Generation of sub-6-fs blue pulses by frequency doubling with quasi-phase-matching gratings

We demonstrate the generation of sub-6-fs pulses centered at 405 nm by freq uency doubling of 8.6-fs Ti:sapphire laser pulses. The frequency doubling i s carried out in a nonlinearly chirped quasi-phase-matching grating fabrica ted in a lithium tantalate substrate. This device simultaneously provides f requency conversion and pulse compression of the positively prechirped fund amental pulses. The second-harmonic pulses are characterized in a cross-cor relation setup, and their pulse shapes are retrieved by two iterative phase -reconstruction algorithms. The generated second-harmonic spectrum spans a bandwidth of 220 THz. To our knowledge, these are the shortest pulses ever generated in the blue spectral region. (C) 2001 Optical Society of America.