Colloidal quantum wells (CQWs), which are also known as colloidal nanoplatelets, are appealing light-emitting materials exhibiting unique optical properties. These include narrow photoluminescence full-width at half-maxima, quantum-well like density of states and absence of inhomogeneous broadening which make these CQWs as promising optical gain materials. However, there was no demonstration of lasing in the CQWs to date. In this work, we demonstrate ultrahigh efficiency amplified spontaneous emission (ASE) and lasing in CdSe core and CdSe/CdS core/crown CQWs. Core/crown CQWs, which have been recently introduced, show enhanced optical properties owing to the laterally grown CdS layer that also acts as a light-absorbing antenna boosting the absorption cross-section.[2, 3] Here, we synthesize various core/crown CQWs having varying lateral crown size, then we study single- and two-photon absorption pumped ASE in these CQWs. Core/crown CQWs exhibit enhanced optical gain performance as compared to the only core CQWs thanks to the increased absorption cross-section and enhanced passivation by the crown layer. We achieve single-photon absorption (?exc = 400 nm) pumped ASE with a threshold as low as 41 ?J/cm2 in the core/crown CQWs, which represents the best reported ASE threshold among the colloidal nanocrystals emitting in the green region of the visible spectrum. Then, we also investigate two-photon absorption (?exc = 800 nm) pumped ASE in these CQWs exhibiting a record low threshold of 4.5 mJ/cm2 better than that of the best reported threshold in quantum dots and rods. Moreover, we measure the gain coefficient via variable stripe length technique, using which the gain coefficient of the core/crown CQWs is found to be as high as 650 cm-1. This represents more than 2- and 4-fold enhancement over the best reported gain coefficient in colloidal quantum rods and dots.[4, 5] Finally, we develop an all solution processed vertical cavity surface emitting laser (VCSEL) of the CQWs that are sandwiched between two distributed Bragg reflectors, which were fabricated by colloidal nanoparticles of SiO2 and TiO2. The VCSELs exhibit surpassing two-photon absorption pumped lasing performance with a lasing threshold of 2.5 mJ/cm2 with a Q-factor of 300. Overall, these remarkable results of ultralow threshold ASE and extremely high optical gain coefficient make these CQWs highly promising materials as colloidal optical gain media for lasers.
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