Influence of counterpropagating light on phase matching in second-harmonic generation

Here we show an explicit measurement of the microscopic disruption to the phase-matching conditions for second-harmonic generation caused by a counterpropagating light field. This microscopic phase disruption has been assumed as the mechanism behind successful implementations of all-optical quasi-phase matching of high-order harmonic generation but has never been previously observed. Numerical simulations reproduce the features of the observed disruption and indicate pathways for implementing in situ probing and quasi-phase matching of second-harmonic generation with counterpropagating fields.