The result brings the tiny flakes, called nanocrystals, a crucial step closer to fulfilling their promise of tailoring a laser's color just by changing the size of the crystal, which could lead to more powerful tools for detecting chemicals or sending information via flickers of light.
The color of laser light depends on the light-emitting material. Specifically, a semiconductor offers electrons a choice of two energy states, lower or higher, sort of like a ladder with two rungs. The band gap, or difference in energy between rungs, determines the wavelength of light emitted. "In nanocrystals, the gap changes with their size—the smaller the size, the larger the gap," says physicist Victor Klimov of Los Alamos National Laboratory in New Mexico.
The challenge was getting the nanocrystal's electrons to cooperate. Laser light occurs when most of a material's electrons are in an excited, or higher, energy state. An incoming photon will then knock an electron from the top to bottom energy rungs, kicking out two identical photons in the process and causing amplification of light.
In the nanocrystals, which range from two to 150 nanometers wide, only two electrons per crystal may hop between energy rungs. When both members of the pair became excited, one of them would normally fall to the lower rung before being struck by an incoming photon, producing no photon along the way and leaving too few excited electrons to make laser light.
Klimov and colleagues overcame this hurdle by splitting the nanocrystal into a cadmium sulfide core and a zinc selenide shell. The core trapped excited electrons for two nanoseconds—50 times longer than normal and long enough to be hit by an outside photon—according to a report published this week in Nature.
Before this result, nanocrystal lasers needed their own superpowerful laser to get them going, says chemist Todd Krauss of the University of Rochester. He adds that if such two-layer crystals are long lasting and efficient enough to be melded with electronics, "you open up a complete array of applications."
Nanocrystal lasers might be cheaper, more efficient and versatile compared with today's kind, Krauss says, perhaps leading to chemical sensors or optical communications devices capable of rapidly switching between laser colors. "The potential is there," he says.