Phase conjugation and four-wave mixing in a colloidal medium

We demonstrate phase conjugation by means of degenerate four-wave mixing from a colloidal crystal. The nonlinear medium is provided by a periodic spatial refractive index grating created in a colloidal suspension of dielectric microparticles trapped in the intensity distribution of two nearly copropagating interfering laser beams. Phase conjugation is achieved for a probe beam carrying orbital angular momentum as evidenced by the inversion of the topological charge of a phase singularity within the beam. The efficiency and nonlinear parameters of the colloidal crystal as well as its lattice properties are measured and compared to theoretical predictions and previous experimental work.