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. Credit: Alex Voznyy
Research team ‘squashes’ the shape of nanoparticles, enabling inexpensive lasers that continuously emit light in a customized rainbow of colors. Fresh insights into living cells, brighter video projectors and more accurate medical tests are just 3 of the innovations that could result from a new way of fabricating lasers. The new method produces continuous laser light that is brighter, less expensive and more tuneable than current devices by using nanoparticles known as quantum dots.
“Quantum dots are well-known bright light emitters,” says Alex Voznyy, a senior research associate in Sargent’s lab. “They can absorb a lot of energy and re-emit it at a particular frequency, which makes them a particularly suitable material for lasers.” By carefully controlling the size of the quantum dots, the researchers in Sargent’s lab can ‘tune’ the frequency, or colour, 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 they are made from.
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 offering new tools to probe biochemical reactions. They could also enable laser display projectors that would be brighter and more energy efficient than current LCD technology. But a problem has been that 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,” says Voznyy. “Eventually it gets so hot that it just burns.” Most quantum dot lasers are limited to pulses of light lasting just a few nanoseconds – billionths of a second. The team overcame this problem by changing the shape of the quantum dots, rather than their size. They were able to create quantum dots with a spherical core and a shell shaped like a Skittle, an M&M or a flying saucer – 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. This means that the quantum dots are no longer in danger of overheating, so the laser can fire continuously.
While quantum dots are often built by depositing molecules one at a time in a vacuum, Sargent’s team mixes together liquid solutions that contain various quantum dot precursors. When the solutions react, they produce solid quantum dots that stay suspended in the liquid, ie colloidal quantum dots. The team’s key innovation was to add specific capping molecules into the mix, which allowed them to control the shape of the particles to obtain the desired properties, an approach Fan calls ‘smart chemistry’. “Solution-based processing greatly reduces the cost of making quantum dots,” says Fan. “It will also make it easier to scale up production, because we can use techniques already established in the printing industry.”
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 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.” http://news.engineering.utoronto.ca/flying-saucer-quantum-dots-hold-secret-brighter-better-lasers/
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