Mesoporous-based GHz acoustic resonators with different pore size distributions

The manipulation of acoustic phonons in the gigahertz range has potential for technological breakthroughs. Novel approaches for tunable nanophononics resonators responsive to environmental conditions add an extra control and functionality for these devices. Mesoporous materials for example, with pore sizes at the nanoscale are responsive to humidity. 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. One particularity of liquid infiltration at the nanoscale is the nonlinear response of the liquid infiltration within the pores with the ambient relative humidity, and is strongly dependent on the pore size distribution of the materials. Here, we present a comparison between two open-cavity acoustic resonators of SiO₂ MTFs with different pore size distributions. The resonators are composed of a nickel acousto-optical transducer deposited on a glass substrate, and the MTF on top. We characterize the resonators by transient reflectivity experiments for coherent acoustic phonon generation and detection. Our findings show that the acoustic modes confined in the MTF are independent on the pore size distribution, but mainly on the material properties and layer thickness. This allows for a straightforward comparison when humidity infiltration will be investigated, as both sizes should present different infiltration curves with the relative humidity levels.