A standard lasing material, when stimulated by a pump, absorbs light for a time before it starts to lase. On the way to lasing, the material transitions through the state of “optical transparency” when light is neither absorbed nor amplified. By adding extra charge carriers to their quantum dots, the Los Alamos researchers were able to block absorption and create the state of transparency without external stimulation.
The researchers applied photodoping to specially engineered quantum dots with impeded Auger decay, negatively charging the quantum dots. By injecting approximately one electron per dot on average, the scientists observed a more than twofold reduction in the amplified spontaneous emission threshold, which they attribute to the suppression of ground-state absorption by pre-existing carriers. Pending further investigation, they hope to be able to achieve a ‘zero-threshold’ gain media, meaning the lasing action could be initiated with extremely week optical or electrical stimulation.
The new quantum dots are designed to maintain the lasing-active state for much longer than standard particles do. Normally, the presence of extra electrons would suppress lasing because quantum dot energy is quickly released not as a photon stream but wasteful heat. 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,” said Victor Klimov, leader of the Nanotech team.
Los Alamos National Laboratory - www.lanl.gov