Yuntao Zhu

Light in a thin film of lithium niobate can be made to change color — for instance, turning an invisible infrared beam into green. To do that, the crystal needs to be 'marked': in some places its polarity points up, in others down. Previously, this marking was done by applying a voltage and watching the domain growth in real time. The slightest delay, and the pattern smears.

A tuning fork sounds incredibly pure, but touch it and the pitch changes almost instantly. Engineers faced a similar challenge: creating a laser that simultaneously possesses two seemingly incompatible properties — exceptional spectral purity (a very narrow emission line, like a tuning fork producing a single note without wobble) and the ability to rapidly tune that frequency.