Quantum dots–tiny semiconductor crystals 2 to 10 nanometers in size–emit bright, fluorescent light in different hues. Scientists can make them using simple, inexpensive chemical processes and change the emitted color easily just by tweaking the size of the nanocrystals. While quantum dots have found use in medical imaging and are close to being used in photovoltaic cells and LEDs, researchers have been trying for a decade to use the semiconductor nanocrystals to make lasers. It is crucial for any lasing material to be able to amplify light, and nanocrystals have proved exceedingly stubborn in their refusal to do so.
In a significant breakthrough in creating a nanocrystal laser, Victor Klimov and his colleagues at the Los Alamos National Laboratory have engineered a new type of nanocrystal that can amplify light. The nanocrystal, as reported in last week’s Nature, has a core and a shell made of different materials so that it can separate the electrons from the positively charged holes: the core traps the electrons, while the shell traps the holes. Without the separation in the tiny nanocrystals, the charged particles interact and annihilate each other in less than 50 picoseconds–not enough time for the material to amplify light.
“This opens the door for a laser based on nanocrystal quantum dots,” says Alexander Efros, a theoretical physicist at the Naval Research Laboratory, in Washington, DC, and a pioneer in nanocrystal quantum dots.
As opposed to current gas or diode lasers, which emit at a single wavelength, nanocrystal lasers could emit light ranging from violet to green. And since nanocrystals are made in the form of bright solutions, the lasers could be built right into optical telecommunication fibers or deposited on lab-on-a-chip devices and silicon-based medical and chemical sensors. “You can spray [nanocrystals] on things, drop them into a little spot … manipulate them very easily,” says Todd Krauss, a chemistry professor who studies semiconductor nanocrystals at the University of Rochester, NY. “That’s a lot easier than building an x-million-dollar fabrication line.”
For a material to emit light, its electrons need to be excited, either by light or electric current, so that they move from their normal low-energy state to a higher energy level, leaving behind a positively charged hole. If a photon of a specific energy comes along, it stimulates the excited electron and the hole to recombine, a process that emits two photons. The photons either leave the material, creating light, or they can get reabsorbed by unexcited electrons in the material. If a material is to amplify light, the key is that it needs to have more excited electrons than unexcited ones, which means in the end that more photons come out of the material than originally went in.
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.”