Ultra-coherent Fano laser based on a bound state in the continuum

It is an important challenge to reduce the power consumption and size of lasers, but progress has been impeded by quantum noise overwhelming the coherent radiation at reduced power levels. Thus, despite considerable progress in microscale and nanoscale lasers, such as photonic crystal lasers, metallic lasers and plasmonic lasers, the coherence length remains very limited. Here we show that a bound state in the continuum based on Fano interference can effectively quench quantum fluctuations. Although fragile in nature, this unusual state redistributes photons such that the effect of spontaneous emission is suppressed. Based on this concept, we experimentally demonstrate a microscopic laser with a linewidth that is more than 20 times smaller than existing microscopic lasers and show that further reduction by several orders of magnitude is feasible. These findings pave the way for numerous applications of microscopic lasers and point to new opportunities beyond photonics.