Second-harmonic generation with femtosecond pulses requires the use of extraordinarily thin nonlinear media due to the group velocity mismatch (GVM) between the fundamental and second harmonic waves. This limitation is especially severe for the visible (ω1) to UV (ω2) wavelengths. Recently a 55 μm thick BBO crystal was used1 internal to the cavity of a CPM dye laser to generate 49 fs duration pulses at 315 nm. The value of the GVM, Δτ12/L=360 fs/mm, precludes the generation of significantly shorter pulses even with such difficult to make crystals. We report here the doubling of 14 fs long 630 nm pulses using a poled polymeric film of 2.5 μm thickness. The efficiency is high for such a thin material: 0.16% has been measured and 0.5% has been determined to be possible. Since GVM is negligible in the film, this allows the generation of UV pulses as short as a few femtoseconds.
© 1991 Optical Society of AmericaPDF Article