The role of multiphoton excitation in ultrafast white-light continuum generation

When a powerful ultrashort laser pulse is focused into a transparent medium, it can undergo severe spectral broadening and be transformed into a white-light spectral continuum. Despite the continuum's widespread use as a tunable ultrafast light source, the mechanisms of continuum generation are still poorly understood. We report an experimental investigation of white-light continuum generation in transparent media using 100-fs pulses from a Ti:sapphire laser system. We measured the continuum's spectral width for a variety of materials and pump wavelengths. The materials were selected to cover a range of band gaps and the laser wavelength was tuned using a seeded optical parametric amplifier. We found that the generation of a continuum as well as the width of the continuum depends on the ratio of the material's band gap energy to the pump photon energy. Because this ratio is also a key parameter governing multiphoton excitation of electrons, we propose that multiphoton excitation plays a dominant role in continuum generation. The sudden appearance of free electrons by multiphoton excitation causes a rapid change in the index of refraction, which in turn causes a strong phase modulation and thus a large spectral broadening.