Inside any laser is a cavity with a “gain medium” that gives the laser its energy to emit light. A typical gain medium contains atoms that can be excited by using an external energy source and is sandwiched between a pair of mirrors. The mirrors impose a periodicity on the light inside the cavity—similar to how the length of a guitar string limits what musical notes can be played—and allows the medium to pack more energy into the light each time it passes through the gain medium. On page 425 of this issue, Lyubarov et al. (1) propose a radically new approach to making a laser in which the cavity is replaced by a medium with no mirrors. Instead, the optical properties of the medium are periodically modulated in time.
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