Quantum Dots Amplify Light

A collage showing contains a transmission electron microscopy image of the improved quantum dot and its representation (left), the schematic of the device (middle), and the device under operation. (Source: LANL)

Los Alamos scientists have shown that they can success­fully amplify light using elec­trically excited films of the chemically synthesized semi­conductor nano­crystals or quantum dots. The quantum dot films are inte­grated into devices much like the now-ubi­quitous light-emitting diodes (LEDs), but, in this case designed to sustain the high current densities required for achieving the optical-gain regime. One sees laser diodes every day in laser pointers, barcode readers and the like, and a key element of such devices is an optical-gain medium, which instead of absorbing incident light, amplifies it.

“Optical gain with elec­trically excited quanum dots is now a reality,” said Victor Klimov, head of the quantum dot team at Los Alamos. “We have been working to develop new lasing media, using chemi­cally syn­thesized quantum dots, although it had been widely believed that quantum dot lasing with elec­trical stimu­lation is simply impos­sible,” he said. “By using our specially designed dots, we can avoid energy losses created by Auger recom­bination.”

These results demon­strate the feasi­bility of a new gene­ration of highly flexible, elec­trically pumped lasers processible from solu­tions that can complement or even even­tually displace existing laser diodes fabricated using more complex and costly vacuum-based epitaxial tech­niques. These prospec­tive devices can enable a variety of applications, from RGB laser modules for displays and projectors, to multi-wave­length micro-lasers for bio­logical and chemical diag­nostics.

The Los Alamos team demon­strates that using their “designer” quantum dots, they can achieve light amplification in a nano­crystal solid with direct-current electrical pumping. The key property of the novel quantum dots, under­lining the success of the conducted study, is a carefully engineered particle interior in which the material’s compo­sition is conti­nuously varied along a radial direction. This approach eli­minates sharp steps in the atomic composition which would normally trigger Auger recom­bination. As a result, the engi­neered quantum dots feature nearly complete suppres­sion of Auger effect’s heat loss, and this allows for redirecting the energy released by the electrical current into the light-emission channel instead of wasteful heat.

Another important element of this work is a special “current-focusing” device archi­tecture which allows the high current densities neces­sary for achieving optical gain. The method used by Los Alamos researchers was to taper one of the charge-injec­tion electrodes, limitingthe size of the current-conducting area to less than 100 microns. Using this strategy, they were able to produce current concen­tration sufficient to reach the regime of light ampli­fication without damaging either the dots or the injec­tion layers. (Source: LANL / DOE)

Reference: J. Lim et al.: Optical Gain in Colloidal Quantum Dots Achieved with Direct-Current Electrical Pumping, Nat. Mat., online 20 November 2017; DOI: 10.1038/nmat5011

Link: Nanotechnology and Advanced Spectroscopy (V. Klimov), Los Alamos National Lab., Los Alamos, USA

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