The manipulation of ultra-high-frequency acoustic phonons in the tens of gigahertz to sub-terahertz range offers a promising avenue for technological advancements. This control can be achieved by means of semiconductor superlattices, such as GaAs/AlAs, which requires costly fabrication processes in order to achieve atomically flat interfaces to avoid phonon scattering and dephasing. In addition, acoustic resonators based on such systems are bound to fixed frequency operations. Mesoporous materials, with pores at the nanoscale, are good candidates to overcome such limitations. It has been shown that multilayered resonators based on mesoporous SiO2 and TiO2 support acoustic resonances in the 5-100 GHz range. These materials are susceptible to liquids and vapors infiltration into the pores, causing the modification of the effective optical and elastic parameters. Here we present a design of open-cavity acoustic resonators of SiO2 Mesoporous Thin Films (MTFs) responsive to the environment. The resonator is composed of an acoustic distributed Bragg reflector, to avoid phonon diffusion towards the substrate, followed by a nickel acousto-optical transducer, and the MTF on top. We characterize the resonator by using coherent phonon generation and detection experiments in a reflectivity pump-probe setup and compare it with a simplified structure. The experimental results show that the acoustic signals in the open-cavity resonator have improved performance. Our findings unveil a promising platform for nanoacoustic reconfigurable optoacoustic devices based on soft, inexpensive fabrication methods. As a perspective to this work, we aim at studying the dependence of the acoustic resonances on the environment humidity.
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