The role of pore size in the performance of mesoporous-based gigahertz nanoacoustic resonators

Traditionally investigated in semiconductor superlattices, acoustic phonons in the tens of gigahertz to sub-terahertz range require costly fabrication and lack tunability [1]. Mesoporous materials, with nanoscale pores, present a promising alternative. Mesoporous SiO2 and TiO2-based acoustic resonators support gigahertz-range resonances, and liquid/vapor infiltration can alter their optical and elastic properties. This study proposes open-cavity acoustic resonators using SiO2 mesoporous thin films (MTFs) responsive to ambient humidity. The design includes an acoustic distributed Bragg reflector, a nickel acousto-optical transducer, and an MTF layer. Porosity's influence on acoustic resonances is explored by comparing different pore sizes. Using a transient reflectivity pump-probe setup, where the pump beam induces thermoelastic stress and the probe detects transient reflectivity, these findings advance cost-effective, reconfigurable nanoacoustic devices. [1] P. Priya, Cardozo de Oliveira, E. R., and Lanzillotti-Kimura, N. D., Applied Physics Letters 122, 140501 (2023)