Nanomaterial Yields Many Laser Colors

Nanocrystals that can produce red, green and blue laser light from a single material could open the door to digital displays and other devices that employ a variety of laser colors all at once.

Red, green and blue lasers have become small and cheap enough to find their way into products ranging from Blu-Ray DVD players to fancy pens, but each color is made with different semiconductor materials and by elaborate crystal growth processes.

Now, a new prototype technology developed at Brown University and QD Vision of Lexington, Mass., can achieve all three colors using materials consisting of nanometer-sized semiconductor particles called colloidal quantum dots, or nanocrystals. The colloidal quantum dots have an inner core of cadmium and selenium alloy and are coated with zinc, cadmium and sulfur alloy and proprietary organic molecular glue.

“Today in order to create a laser display with arbitrary colors, from white to shades of pink or teal, you’d need these three separate material systems to come together in the form of three distinct lasers that would not have anything in common,” said Arto Nurmikko, professor of engineering at Brown University. “Now enter a class of materials called semiconductor quantum dots.”

Colloidal quantum dots — nanocrystals — can produce lasers of many colors. Cuong Dang manipulates a green beam that pumps the nanocrystals with energy, in this case producing red laser light (at left). (Image: Mike Cohea/Brown University)

Chemists at QD Vision synthesized the nanocrystals using a wet chemistry process that allows precise variation of the nanocrystal size by altering the production time. Size is all that must change to produce different laser light colors: 4.2-nm cores produce red light, 3.2-nm ones emit green light, and 2.5-nm ones shine blue. Other sizes would produce other colors along the spectrum.

Because of their improved quantum mechanical and electrical performance, the coated pyramids require 10 times less pulsed energy or 1000 times less power to produce laser light than previous attempts at the technology.

When a batch of colloidal quantum dots is brewed to the Brown-designed specifications, Nurmikko and Cuong Dang, a senior research associate in Nurmikko’s group, get a vial of viscous liquid that somewhat resembles nail polish. This liquid is used to coat a square of glass or a variety of other shapes to make a laser. When the liquid evaporates, what’s left on the glass are several densely packed solid, highly ordered layers of nanocrystals. Sandwiching the glass between two specially prepared mirrors enables the researchers to create a vertical-cavity surface-emitting laser (VCSEL). This is the first working VCSEL with colloidal quantum dots, the Brown researchers said.

The alloy in the nanocrystal’s outer coating reduces an excited electronic state requirement for lasing and protects the nanocrystal from a kind of crosstalk that makes it hard to produce laser light, Nurmikko said. In addition to reducing crosstalk, the nanocrystal’s structure and outer cladding reduce the amount of energy needed to pump the quantum dot laser. The new approach’s structure enables the dots to act more quickly, releasing light before heat is lost as a result of a phenomenon known as the Auger process.

“We have managed to show that it’s possible to create not only light, but laser light,” Nurmikko said. “In principle, we now have some benefits: using the same chemistry for all colors, producing lasers in a very inexpensive way, relatively speaking, and the ability to apply them to all kinds of surfaces, regardless of shape. This makes possible all kinds of device configurations for the future.”

The method was described in Nature Nanotechnology.

For more information, visit: www.brown.edu