One of the key parts of making tiny laser devices, like ophthalmic surgery scalpels, work efficiently is the use of tiny semiconductor particles, called quantum dots. A nanotech research team at Los Alamos National Laboratory doctored, or “doped,” nanometer-sized dots with additional electrons. This treatment nudged the dots closer to producing the desired laser light with less stimulation and energy loss.
"When we properly tailor the compositional profile within the particles during their fabrication and then inject two or more electrons in each dot, they become more able to emit laser light. Importantly, they require considerably less power to initiate the lasing action," said Victor Klimov, leader of the Nanotech team.
Los Alamos National Laboratory Chemist Jaehoon Lim works on an apparatus that synthesizes quantum dots along with Los Alamos researcher Young-Shin Park (also with the University of New Mexico Center for High-Technology Materials). In a paper published in Nature Nanotechnology, Los Alamos colleagues Kaifeng Wu and Victor Klimov worked with Lim and Park to demonstrate that negatively charged quantum dots show promise for low-power laser applications or quantum dot laser diodes. (Los Alamos National Laboratory)
To force a material to emit laser light, researchers had to work toward a “population inversion.” Population inversion is making the number of electrons in a higher energy electronics state exceed the number that is in a lower energy state. In order to achieve this condition, they used external stimulus (optical or electrical) of a certain power, which should exceed a critical value, called “optical-gain threshold.” In a recent paradigm-changing advance, Los Alamos researchers demonstrated that adding extra electrons into their specially designed quantum dots, they can reduce the threshold to virtually zero.
A standard lasing material, when stimulated by a pump, absorbs light for a time before it starts to lase. Before lasing, the material transitions to the state of “optical transparency” when the light is not absorbed or amplified. By adding extra charge carriers to quantum dots, the Los Alamos researchers could block absorption and create the state of transparency without external stimulation. This implies that extremely weak pumping can initiate lasing emission.
Another important part of this research is a new type of quantum dots with their interiors designed to maintain the lasing-active state for longer than standard particles do. Normally, extra electrons would stop lasing because quantum dot energy is quickly released as wasteful heat, not a photon stream. The new Los Alamos particle design eliminates these parasitic losses, redirecting the particle’s energy into the emission channel.
"These studies open exciting opportunities for realizing new types of low-threshold lasing devices that can be fabricated from solution using a variety of substrates and optical cavity designs for applications ranging from fiber optics and large-scale lasing arrays to laser lighting and lab-on-a-chip sensing technologies," Klimov said.
A paper on this research was published in the journal Nature Nanotechnology.
