Two-Band Optically Pumped Amplified Spontaneous Emission in an Ultrahigh-Current-Density Colloidal Quantum Dot LED

The demonstration of a new-generation of laser diodes employing solution-processable materials has been actively pursued across multiple fields including organic molecules, perovskites, and colloidal quantum dots (QDs). The latter structures are especially attractive due to their outstanding light-emission properties and sparce atomic-like electronic states with size-controlled energies. A primary obstacle towards a QD-based laser diode is very fast Auger decay of multiexciton states required for a lasing action. Recently, continuously graded QDs (cg-QDs) with suppressed Auger recombination demonstrated optical gain achieved with electrical injection in a “current-focusing” light-emitting diode (LED)^1,2. However, light amplification from QDs in a complete LED stack has not been yet realized because modal gain has not been sufficiently high to outcompete optical losses. Here, we demonstrate a new LED architecture, which employs a newest generation of rigorously optimized cg-QDs incorporated into a two-dimensional current focusing device. By tuning the thicknesses and refractive indices of various elements of the LED stack, we maximize a mode confinement factor for the gain-active QD layer and simultaneously minimize the extent of the optical mode into lossy charge transport layers. Using this approach, we are able to achieve the regime when modal gain exceeds optical losses and realize amplified spontaneous emission (ASE) within both the band-edge (1S) and the higher-energy (1P) transitions using pulsed optical excitation. The same devices also demonstrate strong optical gain under electrical pumping. In particular, we are able to invert both the 1S and 1P transitions using current densities of &gt;500 A cm^-2, which leads to generation of more than 4 excitons per dot on average. The experimental observations are rationalized through numerical COMSOL modeling with input optical-gain and loss parameters derived from variable stripe length measurements. These studies prove once again the feasibility of colloidal QD laser diodes and clear some of the last obstacles on the path to a functional laser diode based on colloidal QDs.<br/>1. J. Lim, Y.-S. Park, V. I. Klimov, <i>Nature Materials</i>, <b>17</b>, 42-49 (2018)<br/>2. H. Jung, N. Ahn, V. I. Klimov, <i>Nature Photonics</i>, <b>15</b>, 643-655 (2021)