Abstract

In recent years, high harmonic generation in noble gases has become one of the most promising ways for realizing coherent radiation sources in the VUV-XUV region. It is well known that by irradiating a noble gas sample of finite extension with a high energy laser pulse, odd harmonics of the fundamental laser frequency are generated with considerable efficiency We have investigated the possibility of employing low energy pulses from a high repetition rate laser. After suitable focusing, intensity in excess of 10¹⁵ Wcm^-2 are now available also with commercial lasers operating at high repetition rate (1 kHz) and with low energy pulses (1 mJ). The above intensity is sufficient for efficient generation of harmonics lying in the spectral range 30÷200 nm, namely a region of interest for several experiments of atomic and molecular physics. In this paper we describe a sub-picosecond source of coherent, partially tunable, VUV radiation based on generation of harmonics of a Titanium-Sapphire pump laser (λ=800 nm) delivering 1 mJ - 80 fs pulses at 1 kHz repetition rate. In spite of the low energy, intensities above 10¹⁵ Wcm^-2 are achievable due to the short pulse duration and good optical quality of the laser beam. Such intensity level corresponds to well above the saturation intensity of Xe for 100 fs, 800 nm pulses. We observed odd harmonics from the 3rd (λ=267 nm) to the 13th (λ=61 nm) order (plateau). In this spectral range, of great interest for application in VUV ultrafast spectroscopy, we have studied the optimization of harmonic conversion efficiency with evidence of blue-shift and broadening of the harmonic wavelength due to the time dependent photoelectron density.

© 1996 IEEE