Quasi Phase Matching and Quantum Path Control of High Harmonic Generation using Counterpropagating Light*

We demonstrate the first use of a 3-pulse train of counterpropagating pulses to enhance the coherent upconversion of an intense ultrashort laser pulse into the extreme ultraviolet region of the spectrum. This all-optical quasi-phase-matching technique uses interfering beams to scramble the quantum phase of the generated high-order harmonics, to suppress emission from out-of-phase regions. A wavelength selective enhancement in the flux of up to $\approx $ 300x is observed at photon energies around 70 eV in Argon, that cannot otherwise be phase matched. We also show that further very large enhancements are possible, presenting a real prospect for orders-of-magnitude improvement in coherent upconversion of lasers into the soft x-ray region of the spectrum. Finally we show that by adjusting the intensity of the counterpropagating light, we can selectively enhance different electron quantum path trajectories, demonstrating attosecond time-scale coherent control of the radiating electron wavefunction.