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  • Single nanomaterial yields a laser rainbow

Photonics Spectra
Jul 2012

PROVIDENCE, R.I. – Nanocrystals that produce red, green and blue laser light from a single material could lead to digital displays and other devices that employ a variety of laser colors simultaneously. 

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

  Now, a prototype technology developed at Brown University and at QD Vision of Lexington, Mass., can achieve all three colors using materials consisting of colloidal quantum dots. 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. 

The method was described online in Nature Nanotechnology (doi: 10.1038/ nnano.2012.61).

“We are actively working with cadmium-free colloidal quantum dots such as indium phosphide,” Brown senior research assistant Cuong H. Dang told Photonics Spectra. “Benefits are obvious with nontoxic materials.”

To create a laser display with arbitrary colors such as shades of pink or teal, three separate material systems would need to come together in the form of three distinct lasers that would not have anything in common, according to Arto Nurmikko, professor of engineering at Brown. Instead, a new class of materials called semiconductor quantum dots was introduced.

QD Vision chemists synthesized the nanocrystals using a wet chemistry process that enables precise variation of size by altering production time. To produce different laser light colors, the only variable that must change is size: 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.

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). Courtesy of Mike Cohea/Brown University.

“I don’t see any problem to produce all visible colors with our technology,” Dang said. “But I did receive a request for the infrared range: 1- to 2-µm wavelength, which we have not yet achieved.”

The coated pyramids, with improved quantum mechanical and electrical performance, require 10 times less pulsed energy, or 1000 times less power, to produce laser light than previous attempts.

A batch of colloidal quantum dots prepared to the Brown-designed specifications yields 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, several densely packed solid, highly ordered layers of nanocrystals remain on the glass. By sandwiching this glass between two specially prepared mirrors, the researchers created a vertical-cavity surface-emitting laser (VCSEL) – the first working VCSEL with colloidal quantum dots.

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. Besides 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.

“The alloy for shell zinc cadmium sulfide was rooted from our experience with II-VI semiconductor materials in bulk and thin-film forms a while ago,” Dang said. “There are a number of ligands involved in optimizing the process. We tried both aromatic and aliphatic ligands.”

Next, the scientists hope to tackle their system’s heating problem and to enable electrical injection as opposed to the current optical injection to provide final products, Dang said.

“We have managed to show that it’s possible to create not only light, but laser light,” Nurmikko said in a university release. “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.”

quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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