Type-(I+II) Colloidal Quantum Dots for Liquid- and Solid-State Semiconductor Lasers

Due to their tunable emission wavelengths, high emission quantum yields, and low optical gain thresholds, colloidal quantum dots (QDs) are attractive materials for applications in laser technologies. However, the development of technologically viable QD lasing devices has been hindered by nonradiative Auger recombination, leading to rapid optical gain relaxation on 10s-to-100s ps time scales. Short optical gain lifetimes pose an especially serious problem in the case of dilute optical-gain media such as QD solutions or QD/glass (polymer) composites. Fast optical gain decay also complicates the realization of lasing with continuous wave (<i>cw</i>) optical pumping or electrical injection.<br/> <br/>To address the ‘Auger recombination challenge’, we develop type-(I+II) QDs using which we implement a novel gain mechanism which relies on stimulated emission from hybrid biexcitons composed of a direct and an indirect exciton. Due to the reduced number of recombination pathways, the hybrid biexciton exhibits slow trion-like relaxation dynamics. As a result, the type-(I+II) QDs exhibit extended optical gain lifetimes of several nanoseconds, due to which a critical QD concentration needed for lasing is reduced to levels accessible with QD solutions. When integrated into a Littrow optical cavity, 'static' (non-circulated) solutions of type-(I+II) QDs exhibit narrow-line (&lt;0.4 meV) lasing, tunable from 634 nm (red) to 590 nm (orange-yellow), which covers the lasing windows of two popular laser dyes, Rhodamine B and Rhodamine 101.<br/> <br/>Type-(I+II) QDs also provide a distinct advantage in the case of <i>cw</i> lasing. Single excitons generated in these QDs occupy the long-lived indirect state which simplifies reaching the optical gain threshold (the regime where all QDs in the sample contain single excitons) with <i>cw</i> pumping. This further helps maintain the QDs in the ‘pre-lasing’, optical-transparency state. The resulting effect is a considerable reduction of the lasing threshold compared to standard type-I QDs with all-direct biexcitons. Importantly, this does not lead to the reduction of the gain coefficient (which would have happened in type-II QDs with all-indirect biexcitons) as stimulation emission of the hybrid direct/indirect biexciton occurs preferentially <i>via</i> the direct transition. Indeed, solid-state films of type-(I+II) QDs integrated with a distributed feedback (DFB) cavity, readily exhibit narrow-line (&lt;0.8 meV) <i>cw</i> lasing with a threshold of only 560 W cm^-2.<br/> <br/>The reported studies point towards a considerable potential of the novel type-(I+II) QDs as optical gain media for technologically viable liquid- and solid-state QD lasers.<br/> <br/>This work was supported by the Laboratory Directed Research and Development (LDRD) program at Los Alamos National Laboratory under project 20230352ER. V.P. and N.A. acknowledge support by a LANL Director’s Postdoctoral Fellowship.