Induced spectral broadening of a second harmonic pulse in ZnSe crystals generated by an intense ultrashort laser pulse

Abstract

We report on the first observation of Induced spectral broadening (ISB) of a weak nonphase matching second harmonic pulse produced from the propagation of an intense primary picosecond laser pulse through ZnSe crystals. A mode-locked Nd:glass laser with a single 8-ps pulse at the 1060-nm wavelength was weakly focused Into the sample. Output signals were sent through a spectrograph to measure the spectral distribution. Measured spectra for nonphase matching SHG and ISB generated in ZnSe crystal by a 1060-nm laser pulse indicated the extent of the spectral broadening about the second harmonic line. When the Incident laser pulse energy was 2 mJ, there was spectral broadening of ~1,100 cm−1 on the Stokes side and 770 cm−1 on the anti-Stokes side. When pulse energy was <1 mJ, the ISB was found to be linearly dependent on the pulse energy of 1060 nm. The pulse duration of the ISB measured by a streak camera at 550 nm using a 10-nm narrowband filter was ~9 ps. The Pockels term of ZnSe is responsible for the sharp line ~100 cm−1 at the SHG frequency. This weak SHG pulse travels through the crystal together with the primary pulse. Due to the large n2 of ZnSe and the Intense incident 1060-nm laser, the spectral line of the weak 530-nm laser broadens. The ISB generated around the SHG 2w0 in ZnSe crystals is attributed to the induced phase modulation by an intense picosecond laser pulse at w0.