustic barriers,
enhancing the mechanical coherence of plasmonic resonators. The resulting f*Q values,
although still below the decoherence threshold (~6×10¹² Hz) at room temperature [2], indicate
promising progress toward integrated quantum optomechanical systems based on plasmonic
elements.

References:
[1] A. V. Bragas et al., J. Opt. Soc. Am. B 40, 1196 (2023).
[2] J. Wang et al., Nat. Commun. 10, 1146 (2019).