Towards an on-chip terahertz acoustic wave source based on graphene devices.

In graphene devices, the electronic drift velocity can easily exceed the speed of sound in the material at moderate current biases. Under this condition, stimulated phonon emission dominates over absorption and can produce an exponential growth of the phonon population in the direction of the carrier flow. Here, we demonstrate that phonon amplification can significantly affect the electrical properties of long clean graphene devices, increasing its resistivity up to 7 times over a distance of 8 microns. These effects are observable at a wide range of carrier densities (0.5×10¹² to 4×10¹² cm-²) and at temperatures from 1.5 to 280 K. Due to the ease of reaching the emission condition, phonon amplification should be considered when measuring electrical transport in long graphene devices. These findings could lead to a new method of room temperature on-chip generation and detection of acoustic waves in the THz frequency range.