Fig. 3.

Scaling of phase matching parameters. (A) Experimental macroscopic HHG signal growth versus pressure at photon energies close to the phase-matching cutoffs for 1.3-μm lasers. (B) Theoretical predictions of the pressure required for phase matching by using 0.8- and 1.3-μm lasers, based on a simple semianalytical model (solid lines). Quantum calculations for He are shown in dashed lines. (C) Evolution of the laser field in He, represented as a temporal overlap of the field at different distances along the wave guide for a pressure of 1,500 torr and for a propagation distance of 1 cm. The macroscopic in-phase HHG emission is confined within the central cycle of the laser pulse.