Nonlinear atom optics

The availability of coherent matter waves from Bose-Einstein condensates had led to the extension of atom optics from the linear to the nonlinear regime, in analogy with the developments in optics following the invention of the laser. We provide a tutorial introduction to this emerging field, emphasizing the role of collisions as a "nonlinear medium" in the focusing and defocusing of matter waves, the launching of atomic solitons, the generation of vortices, and the demonstration of four-wave mixing both among matter waves and between optical and matter waves.