In semiconductor nanocrystals, two electrons normally sit in the low-energy level. So in order to make the crystals amplify light, researchers have attempted to excite both of these electrons, creating two electron-hole pairs. But in the tiny crystals, the two electron-hole pairs interact, and one annihilates the other. “You have basically 50 picoseconds or so before one [electron-hole pair] will find the other one and one of them gets killed, and once one gets killed, you can’t get any lasing or amplification,” Krauss says.
One could make the most of those 50 picoseconds by bombarding the nanocrystals with short, intense bursts of light, Klimov says. That way, they’re quickly hit with a lot of photons before the two electron-hole pairs have had a chance to interact. For that, a researcher needs a femtosecond laser, says Klimov. Such an approach, he says, “is not practical.”
Klimov and his colleagues have found a better trick: they have made a nanocrystal that can amplify light with just one electron-hole pair. The nanocrystal has a cadmium-sulfide core wrapped with a zinc-selenide shell. “Turns out that this leads to charge separation,” Klimov says. “The electrons want to stay in the core, but the holes want to go to the shell.” This separation changes the properties of the nanocrystal. Out of the two electrons sitting in the low-energy level in the crystal, one now needs a much larger energy boost than the other does to get excited, so it tends to stay put in the lower level. As a result, only one electron gets excited and forms an electron-hole pair. Now when this pair recombines in the presence of a photon and generates two identical photons, both the photons leave the material.
The researchers can now bombard the nanocrystals using less energy, which means a less powerful laser light, and still make them amplify the light. The single excited electron in a nanocrystal also stays excited for almost two nanoseconds. This means that one could potentially use slower lasers to get the light amplification, Klimov says. “Eventually, it would be great if we could pump [nanocrystals] electrically.”
Indeed, a practical nanocrystal laser would need to be energized using electricity, Krauss says, just like the lasers used in telecommunications systems, laser pointers, and CD players. “Right now, it takes a $300,000 laser to make a nanocrystal laser,” he says. “But if you could plug it into the wall–now we’re getting somewhere. The Holy Grail really is to get [lasers] to be electrically pumped, and this [new method] is a big step towards that.”