Engineered Colloidal Quantum Dots as Universal Optical Gain Media for Solution- and Solid-State Lasers and Laser Diodes

Colloidal quantum dots (QDs) are attractive materials for applications in laser technologies. In addition to being compatible with inexpensive and easily scalable chemical techniques, they offer multiple advantages derived from a zero-dimensional (0D) character of their electronic states. These include a size-tunable emission wavelength, low optical-gain thresholds, and high temperature stability of lasing characteristics.<br/><br/>So far, all validated reports on colloidal QD lasing employed optically excited close-packed solid-state QD films. Two classes of QD lasers that are yet to be realized are liquid-state optically pumped devices and electrically pumped solid-state lasers (laser diodes). In both cases, a principal obstacle is very fast Auger recombination of optical-gain active multicarrier state. One consequence of fast Auger decay is the existence of a minimal (critical) QD concentration required for lasing. Due to solubility limits, this concentration is difficult to attain with liquid-state samples. Therefore, up to now, QD lasing devices have employed high-density solid-state films. Fast Auger delay also creates a serious problem when one tries to realize lasing with electrical pumping. In particular, it mandates the use of prohibitively high current densities for maintaining QDs in the inverted multicarrier state.<br/><br/>Here we describe how we overcome the challenge of fast Auger recombination using meticulously engineered hetero-QDs. To impede Auger decay, we modify an optical gain state so as to reduce the number of available Auger pathways. Further, we employ compositional grading of the QD interior to smoothen carrier confinement potentials and whereby suppress the strength of individual Auger transitions. As a result, our engineered QDs exhibit long optical gain lifetimes (nanosecond time scales) while still maintaining strong 3D quantum confinement.<br/>We use our novel QDs to demonstrate new types of lasing devices including prototype laser diodes and widely tunable dye-like liquid-state lasers.