Enhanced high harmonic generation driven by two-color laser pulses with two foci

摘要

We demonstrate a enhancement of high harmonic generation in CO2 by using two color laser pulses with two foci. The intensity of harmonic H23 increased by a factor of 65 compared to a single focus. High harmonic generation (HHG) is an attractive high-brightness coherent extreme-ultraviolet (XUV) source [1], which is an efficient tool for a large number of applicantions: imaging molecular structure and tracking electron dynamics [2,3]. For this purpose, it is important to enhance the efficiency of HHG. The harmonic radiation efficiency could be enhanced using two-color laser field consisted of fundament field and its second harmonic field [4-6]. In addition, using fundamental laser pulse with two foci, the high conversion efficiency of HHG can be obtained [7]. In this paper, we report experimentally the significant enhancement of HHG in CO2 by using two-color laser field with two foci. Compared to single foci, the harmonic intensity is increased by a factor of 65. The experiments are carried out using a kHz Ti:sapphire-based laser system delivers 45 fs pulses with 1.8 mJ energy at a center wavelength of 800 nm. The fundamental pulse is frequency-doubled with a 100μm thick Beta-Barium Borate (BBO) crystal (type I) which efficiency is 10%. The polarization between the fundamental field and the second harmonic field is orthogonal. We use fused silica lens to relize two foci. When fundamental field and its second harmonic pass lens, the dispersion effect of lens results in the appearance of two foci. The place separation between two foci is 12 mm along the propagation axis. By removeing BBO crystal, we can get one focal point case. The experimentally obtained HHG spectrum using fundamental field and frequency-doubled beam are shown in Fig. 1. The harmonic signals in fundamental field are much lower than those in two-color field with two foci. The harmonic intensity is enhanced by a factor of more than 65 in 32 nm region. This indicates that - wo-color laser pulses with two foci can improve remarkably conversion efficiency of harmonics. The result may be interpreted as the imporved phase matching and increased interaction volume. Our result is particularly significant for promoting high harmonic to be a high brightness XUV source.