Experimental study on the propagation of high intense laser pulse in an underdense plasma with pulse length comparable with the background plasma wavelength

Optical investigation are reported of 2TW, 45fs Ti:sapphire laser pulses in an optically ionized helium gas. The stimulated Raman backscattering (BSRS) spectrum, together with ion energy spectrum, are measured as a function of the electron density from 4.8x1018/cm3 to 1.4x1019/cm3. The SRS reflectivity and the phenomena of transition from strongly coupled BSRS to weakly coupled BSRS were studied and compared with theory. Second harmonic emission in forward direction was also observed, and its origin is due to the radial electron density gradient caused by the ionization process. A good agreement is obtained between theory and experimental results