  We present a microscopic theory for ultrafast control of solids with
high-intensity terahertz frequency optical pulses. When resonant with selected
infrared-active vibrations, these pulses transiently modify the crystal
structure and lead to new collective electronic properties. The theory predicts
the dynamical path taken by the crystal lattice using first-principles
calculations of the energy surface and classical equations of motion, as well
as symmetry considerations. Two classes of dynamics are identified. In the
perturbative regime, displacements along the normal mode coordinate of
symmetry-preserving Raman active modes can be achieved by cubic
anharmonicities. This explains the light-induced insulator-to-metal transition
reported experimentally in manganites. We predict a regime in which ultrafast
instabilities that break crystal symmetry can be induced. This nonperturbative
effect involves a quartic anharmonic coupling and occurs above a critical
threshold, below which the nonlinear dynamics of the driven mode displays
softening and dynamical stabilization.

Strongly Correlated Electrons