Two-color Amplified Spontaneous Emission from Liquid Suspensions of Engineered Quantum Dots with Suppressed Auger Recombination.

Colloidal quantum-dot (QD) lasing is normally achieved in close-packed solid-state films, as a high QD volume fraction is required for stimulated emission to outcompete fast Auger decay of optical-gain-active multiexciton states. Here we demonstrate a new type of optical-gain medium in which QDs are liquefied using a small amount of a solvent. This high-density QD suspension behaves as a transparent liquid which can be handled as a traditional laser dye. To test the utility of these liquid media as lasing materials, we use recently developed compact compositionally-graded QDs (ccg-QDs) that feature strong suppression of Auger decay. Transient absorption measurements of ccg-QD liquid suspensions reveal broad-band optical gain spanning a wide spectral range from 560 nm (green) to 675 nm (red). The gain magnitude is sufficient to realize a two-color amplified spontaneous emission (ASE) at 637 nm and 594 nm due to the band-edge (1S) and the excited-state (1P) transition, respectively. Importantly, the ASE regime is achieved using quasi-continuous excitation with nanosecond pulses. Furthermore, the ASE is highly stable under prolonged excitation. This stands in contrast to traditional dyes that exhibit strong degradation under identical excitation conditions. These observations point towards a considerable potential of high-density ccg-QD suspensions as liquid, dye-like optical gain media that feature readily achievable spectral tunability and stable operation under intense photoexcitation. This article is protected by copyright. All rights reserved.