This thesis takes advantage of both the ultrashort duration and high peak power of femtosecond laser pulses to explore two essential frontiers in condensed-matter and strong-field physics. First, it resolves the long-debated insulator-to-metal transition mechanism in monoclinic (M1 phase) vanadium dioxide (VO2) by using a single-cycle optical pulse excitation. The optical pulse first drives the system to an excited metallic rutile state, followed by electron thermalization and lattice relaxation that unfold over tens to hundreds of femtoseconds. Isolation of the structural and electronic dynamics of the system is achieved through this methodology. The extracted pure population dynamics allow for the first time to visualize the highly anharmonic vanadium–vanadium bond oscillations, which play a key role in the physics of the insulating monoclinic to rutile metal phase transition. Second, the thesis demonstrates the development of an advanced solid-state high harmonic generation and spectroscopy apparatus, which combines stable broadband harmonic detection under precisely characterized light fields. This capability enables for the first time, the precise study of the transition from perturbative to nonperturbative or extreme nonlinear optics in bulk crystals. The transition to the extreme regime is probed by tracking the scaling of the harmonic yield with the applied field strength over a broad range of crystalline solids—including diamond, MgO, MgF2, CaF2, and LiF, thereby providing a platform for investigating the physics of this transition.