  Strain engineering has been extended recently to the ultrafast timescales,
driving metal-insulator phase transitions and the propagation of ultrasonic
demagnetization fronts. However, the non-linear lattice dynamics underpinning
interfacial optoelectronic phase switching have not yet been addressed. Here we
focus on the lattice dynamics initiated by impulsive resonant excitation of
polar lattice vibrations in LaAlO$_3$ single crystals, one of the most widely
utilized substrates for oxide electronics. We show that ionic Raman scattering
drives coherent oxygen octahedra rotations around a high-symmetry crystal axis
and we identify, by means of DFT calculations, the underlying phonon-phonon
coupling channel. Resonant lattice excitation is shown to generate longitudinal
and transverse acoustic wavepackets, enabled by anisotropic optically-induced
strain in and out of equilibrium. Importantly, shear strain wavepackets are
found to be generated with extraordinary efficiency at the phonon resonance,
being comparable in amplitude to the more conventional longitudinal acoustic
waves, opening exciting perspectives for ultrafast material control.
