Colorful Lasers from Q Dots

A new type of nanometer-size semiconductor crystal that can amplify light marks an important step toward cheap, tunable lasers.

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.

Crystal light: A semiconductor nanocrystal made by researchers at the Los Alamos National Laboratory is able to amplify light. Electrons get trapped in the cadmium-sulfide core while positively charged holes move to the zinc-selenide shell. This keeps the particles from interacting and annihilating each other, letting the nanocrystal amplify light, which is a crucial requirement in making a nanocrystal laser. Credit: LANL

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.

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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.