Visualizing Nonlinear Phononics in Layered ReSe2

Nonlinear phononics has recently been demonstrated as a viable approach for dynamically modifying materials' properties. Conventionally, nonlinearity in the lattice dynamics is introduced via the ionic Raman scattering, in which infrared-active phonons (i.e., coherent ionic vibrations) serve as the intermediate state for transferring energy to Raman-active phonons. Here we report that it is also possible to achieve phononic nonlinearity through the electronic route, a process that relies on excited electronic states to initiate energy exchange among Raman-active phonons. Taking layered ReSe2 as a model system, we use coherent phonon spectroscopy with a pump energy larger than the band gap to follow lattice dynamics and observe the nonlinear coupling between both Raman-active intralayer atomic oscillations and interlayer breathing modes. In addition, we show that such nonlinear phononic coupling is highly dependent on the environment temperature. This work, which demonstrates a different and novel mechanism, may enrich the toolkit for controlling material properties by means of nonlinear phononics.

Automated summary

Research demonstrates that nonlinear phononics can be achieved through the electronic route, in addition to introducing nonlinearity via ionic Raman scattering. By using coherent phonon spectroscopy with a pump energy larger than the band gap in layered ReSe2, nonlinear coupling between Raman-active phonons was observed.