Electrically driven amplification of terahertz acoustic waves in graphene

Dr. Aaron Barajas, Postdoctoral Scholar, UC Irvine

In graphene devices, the electronic drift velocity can easily exceed the speed of
sound in the material at moderate current biases. Under this condition, the
electronic system can efficiently amplify acoustic phonons, leading to the
exponential growth of sound waves in the direction of the carrier flow. In this
talk, I will discuss our findings about how such phonon amplification can
significantly modify the electrical properties of graphene devices. We observe a
superlinear growth of the resistivity in the direction of the carrier flow when the
drift velocity exceeds the speed of sound, resulting in a sevenfold increase over a
distance of 8 µm. The resistivity growth is observed at carrier densities away from
the Dirac point and is enhanced at cryogenic temperatures. These observations
are explained by a theoretical model for the electrical amplification of acoustic
phonons — reaching frequencies up to 2.2 THz — where the wavelength is
controlled by gate-tunable transitions across the Fermi surface. These findings
provide a route to on-chip high-frequency sound generation and detection in the
THz frequency range.