In the high harmonic (HH) process, an intense femtosecond laser pulse is nonlinearly up-converted from the visible or near-infrared to much higher photon energies, spanning from the EUV into the soft X-ray region of the spectrum. High harmonics have many unique properties, such as their high degree of temporal and spatial coherence, the wide spectral range they cover and the sub-femtosecond time scale of the XUV bursts. These make them promising tools for various applications like being a probe tool for ultrafast dynamics and a diffractive imaging tool for nano-scal structures. However, reported yield of the harmonics are relatively low (conversion efficiency of about 10^-5-10^-6). There are mainly two factors which limiting the conversion efficiency; the first is the low recombination cross-section of the recolliding electron, from the continuum to the ground state; the second is due to the phase mismatch problem - causing a destructive interference of the high harmonics. such low yield restricts the applicative use of HHG.

In this presentation I will describe the generation of HH during the propagation of short laser pulses through low-ionized plasma, produced on the surfaces of different targets. In such plasmas, resonance conditions with an autoionizing level show a single harmonic enhancement over the neighboring harmonics, thus, allows enhanced conversion efficiency. Another direction to increase the harmonic yield is to improve phase matching condition by modulation of the pre-formed plasma (quasi phase matching).