THz Phonon Generation in Graphene Driven Out of Equilibrium

In ultraclean graphene devices driven out of equilibrium, electrons can reach high drift velocities with relative ease. When the drift velocity of electrons exceeds the speed of sound in a material, stimulated phonon emission dominates over absorption. This can result in an amplification of THz frequency phonons in the direction of carrier flow. We explore the local electrical properties along long graphene devices, observing a 7-fold increase in resistivity over a distance of 8 microns. Phonon amplification is demonstrated under a wide range of carrier densities (0.5 to 4*10^12 cm^-2) and temperatures (1.5 to 280K). The resulting inhomogeneous resistivity could modify electron transport measurements in similar devices and act as another effect for consideration for long graphene devices. Our findings suggest future applications as a novel on-chip THz phonon generator, which could be coupled to other 2D materials to modify crystal structure on the order of the atomic lattice.