Two photon frequency conversion.

We study the case of two simultaneous three-wave-mixing processes, where one frequency is converted to another through an intermediate frequency. The common assumption is that these processes can occur only when the material is transparent at all participating frequencies. Here we show experimentally that, under appropriate conditions, the intermediate frequency remains dark throughout the interaction. This means that even if the material is opaque at the intermediate frequency, the conversion will remain efficient. New possibilities of frequency conversion are therefore available, e.g. through absorptive bands in the ultraviolet or mid-infrared. Moreover, though it was hitherto assumed that the phase mismatch value is governed only by dispersion, we show here that phase matching also depends on light intensity. These findings promise novel all optical switching techniques.