These quantum dots are type I, in which an electron-hole pair, or exciton, is confined inside the core. In type II quantum dots, the electron and hole are separately confined in the core and the shell. When excitons recombine upon excitation of the quantum dots, they release energy in the form of light or heat. In the past, type I quantum dots have typically been excited in such a way that multiple excitons per crystal must form for lasing to occur. The excitons can interact with one another, causing the quantum dot to lose as much as 99% of its energy as heat, Dang says. Other researchers previously demonstrated single-exciton lasing with type II quantum dots, but type II quantum dots are inefficient light emitters. The Brown team now shows that single-exciton lasing is also possible with the more efficient type I quantum dots. They incorporated the quantum dot films into a device called a vertical-cavity surface-emitting laser and achieved red, green, and blue lasing. Instead of producing multiple excitons per dot, the low-energy excitation produced on average 0.8 excitons per dot.
by Celia Henry Arnaud |
May 07, 2012