Transform-limited pulses are not optimal for resonant multiphoton transitions

Maximizing nonlinear light-matter interactions is a primary motive for comp ressing laser pulses to achieve ultrashort transform limited pulses. Here w e show how, by appropriately shaping the pulses, resonant multiphoton trans itions can be enhanced significantly beyond the level achieved by maximizin g the pulse's peak intensity. We demonstrate the counterintuitive nature of this effect with an experiment in a resonant two-photon absorption, in whi ch, by selectively removing certain spectral hands, the peak intensity of t he pulse is reduced by a factor of 40, yet the absorption rate is doubled. Furthermore, by suitably designing the spectral phase of the pulse, we incr ease the absorption rate by a factor of 7.