Nonlinear optics makes it possible for light waves at varying frequency to interact and transfer energy. Its study was made possible with the invention of the laser, and within a few years of this it revolutionized our ability to create and manipulate directed, laser-like, beams in the ultraviolet, visible, and infrared regions of the spectrum, particularly where lasers based on conventional population inversion are not practical. These advances were based on a theoretical framework for NLO in the perturbative regime that was outlined in great detail already as early as 1962.
However, nonlinear optics itself is not restricted to regions where perturbation theory is valid, and in recent years a frontier area has been in nonperturbative nonlinear optics. In a dramatic recent advance, an international team led by JILA demonstrated that the high harmonic generation process driven by mid-infrared lasers can be used to generate coherent light at keV photon energies through a >5000th-order nonlinear process— while still maintaining the full phase matching that is necessary for optimum conversion efficiency. This work represents the most extreme coherent upconversion process in the 50-year history of nonlinear optics, and essentially realizes the fully temporally and spatially coherent version of the Roentgen X-ray tube in the soft X-ray region. Most importantly, the limits of this extreme process are still not understood, neither in theory nor experiment—it may be possible to generate coherent hard X-rays using a tabletop-scale apparatus.