Counterpropagating light as a means for all-optical phase matching

Frequency conversion through nonlinear optical processes is the core technology behind a new generation of powerful tools, including tunable optical parametric oscillators for spectroscopy and attosecond-duration pulses for studying electronic processes in atoms. Conversion efficiency is often the limiting factor in the power of such sources, due to chromatic dispersion of the nonlinear medium. The phase mismatch is typically compensated by exploiting birefringence or constructing layered media, but these are limited to nonlinear materials amenable to this type of engineering. Recently, it has been shown that a medium-independent, all-optical method for correcting the phase mismatch is possible. A sequence of counterpropagating pulses locally disrupts the phase-matching conditions on a microscopic level. We show how this microscopic disruption is possible in second harmonic generation and test our current theoretical understanding by comparing experimental observations to a numerical model. We also describe the remaining engineering challenges to implementing the use of counterpropagating light for phase matching and other precision optical measurements.