Discrete Time Crystals in Actively Mode-Locked Lasers

We report the first experimental observation of discrete time crystal phases and crystallites in an actively mode-locked semiconductor laser. By tuning either the bias current or the modulation frequency, the system undergoes a spontaneous symmetry-breaking transition from the harmonically mode-locked state toward robust, highly coherent time crystal states that persist indefinitely. Two equivalent time crystal configurations can coexist as domains separated by sharp, long-lived boundaries analogous to domain walls. The phenomenon is successfully reproduced by a time-delayed model. Our findings demonstrate that mode-locked semiconductor lasers offer a readily accessible platform to explore and control nonequilibrium phases of light, enabling practical implementations of time crystal physics in photonic systems.