Researchers at Los Alamos National Laboratory (LANL) have assessed the status of research into colloidal quantum dot lasers with a focus on prospective electrically pumped devices, or laser diodes.
Their review analyzes the challenges for developing lasing with electrical excitation, and presents approaches to overcome them.
Semiconductor lasers, or laser diodes, are a key part of many common consumer products as well as equipment used in telecommunication, scientific research, medicine, and space exploration. Usually, these devices employ ultrathin semiconductor films, or quantum wells, grown via vacuum-based layer-by-layer atomic deposition.
While allowing for precise control of the materialâs properties, this growth method requires a clean-room environment. In addition, it is restricted to a rather small number of mutually compatible materials used as a lasing medium and an underlying substrate. Specifically, compatibility issues greatly complicate integration of existing semiconductor lasers with standard silicon-based microelectronics, the LANL team notes.
âThese problems can, in principle, be resolved with inexpensive solution-processable light emitters,â said Klimov. âIn particular, an attractive alternative to standard quantum wells is semiconductor particles prepared via bench-top colloidal chemistry.â
Colloidal quantum dots can be synthesized in bulk quantities in a standard wet-chemistry lab using inexpensive, readily available precursors. Further, they can be combined with virtually any substrate that would resolve the issue of compatibility with silicon microelectronics and open new areas of applications not accessible with traditional laser diodes.
There are also additional advantages derived from the unique quantum nature of colloidal nanocrystals. In particular, due to their ultrasmall sizes, their emission wavelength can be readily tuned by varying nanocrystal dimensions. This could enable laser diodes with an ultra-wide range of accessible wavelenghths.
âDespite these potential benefits, the colloidal quantum dots are difficult lasing materials,â Klimov said. âHigh-quality nanocrystals have been available since the early â90s. However, they would not lase until around 2000, when the Los Alamos team demonstrated for the first time the effect of light amplification with cadmium selenide nanocrystals.â
The Los Alamos researchers used densely packed quantum dot solids, which allowed them to boost the rate of stimulated emission so it could outpace Auger decay. Furthermore, they employed very short (about 100 fs) pulses to populate quantum dots with biexcitons before they had a chance to decay. This approach produced a coveted result â the realization of amplified spontaneous emission, proof-of-principle for colloidal quantum dot lasing.
Auger recombination still represents a major obstacle for the realization of technologically viable quantum dot lasers. Another serious challenge is the development of practical devices that can sustain ultra-high current densities of hundreds of amperes per centimeter squared required for lasing.
The realization of such structures is vastly complicated by poor charge-transport properties of granular quantum dot solids and high resistivity of solution-processed charge-transport layers. As a result, devices overheat at high current densities and eventually fail due to heat-induced breakdown.
To resolve the thermal damage problem, LANL developed a new device architecture in which current flow was restricted to a small area of 50 by 300 microns. This current-focusing approach boosts current density and simultaneously reduces the heat generation volume and improves heat exchange with the environment.
Another challenge is incorporation of an optical resonator so as it does not disrupt charge injection pathways and, at the same time, maintains lasing despite the presence of âoptically lossyâ charge transport layers. This problem also has been recently addressed by Los Alamos researchers.
The team applied an interesting approach wherein an optical resonator was prepared as a periodic grating engraved into a layer serving as an electron injector. In this way, they preserved a standard architecture of a light emitting diode but endowed it with an additional function of lasing device. The developed dual-function structures performed as a standard LED operating under electrical pumping and a laser activated optically.
The final step is to combine all of these strategies in a single device capable of lasing with electrical excitation. Given recent advances in ultrahigh current density architectures and successful recipes for cavity integration, this objective seems to be within close reach, suggesting that colloidal quantum dot laser diodes soon may become a reality.