Nonlinear optics for high-order frequency conversion: applied attosecond science

We show that it is possible to use of a train of counterpropagating light pulses to enhance the coherent upconversion of intense femtosecond lasers into the extreme ultraviolet (EUV) region of the spectrum. This all optical quasi-phase-matching uses interfering beams to scramble the quantum phase of the generated EUV light, suppressing the contribution of out-of-phase emission. Selective enhancement of up to 600X is observed at photon energies of ~70 eV using argon gas and ~ 150 eV using helium gas.