Phase-matching effects in the generation of high-energy photons by mid-infrared few-cycle laser pulses

We report on our experimental and theoretical investigations on the generation of high-order harmonics driven by 1500 nm few-cycle laser pulses in xenon. In contrast to the common belief, we found experimental evidence suggesting that harmonic generation driven by mid-infrared laser pulses can be realized with high efficiency; in particular, an enhancement of very high harmonic orders can be achieved under suitable conditions of the laser-medium interaction. The experimental results were simulated by a 3D non-adiabatic model. The theoretical outcomes confirm the experimental findings and provide a physical explanation for the counter-intuitive results. In particular, a time-dependent phase-matching analysis threw light on the generation mechanisms at a timescale of half optical cycle of the fundamental pulse.