a, SEM image of a typical PCC. b, c, Room-temperature cross-polarized reflection taken from this cavity, before (b) and after (c) monolayer WSe2 transfer. As-fabricated cavities (before transfer) of our lasing devices typically have Q-factors ranging from 5,000 to 14,000. After monolayer transfer, the Q-factor is reduced from 8,000 to 1,300 in this device. After cooling down to cryogenic temperatures, the Q-factor recovers to ∼2500.