Quasiparity-Time Symmetric Microdisk Laser

Recent developments in parity-time (PT) symmetric systems have ushered in unique photonic devices with enhanced functionalities. While single-mode laser emission has been demonstrated in such systems, the current designs face severe challenges in applications, either due to their stringent requirement on fabrication precision or nonscalability to larger devices. Here, we demonstrate a general mechanism to achieve single-mode lasing in coupled cavities, which relies on external mode coupling and overcomes these drawbacks. We find significant gain enhancement for selected modes by external coupling, and our experiments have confirmed the resulting single-mode laser emission in size-mismatched photonic molecules (PMs), when only one constituent cavity is pumped. This behavior persists for a wide range of pump power, from transparent threshold to gain saturation, and it is highly tolerant of fabrication imprecisions. In addition, the output intensity of such single-mode lasers also displays enhancement when compared with the same PMs under uniformly pumping. We believe our results will both advance the understanding of different coupling scenarios in coupled cavities and improve the characteristics of onchip laser sources for practical applications.