2, Bilkent University, Ankara, , Turkey
3, Emory University, Atlanta, Georgia, United States
4, Nanyang Technological University, Singapore, , Singapore
Colloidal semiconductor nanocrystals are promising for optically pumped lasers that can practically emit at any wavelength from UV to IR. However, lasing performance of the conventional nanocrystals has been severely limited due to Auger recombination which depletes gain-active excitonic species before they could contribute to gain. To this end, shape-controlled and composition-tuned nanocrystals have been proposed towards low-threshold optical gain since they can suppress Auger recombination. Among these, nanocrystals having Type-II band alignment have been highly promoted for lasing, yet to date their performance has suffered from small modal gain coefficients and short gain lifetimes due to their diminishing oscillator strength and modest absorption cross-section. Overcoming these challenges, here we accomplish unprecedented optical gain performance in Type-II nanocrystals by developing an alloyed colloidal quantum well architecture. By optimizing the composition of the core/alloyed-crown CdSe/CdSexTe1-x quantum wells, we realized amplified spontaneous emission with a threshold as low as 26 µJ/cm2, accompanied with large net modal gain coefficients up to ~930 cm-1 and long gain lifetimes (τgain ~400 ps). The performance of the Type-II quantum wells studied here represents more than an order of magnitude improvement over the previous best reports in Type-II nanocrystals. Also, the measured modal gain coefficient is record high among all colloidal semiconductors. Moreover, we corroborated the underpinning mechanism of this efficient gain via ultrafast transient absorption spectroscopy and revealed that the gain surprisingly arises from the carriers that are localized to the alloyed-crown region. The gain scheme in these alloyed quantum wells resembles a “threshold-less” four-level gain system, which makes them extremely promising towards realization of continuous-wave (CW) pumped and electrically-injected nanocrystal lasers.