Infrared-to-optical image conversion by Bragg reflection from thermally induced index gratings

We have observed efficient reproduction at visible wavelengths of 1.06-μ images. We employed a phase-matched four-wave mixing process in which three waves at ν, ν, and ω mix to generate a fourth wave at ω, with no resonant conditions in either frequency. The image conversion was seen with each of the 14 liquids tried as a nonlinear medium and also with a glass. Thermal index changes were the dominant mechanism.