Quasi-phase-matching and quantum-path control of high-harmonic generation using counterpropagating light

High-order harmonic upconversion of femtosecond lasers produces a unique source of short-wavelength light with femtosecond-to-attosecond pulse duration. However, because the involved nonlinear medium is a partially ionized gas, traditional approaches for phase-matching the conversion process are not applicable. This severely limits the flux from this source. Here, we demonstrate the first use of a train of counterpropagating light pulses to enhance high-harmonic emission. This all-optical quasi-phase-matching technique uses interfering beams to scramble the quantum phase of the generated short-wavelength light, to suppress emission from out-of-phase regions. Selective enhancement of more than 300 is observed at photon energies around 70 eV in argon gas. Finally, we show that by adjusting the intensity of the counterpropagating light, different electron quantum trajectories can be selectively enhanced, demonstrating attosecond-timescale coherent control of the radiating electron wavefunction.