In 1991, L'Huillier in collaboration with Kenneth Schafer and Kenneth Kulander presented numerical simulations of the time-dependent Schrödinger equation in order to understand the generation of high-order harmonics. They first predicted the shape of the high harmonics spectrum and the phase-matching conditions. In 1994, Maciej Lewenstein, L’Huiller and Paul Corkum presented a full quantum theory of high harmonic generation.