Abstract

Due to the large range of molecular systems, femtochimistry requires tunability from the IR to the UV. This involves most of the time, a non-collinear optical parametric amplification [1] with three stages for gas phase femtochemistry to get tunability in the visible, followed by frequency mixing with the fundamental pulse or its second harmonic to reach the ultraviolet (UV). The main parameters for this application are the intensity, stability, quality of the spatial mode and polarisation. In general, the intensity is maximized by using crystals slightly oversized in thickness and stability is obtained by saturating the frequency mixing process. However artefacts on pump-probe signals, introduced mostly by a chirp of the pump, have been detected principally as a delay-shift or either on the lifetimes [2]. The majority of published works in femtochimistry are done with a cross-correlation time typically twice larger than what is expected from the respective bandwidth of the pump and probe. In order to minimize these chirps, the phase characteristics of the pulses need to be measured after each non-linear process to optimize the intensity without to destroy drastically the duration. The challenge is to get a characterization set-up versatile in wavelengths (from 800 nm to 197 nm), input energies (from few hundred of nJ to several 100µJ), divergences and polarisations. We have developed a frequency resolved optical gating based on a transient grating generated in Sapphire blade [3]. The input beam is divided in three parts (see figure 1) that are focused by a parabolic off-axis mirror (f= 50 mm) within the sapphire.

© 2011 Optical Society of America