We experimentally demonstrate mid-IR to visible frequency conversion via simultaneous two photon excitation, while the intermediate frequency remains dark throughout the interaction. The phase mismatch in such mechanism is shown to be intensity dependent.

Details: in the field of nonlinear optics, available lasers are mixed to produce new frequencies, using three-wave-mixing processes. The common assumption is that these processes can occur only when the material is transparent at all participating frequencies. We study the case of two simultaneous processes, where one frequency is converted to another through an intermediate frequency. 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 thus 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.