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Nature Physics 3, 270–275 (1 April 2007) | doi:10.1038/nphys541

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

Xiaoshi Zhang , Amy L. Lytle , Tenio Popmintchev , Xibin Zhou , Henry C. Kapteyn , Margaret M. Murnane & Oren Cohen

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|[thinsp]|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.