‘Flying Saucer’ Quantum Dots Hold Secret to Better, Brighter Lasers

quantum dots
This computer-generated model shows the spherical core of the quantum dot nanoparticle (in red) along with the ‘flying saucer’ shape of the outer shell (in yellow). The tension in the core induced by the shell affects the electronic states and lowers the energy threshold required to trigger the laser. (Alex Voznyy/University of Toronto)

Fresh insights into living cells, brighter video projectors and more accurate medical tests are just three of the innovations that could result from a new way of fabricating lasers.

The new method was developed by an international collaboration among researchers from the University of Toronto, Vanderbilt University and Los Alamos National Laboratory. It uses nanoparticles known as quantum dots to produce continuous laser light in a wide range of different colors. The novel nanoparticles could ultimately provide a brighter and less expensive laser light source than current devices.

The University of Toronto team designed and fabricated the new type of quantum dots. Chemists at the Vanderbilt Institute for Nanoscale Science and Engineering performed the structural, chemical and optical characterization of the nanoparticles. And the researchers at Los Alamos tested their lasing ability.

“We’ve been working with quantum dots for more than a decade,” says Ted Sargent, professor of electrical and computer engineering at the University of Toronto. “They are more than five thousand times smaller than the width of a human hair, which enables them to straddle the worlds of quantum and classical physics and gives them useful optical properties.”

quantum dots
Sandra Rosenthal (Vanderbilt)

“This is very exciting research,” said Professor of ChemistrySandra Rosenthal, said Professor of Chemistry Sandra Rosenthal who holds the Jack and Pamela Eagan Chair and headed the Vanderbilt contingent. “Sargent’s team has managed to create quantum dots with a unique and elegant structure. When we examined the structure of their quantum dots at the atomic level, we were impressed not only by the engineered structure itself but also by the level of uniformity they have achieved.”

By carefully controlling the size of the quantum dots, the researchers in Sargent’s lab found that they can ‘tune’ the frequency of the emitted light to any desired value. By contrast, most commercial lasers are limited to one specific frequency, or a very small range, defined by the materials from which they are made.

There is a wide range of potential applications for the tiny lasers. For example, the ability to produce a laser of any desired frequency from a single material would give a boost to scientists looking to study diseases at the level of tissues or individual cells by providing new tools to probe specific biochemical reactions. They could also enable laser display projectors that would be brighter and more energy efficient than current LCD technology.

The new nanoparticle structure solved a problem that has bedeviled previous efforts to produce quantum dot lasers: Until now, the amount of light needed to excite the quantum dots to produce laser light has been very high.

“You have to stimulate the laser using more and more power, but there are a lot of heating losses as well,” said Alex Voznyy, a senior research associate in the Sargent Lab. “Eventually it gets so hot that it just burns.” As a result, most quantum dot lasers are limited to emitting pulses of light lasting just a few nanoseconds — billionths of a second.

quantum dots
James McBride (Vanderbilt)

The Toronto engineers overcame this obstacle by changing the shape of the quantum dots. They did so by creating quantum dots with a spherical core and a shell shaped like a Skittle, an M&M or a flying saucer — a ‘squashed’ spherical shape known as an oblate spheroid.

The mismatch between the shape of the core and the shell introduces a tension that affects the electronic states of the quantum dot, lowering the amount of energy needed to trigger the laser. As reported in a paper titled “Continuous-Wave Lasing in Colloidal Quantum Dot Solids Enabled by Facet-Selective Epitaxy” published March 20 in the scientific journal Nature, the innovation means that the quantum dots are no longer in danger of overheating, so they can produce continuous beams of laser light.

The team has more work to do before they can look to commercialization. “For this proof-of-concept device, we’re exciting the quantum dots with light,” says U of T postdoctoral fellow Randy Sabatini. “Ultimately, we want to move to exciting them with electricity. We also want to scale up the power to milliwatts or even watts. If we can do that, then it becomes important for laser projection.”

The Vanderbilt team’s research was supported by National Science Foundation grants CHE-1506587 and EPS 1004083.