  Recently, two-dimensional (2D) materials have opened a new paradigm for
fundamental physics explorations and device applications. Unlike gapless
graphene, monolayer transition metal dichalcogenide (TMDC) has new optical
functionalities for next generation ultra-compact electronic and
opto-electronic devices. When TMDC crystals are thinned down to monolayers,
they undergo an indirect to direct bandgap transition, making it an outstanding
2D semiconductor. Unique electron valley degree of freedom, strong light matter
interactions and excitonic effects were observed. Enhancement of spontaneous
emission has been reported on TMDC monolayers integrated with photonic crystal
and distributed Bragg reflector microcavities. However, the coherent light
emission from 2D monolayer TMDC has not been demonstrated, mainly due to that
an atomic membrane has limited material gain volume and is lack of optical mode
confinement. Here, we report the first realization of 2D excitonic laser by
embedding monolayer tungsten disulfide (WS2) in a microdisk resonator. Using a
whispering gallery mode (WGM) resonator with a high quality factor and optical
confinement, we observed bright excitonic lasing in visible wavelength. The
Si3N4/WS2/HSQ sandwich configuration provides a strong feedback and mode
overlap with monolayer gain. This demonstration of 2D excitonic laser marks a
major step towards 2D on-chip optoelectronics for high performance optical
communication and computing applications.
