The effect of gas pressure and slit width on the high order harmonic generation in helium-filled slab waveguide

We measured the dependence of the intensity of high harmonics on the gas pressure in slab waveguide. The phase matching of 61st high harmonic was obtained according to the quadratic dependence of the increase in efficiency with pressure. For higher order harmonics such as H91, periodic oscillations of the harmonic intensity with gas pressure was observed. We also studied the effect of the slit width of the slab waveguide on the high harmonics generation. The influence of the slit width on laser transverse mode and phase mismatching conditions can be used to explain this effect.