  Infrared pulses with large spectral width extending from 1.2 to 3.4 um are
generated in the organic crystal DAST (4-N,
N-dimethylamino-4-N-methylstilbazolium tosylate). The input pulse has a central
wavelength of 1.5 um and 65 fs duration. With 2.8 mJ input energy we obtained
up to 700 uJ in the broadened spectrum. The output can be easily scaled up in
energy by increasing the crystal size together with the energy and the beam
size of the pump. The ultra-broad spectrum is ascribed to cascaded second order
processes mediated by the exceptionally large effective chi2 nonlinearity of
DAST, but the shape of the spectrum indicates that a delayed chi3 process may
also be involved. Numerical simulations reproduce the experimental results
qualitatively and provide an insight in the mechanisms underlying the
asymmetric spectral broadening.
