  In this paper, we report on the observation of nonlinear effects in a
nanostrip phononic metasurface (NPM) that enable the tuning of resonance
frequencies at 1.42 GHz. The NPM resonator made of periodic nanostrip array is
fabricated on a lithium niobate substrate. Each of the nanostrip is 250-nm wide
and is made of 680-nm-thick SiO2 layer stacking on 50-nm Al metal electrodes.
Finite element analysis reveals that the device operates in a vertically
polarized (compression) mode with substantial acoustic energy confined in the
nanostrips, leading to a local resonance at low acoustic velocity. Due to the
nonlinearity, the resonance frequency of the device decreases quadratically
with the increase of stimulation power from 10 to 30 dBm. The underlying
mechanism of the nonlinearity is found to be the power-dependent coupling of
the adjacent nanostrips. This coupling induces softening of the substrate
surface region, which reduces the acoustic velocity and hence the bulk
radiation. As a result, the quality factor of the NPM resonator is found to
improve with the increase of stimulation power. The power-dependent coupling of
nanostrips in the NPM resonator demonstrates a reliable method for the
realization of nonlinearity in phononic metasurfaces, which would significantly
enrich the mechanisms for the manipulation of surface acoustic waves at high
frequencies.
