  In this work, we report the first account of monolithically-fabricated
vertical cavity surface emitting lasers (VCSELs) of densely-packed,
orientation-controlled, atomically flat colloidal quantum wells (CQWs) using a
self-assembly method and demonstrate single-mode lasing from a record thin
colloidal gain medium with a film thickness of 7 nm under femtosecond optical
excitation. We used specially engineered CQWs to demonstrate these hybrid
CQW-VCSELs consisting of only a few layers to a single monolayer of CQWs and
achieved the lasing from these thin gain media by thoroughly modeling and
implementing a vertical cavity consisting of distributed Bragg reflectors with
an additional dielectric layer for mode tuning. Accurate spectral and spatial
alignment of the cavity mode with the CQW films was secured with the help of
full electromagnetic computations. While overcoming the long-pending problem of
limited electrical conductivity in thicker colloidal films, such ultra-thin
colloidal gain media can help enabling fully electrically-driven colloidal
lasers.
