Ultracompact multibound-state-assisted flat-band lasers.

Highly compact lasers with a low threshold and stable single-mode operation are in great demand for integrated optoelectronics. However, considerable side leakages and radiation losses in small cavities substantially degrade the quality () factor, posing a substantial obstacle in pursuing high-performance miniature lasers. Here we propose and experimentally demonstrate a flat-band laser supplemented by multiple bound states in the continuum. By simultaneously confining light in all three dimensions, a high factor of ~1,440 in an ultracompact terahertz quantum cascade laser cavity with a lateral size of ~3 is reported. The field confinement makes it possible to realize an electrically pumped single-mode terahertz laser with a low threshold current density, despite the small device footprint. This surface-emitting laser emits a well-defined beam with good directionality. The demonstrated multibound-state-assisted flat-band design is also applicable to other wavelength regimes, offering a route to energy-efficient, monolithically integrated and ultracompact laser sources that suit a wide range of applications.