Status and Prospects of Colloidal Quantum Dot Lasers
Victor Klimov a
a Chemistry Division, Los Alamos National Laboratory, Los Alamos, USA, United States
Proceedings of Internet Conference for Quantum Dots (iCQD)
Online, Spain, 2020 July 14th - 17th
Organizers: Quinten Akkerman, Raffaella Buonsanti, Zeger Hens and Maksym Kovalenko
Keynote, Victor Klimov, presentation 052
Publication date: 3rd July 2020

Colloidal semiconductor quantum dots (CQDs) are attractive materials for realizing highly flexible, solution-processable optical gain media with readily tunable operational wavelengths [1, 2]. However, CQDs are difficult to use in lasing due to extremely short optical gain lifetimes limited by nonradiative multicarrier Auger recombination [3]. This, in particular, is a serious obstacle for realizing cw optically and electrically pumped lasing devices. Recently, we have explored several approaches for mitigating the problem of Auger decay by taking advantage of a new generation of core/multi-shell CQDs with a radially graded composition that allow for considerable (nearly complete) suppression of Auger recombination [4, 5]. Using these specially engineered CQDs, we have been able to realize optical gain with direct-current electrical pumping [4], which has been a long-standing goal in the field of colloidal nanostructures. Further, we have applied these dots to practically demonstrate the viability of a ‘zero-threshold optical gain’ concept using not neutral but negatively charged particles wherein the pre-existing electrons block either partially or completely ground-state absorption [5, 6]. Such charged QDs are optical-gain-ready without excitation, which has allowed us to reduce the lasing threshold to record-low values that are well below the fundamental single-exciton-per-dot limit [6]. Most recently, we have developed CQD devices that operate as both an electroluminescent (EL) structure and a distributed feedback optically pumped laser [7]. By carefully engineering a refractive-index profile across the device stack, we have been able to demonstrate low-threshold lasing even with a very thin EL-active region, which comprises only three monolayers of the QDs.  Yet another recent advance has been the realization of CQD-LEDs that achieve ultrahigh current densities exceeding 1,000 A cm-2. This has allowed us to inject ~10 excitons per dot and thereby realize population inversion of both the ground-state (1S) and the excited-state (1P) transitions. All of these recent developments suggest that CQD laser diodes (QLDs) are just around the corner. The availability of such devices will benefit numerous fields from integrated photonic circuits and optical interconnects to lab-on-a-chip platforms and wearable devices.

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