Colloidal Quantum Dot Laser Diodes

Colloidal quantum dots (CQDs) combine superior light-emission characteristics of quantum-confined semiconductors with chemical flexibility of molecular systems. These properties could, in principle, enable solution processable laser diodes with an ultrawide range of accessible colours. However, the realization of such devices has been hampered by fast optical gain decay due to nonradiative Auger recombination and poor stability of CQD solids at high current densities required for the lasing regime. Recently, we have resolved these problems and achieved a lasing regime with electrically pumped CQDs. The active layer of our devices is made of novel “compact continuously graded CQDs” (ccg-CQDs) that feature strongly suppressed Auger recombination and a large material gain coefficient. The ccg-CQDs are incorporated into an LED-like device stack assembled of low-loss materials and engineered so as to maximize optical field confinement within the CQD layer. The developed design allows us to boost the modal gain coefficient for the active medium and to reduce optical losses in the adjacent charge transport/injection layers. Further, we incorporate an integrated photonic structure which allows for highly efficient light trapping in the CQD waveguiding layer. To lessen the deleterious influence of thermal effects, we reduce the injection area with a small “current-focusing” aperture inserted into a hole injection path and use a pulsed bias to drive our devices. Using this approach, we achieve the lasing effect due to electrically excited stimulated emission from the CQDs. These prototype CQD laser diodes demonstrate a low-threshold laser action at both the band-edge, 1S (637 nm) and the excited-state, 1P (586 nm) transitions at room temperature.