  We experimentally demonstrate a transverse acoustic (TA) phonon transistor.
Phonons are coherently initiated by femtosecond photocarrier screening on
potential gradients. Although translational symmetry within the isotropic plane
normally prohibits optical generation of TA phonons, we show that the combined
application of an external bias in the vertical and lateral directions can
break the selection rules, generating the forbidden TA mode. The amplitude and
on-state time of the TA mode can be modulated by the external field strength
and size of the laterally biased region. The observed frequency shift with an
external bias as well as the strong geometrical dependence confirm the role of
the asymmetric potential distribution in electrically manipulating the crystal
symmetry to control and activate the transistor.

Materials Science