Pressure control of phase matching in high-order harmonic generation in hollow fibers filled with an absorbing weakly ionizing gas

High-order harmonic generation in a hollow fiber filled with a weakly ioniz ing gas is theoretically analyzed within the framework of the slowly varyin g envelope approximation. The gas pressure that corresponds to maximum effi ciency of frequency conversion, the absorption coefficient, the phase misma tch owing to gas dispersion, and the enhancement of harmonic-generation eff iciency owing to waveguide phase matching are estimated for 27th-harmonic g eneration in hollow fibers filled with helium, neon, argon, krypton, or xen on. As a result of the ionization-induced self-phase modulation of the pump pulse in a hollow fiber filled with a weakly ionizing gas, the phase misma tch changes within the pump pulse, decreasing the overall efficiency of har monic generation and making the harmonic-generation efficiency less sensiti ve to the gas pressure in the hollow fiber. (C) 2001 Optical Society of Ame rica.