Colloidal semiconductor quantum wells, also known as nanoplatelets (NPLs), attract an increasingly greater deal of interest as a promising material platform for light-generating applications. The superior optical properties of NPLs including their ultra-large absorption cross-sections, purely homogeneous broadening, and suppressed Auger recombination make them highly attractive for solution-processable color convertors, LEDs and lasers. In this thesis, we studied the optical gain properties and performance levels of tailored heterostructures of such NPLs intimately integrated into various optical cavities. To do so, we systematically measured their amplified spontaneous emission under one- and twophoton absorption excitations. Also, with these hetero-NPLs as the gain media, we have proposed and demonstrated a new class of practical whispering gallery mode (WGM) NPL-fiber architecture with high stability and low lasing thresholds enabled by record low waveguide loss coefficients in its class. Moreover, we have developed a single-mode vertical-cavity surface-emitting laser (VCSEL) of these hetero-NPLs closely integrated into the wedge cavity of a pair of distributed Bragg reflectors, leading to a record low lasing threshold in its class. The findings obtained in these WGM NPL-laser and NPLVCSEL structures indicate that these NPLs are excellent for high-performance colloidal lasing.