  Optical gain in solution, which provides high photostability as a result of
continuous regeneration of the gain medium, is extremely attractive for
optoelectronic applications. Here, we propose and demonstrate amplified
spontaneous emission (ASE) in solution with ultralow thresholds of 30
mikroJ/cm2 in red and of 44 mikroJ/cm2 in green from engineered colloidal
quantum well (CQW) heterostructures. For this purpose, CdSe/CdS core/crown
CQWs, designed to hit the green region, and CdSe/CdS/CdxZn1-xS
core/crown/gradient-alloyed shell CQWs, further tuned to reach the red region
by shell alloying, were employed to achieve high-performance ASE in the
visible. The net modal gain of these CQWs reaches 530 cm-1 for the green and
201 cm-1 for the red, two orders of magnitude larger than those of colloidal
quantum dots (QDs) in solution owing to intrinsically larger gain
cross-sections of these CQWs. To explain the root cause for ultrahigh gain
coefficient in solution, we show that for the first time that the gain
cross-sections of these CQWs is 3.3x10-14 cm2 in the green and 1.3x10-14 cm2 in
the red which are two orders magnitude larger compared to those of CQDs. These
findings confirm the extraordinary prospects of these solution-processed CQWs
as solution-based optical gain media in lasing applications.
