Tunable Exciton-Driven Photoelasticity in 2D Material Acoustic Cavities

While coupling between optical, electronic, and mechanical domains is paramount for high-frequency acoustic devices, materials that offer tunability in the degree of such coupling can be crucially enabling in expanding device functionality. Here, we show that the interaction of photons with coherent acoustic phonons confined in 2D layered semiconducting cavities can be controlled through either modifying the material state via a thermally induced electronic bandgap shift (EBS) or altering the polarization state of the incoming photons when optical birefringence is present in the cavity. We demonstrate temperature-driven EBS as an effective tool to engineer the WSe₂ cavity readout as it allows one to sweep the excitonic energy relative to a chosen probe wavelength. We envision the resulting amplitude and phase modulation of the optical readout as a way of enhancing the cavity's functionality, given that the diminishing heat capacity of the ultrathin suspended films implies an upper limit for the rate of thermo-optic phase switching in excess of 100 MHz. For acoustic cavities that must operate at lower temperatures, we demonstrate a multiexciton extension of the approach where the output signal is controlled by selectively accessing different excitonic states in birefringent ReS₂. Density functional theory calculations indicate that even though the electronic bands of WSe₂ and ReS₂ are shaped predominantly by intralayer electronic interactions, the out-of-plane strain-driven deformation potential (DP), dE_g/dη ∼1 eV (critical for optical transduction of "breathing mode" vibrations), is significant for multiple electronic valleys of interest and is consistent with the experimental results. We anticipate that the demonstrated experimental approach for quantifying the out-of-plane DP in ultrathin films can be extended to heterostructures, in which sophisticated cross-plane interactions can be engineered using combined mechanical and electronic properties of heterogeneous 2D materials.