Quasi-phase matching and characterization of high-order harmonic generation in hollow waveguides using counterpropagating light.

We review recent experimental and theoretical work on the use of counterpropagating light to enhance high-order harmonic generation through all-optical quasi-phase matching. Also presented is a new technique for measuring the coherence of high harmonics in the nonlinear medium. This information is crucial for understanding the process of harmonic generation over extended distances, as well as for effective enhancement using quasi-phase matching techniques.