AUSTIN, Texas (AP) — Researchers can envision a minuscule machine that generates pristine images of the smallest living molecules.
They can imagine a computer chip that runs thousands of times faster than current incarnations, yet remains cool enough to use in a laptop computer.
Those products remain many years and several discoveries into the future, but a collaboration of researchers at the University of Texas at Austin and Taiwan's National Tsing Hua University have recently moved a step closer.
In late July, the group announced the creation of the world's smallest semiconductor-based nanolaser. The device is 500 nanometers long and 60 nanometers wide and high at its thickest points. By comparison, the width of a fine human hair is about 50,000 nanometers.
Such tiny lasers could eventually become crucial building blocks in a range of advanced optical, medical and computer technologies. For example, they could become the foundational component of a new breed of computer chips that would transmit information via pulses of light instead of electrons, thus overcoming the speed, density and heat barriers that limit performance today.
However, creating a functional laser that small presents considerable challenges. At sizes smaller than the wavelength of light, it becomes increasingly difficult for a laser to contain enough of its own photons to sustain a continuous operation.
UT physics professor Chih-Kang "Ken" Shih and Charlotte Sanders, a graduate student on his team, constructed an exceptionally smooth silver film that helps limit the loss of energy while simultaneously tightening the field of laser-generating activity.
Shih has worked on perfecting these types of advanced materials for more than 15 years. He and Sanders created the silver film at their lab in Austin, and the nanolaser was built and tested by one of Shih's former graduate students and his colleagues at the Taiwanese university.
"The main innovation is the kind of silver that they created," said Yeshaiahu "Shaya" Fainman, a University of California, San Diego electrical and computer engineering professor who has worked extensively on nanolasers but wasn't part of this project.
Past films used in this type of nanolaser contained gaps between the tiny atomic-scale clumps of metal, Shih and Sanders said. Those gaps and clumps would scatter the useful wave of energy researchers ultimately wanted to produce — a photon-electron hybrid called surface plasmon polariton.
To give people a sense of the problem, Shih describes it like the difference between off-roading and driving down a freshly paved highway.
"Unless you actively control your steering wheel, you're not going to control your vehicle" if you're driving over rocky terrain, he said. "If you want the wave to travel in a certain direction, you need it to travel in that direction without anything knocking it off the road."
By growing the silver in a single crystalline form, Shih and Sanders eliminated the gaps — essentially building a smooth highway that keeps more of the surface plasmon polaritons moving in a similar direction along the plane of the semiconductor-silver interface.
But the smooth silver film also helped contain the photons that produce the laser activity within a smaller active field. In fact, it helped confine those photos in a space smaller than the wavelength of light.
That solved another problem for nanolaser development, an issue called the three-dimensional diffraction limit.
To sustain lasing, a nanolaser has to keep photons within its active field, where they can interact with excited atoms to produce more photons, said Gennady Shvets, a UT physics professor who carried out the theoretical modeling and testing for the project, along with two of his graduate students.
Before the smooth silver film, devices as tiny as UT's nanolaser couldn't contain enough photons within the active field, so the device wouldn't produce enough new photons and the lasing would grind to a halt. The film helps confine photons within a volume smaller than their wavelength, a feat that allowed researchers to build a smaller device, use a less-intense pumping laser to start the process, and generate a continuous wave.
The next challenge is bringing this whole process up to room temperatures, Shvets and Shih said. Because of the small size and energy involved to pump the nanolaser in its current form, the process must occur at very cold temperatures.
Some types of nanolasers already work at room temperature, but they typically don't create the plasmonic effects that work especially well on a planar semiconductor.
That will involve some experimentation with various materials, designs and sizes, Shvets said. "It's probably as small as it's going to get," he said. "What would be really nice is to make this device work at room temperature."