Single-mode laser by parity-time symmetry breaking

Effective manipulation of cavity resonant modes is crucial for emission control in laser physics and applications. Using the concept of parity-time symmetry to exploit the interplay between gain and loss (i.e. light amplification and absorption), we demonstrate a parity-time synthetic laser with resonant modes that can be controlled at will. In contrast to conventional ring cavity lasers with multiple competing modes, our parity-time microring laser exhibits robust broadband single-mode lasing regardless of the gain spectral bandwidth. Thresholdless parity-time symmetry breaking due to the rotationally symmetric structure leads to stable single-mode operation with the selective whispering-gallery mode order. Exploration of parity-time symmetry in laser physics may develop a new paradigm of strategically utilizing optical losses and open a door to next-generation optoelectronic devices for optical communications and computing.