Mazur's early work at Harvard focused on the use of short-pulse lasers to carry out spectroscopy of highly vibrationally excited molecules. Mazur and his group have made many pioneering contributions to the field of ultrashort laser pulses and their interactions with matter ("femtosecond material science"). In 1989 his group was one of the first in academia to build a colliding-pulse mode-locked laser, which generated pulses of only 70 femtosecond duration. After early measurements by Mazur's group demonstrated conclusively that solids can undergo a structural phase transition without appreciable heating of the lattice, Mazur's group developed a technique to measure the full dielectric function of highly excited semiconductors. Since then the group's use of this technique and various nonlinear optical probes to study laser-induced structural phase transitions.