In the 1960s, physicists showed how light could interact with itself in intriguing ways thanks to a process known as second-harmonic generation. In the right kind of material, two photons of a certain frequency can interact to produce another photon of twice the frequency, turning red light into green, for example. Since then, physicists have exploited these non-linear optics in techniques ranging from precision measurement to quantum computing.
Yet, one persistent challenge has remained. Nonlinear optical devices must be crafted with a single, unchangeable function determined during fabrication. And this limits their utility and function.
