Towards zero-threshold optical gain using charged semiconductor quantum dots

• Published in: Nature nanotechnology Vol. 12; no. 12; pp. 1140 - 1147
• Main Authors: Wu, Kaifeng, Park, Young-Shin, Lim, Jaehoon, Klimov, Victor I.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.12.2017; Nature Publishing Group
• Subjects: 140/125; 639/624/1020/1093; 639/925/357/1017; Augers; Electrons; Lasers; Lasing; Material Science; MATERIALS SCIENCE; Nanotechnology; Nanotechnology and Microengineering; Quantum dots; Quantum theory; Recombination; Reduction; Spontaneous emission; Zero-threshold optical gain, colloidal quantum dot, lasing
• ISSN: 1748-3387; 1748-3395
• DOI: 10.1038/nnano.2017.189

Colloidal semiconductor quantum dots are attractive materials for the realization of solution-processable lasers. However, their applications as optical-gain media are complicated by a non-unity degeneracy of band-edge states, because of which multiexcitons are required to achieve the lasing regime. This increases the lasing thresholds and leads to very short optical gain lifetimes limited by nonradiative Auger recombination. Here, we show that these problems can be at least partially resolved by employing not neutral but negatively charged quantum dots. By applying photodoping to specially engineered quantum dots with impeded Auger decay, we demonstrate a considerable reduction of the optical gain threshold due to suppression of ground-state absorption by pre-existing carriers. Moreover, by injecting approximately one electron per dot on average, we achieve a more than twofold reduction in the amplified spontaneous emission threshold, bringing it to the sub-single-exciton level. These measurements indicate the feasibility of ‘zero-threshold’ gain achievable by completely blocking the band-edge state with two electrons. Blocking band-edge absorption of compositionally graded quantum dots with suppressed Auger recombination by pre-existing electrons allows for demonstrating near-zero-threshold optical gain and amplified spontaneous emission at sub-single-exciton pump levels.