Toward Environmentally Responsive Hypersound Materials.

Engineering gigahertz (GHz) acoustic phonons holds promise for data processing, sensing, and quantum technologies. However, conventional nanophononic resonators lack adaptability to environmental changes. We introduce an open-cavity nanoacoustic resonator built from mesoporous SiO2 thin films (MTFs), whose ordered nanopores make them intrinsically sensitive to humidity. Transient reflectivity measurements reveal pronounced resonance frequency shifts with varying relative humidity, providing a straightforward means to tune the hypersound confinement. Systematic comparisons across pore sizes and film thicknesses show that resonances are governed primarily by thickness and intrinsic material properties rather than pore geometry. The pore size, however, governs the tunability dynamics due to the capillary action. By direct exposure of the mesoporous layer to its environment, this design establishes a versatile platform for coupling nanoscale mechanics with liquids and vapors. Our results highlight a simple route toward environmentally responsive hypersound devices with a potential impact in sensing and adaptive nanophononics.