  Acoustic phonons can be coherently generated by ultrafast displacive
screening of potential gradients, often enhanced by the strong built-in
piezoelectric fields, in wurtzite semiconductors. In such structures,
transverse symmetry within the c plane hinders both the generation and
detection of the transverse acoustic (TA) modes, and only longitudinal acoustic
(LA) mode is generated. We show that even for c-GaN, the application of
asymmetric potential distributions in the c plane can break the symmetry and
selection rules, thus switching on the normally forbidden TA mode. This is in
contrast to the LA mode, the strength of which varies with the symmetric
potential distributions. By comparing transient differential reflectivity
spectra in structures with and without asymmetric potential distributions, the
role of the electrically attained anisotropy was further revealed by the
digitized appearance of the TA mode, in clear contrast to the monotonic LA
mode, and by modulations in the propagation velocities, optical birefringence,
and geometrically varying sensitivities, the underlying mechanisms of which are
modeled by electric-field-dependent perturbations of the dielectric tensors,
incorporating the results of elastic modulations.
