Enhancement of intermediate-field two-photon absorption by rationally shaped femtosecond pulses
Lev Chuntonov, Leonid Rybak, Andrey Gandman, and Zohar Amitay

We extend the powerful frequency-domain analysis of femtosecond two-photon absorption to the intermediate-field regime of considerable absorption yields, where additionally to the weak-field non-resonant two-photon transitions also four-photon transitions play a role. Consequently, we rationally find that the absorption is enhanced over the transform-limited pulse by any shaped pulse having a spectral phase that is anti-symmetric around one-half of the transition frequency and a spectrum that is asymmetric around it (red- or blue-detuned according to the system). The enhancement increases as the field strength increases. The theoretical results for Na are verified experimentally.

© 2008 The American Physical Society.