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SALT LAKE CITY — University of Utah physicists have discovered a way to fix a defect occurring in lasers composed of quantum dots, which they believe will be important in photonics research and the creation of microchips that code information using light instead of electrons.
Lasers are devices that create single-wavelength light (only one color) that is directional, meaning that it stays focused for long distances. The material lasers are composed of has a notable impact on the strength of the beam as light travels through the material.
Quantum dots are one substance lining the inside of lasers. Evan Lafalce, research assistant professor of physics and astronomy at the University of Utah and lead author of the study, explained that, “Quantum dots are tiny crystals only a few hundred atoms in diameter. They have interesting properties such as strong light absorption and emission, and the frequencies (colors) of light that they absorb or emit can be changed by what material they are made out of and the size of the crystals. The properties of these quantum dots were thoroughly optimized to achieve high gain and emit and amplify red light.”
Gain refers to the medium in a laser which amplifies the light. Properties can be tuned very easily in quantum dot lasers, making them very interesting to scientists. Unfortunately, they do frequently have small defects that split light into multiple different wavelengths, making it less concentrated and powerful. Focusing energy into one wavelength is ideal in photonic research.
Collaborators from the University of Utah and the Georgia Institute of Technology have sought to correct this defect. First, the GIT researchers built 50 microscopic disk-shaped quantum dot lasers out of cadmium selenide, according to the press release. U. researchers then coupled two lasers together to see if the adjustment would correct wavelength splitting.
One laser was started with the maximum energy possible to achieve full gain. A strong green light was shone into the two lasers, causing their gain to increase. Once the gain of the two layers stabilized to a similar level, the interaction between them was able to correct the splitting and focus energy into one wavelength.
The researchers are the first to observe this phenomenon. “The wavelength-splitting that occurs within a disk is actually itself a kind of coupling, or interaction, between the light fields traveling in different directions,” Lafalce told KSL.com. “So we now know we can change internal features of the laser to get different performance and also couple lasers together and expand the range of possible behavior even more.”
The findings are very relevant to research on optics and photonics. Traditionally, electrons in electronics are used to carry information. For the past 30 years, researchers have been investigating whether it’s possible to use light to carry information instead.
“The advantage of using light over electronic cables is light has larger bandwidth, which allows you to carry information in parallel channels,” Lafalce explained. “It can also be less costly and consume less energy because you don't need metals and it can generate less heat.”
Lasers would be a large part of that process, and quantum dots would make optical telecommunication a lot more doable.
“It’s not impossible that someone could make a defect-free laser with quantum dots, but it would be expensive and time-consuming. In comparison, coupling is a quicker, more flexible, cost-effective way to correct the problem,” said Lafalce. “This is a trick so that we don’t have to make perfect quantum dot lasers.”
Lafalce also considers their quantum dot research an important materials achievement. He told KSL.com that, "because quantum dots can be kept in a liquid solution or ‘ink,’ they can be easily processed to form a variety of different kinds of structures.”
Next, the researchers plan to investigate how differences in the shapes of the microdisks (elliptical rather than circular) affect the results of laser coupling. Learn more about the experiment on their scholarly publication.