Continuously Emitting Microlasers

At left, a tiny bead struck by a laser produces optical modes that circulate around the interior of the bead. At right, a simulation of how the optical field inside a 5-micron bead is distributed. (Source: A. Fernandez-Bravo, Berkeley Lab / K. Yao)

Researchers have found a way to convert nano­particle-coated micro­scopic beads into lasers smaller than red blood cells. These micro­lasers, which convert infrared light into light at higher frequencies, are among the smallest conti­nuously emitting lasers of their kind ever reported and can constantly and stably emit light for hours at a time, even when submerged in biolo­gical fluids such as blood serum.

The inno­vation, discovered by an inter­national team of scientists at the U.S. Department of Energy’s Lawrence Berkeley Labora­tory, opens up the possi­bility for imaging or controlling biological activity with infrared light, and for the fabrication of light-based computer chips. The unique proper­ties of these lasers, which measure 5 microns across, were discovered by accident as researchers were studying the potential for the polymer beads, composed of a translucent substance known as a colloid, to be used in brain imaging.

Angel Fernandez-Bravo, a postdoctoral researcher at Berkeley Lab’s Molecular Foundry, mixed the beads with sodium yttrium fluoride nano­particles doped with thulium, an lan­thanide. Emory Chan, a Staff Scientist at the Molecular Foundry, had in 2016 used computational models to predict that thulium-doped nano­particles exposed to infrared laser light at a specific frequency could emit light at a higher frequency than this infrared light in a counter­intuitive upcon­version-process. Also at that time, Elizabeth Levy, then a participant in the Lab’s Summer Under­graduate Labora­tory Internship (SULI) program, noticed that beads coated with these upcon­verting nano­particles emitted unexpec­tedly bright light at very specific wave­lengths.

“These spikes were clearly periodic and clearly reproducible,” said Emory Chan, who co-led the study along with Foundry Staff Scientists Jim Schuck and Bruce Cohen. The periodic spikes that Chan and Levy had observed are a light-based analog to whis­pering gallery acoustics that can cause sound waves to bounce along the walls of a circular room so that even a whisper can be heard on the oppo­site side of the room. In the latest study, Fernandez-Bravo and Schuck found that when an infrared laser excites the thulium-doped nano­particles along the outer surface of the beads, the light emitted by the nano­particles can bounce around the inner surface of the bead.

Light can make thousands of trips around the circum­ference of the micro­sphere in a fraction of a second, causing some frequen­cies of light to interfere with themselves to produce brighter light while other frequen­cies cancel themselves out. This process explains the unusual spikes that Chan and Levy observed. When the intensity of light traveling around these beads reaches a certain threshold, the light can stimu­late the emission of more light with the exact same color, and that light, in turn, can stimulate even more light. This ampli­fication of light produces intense light at a very narrow range of wave­lengths in the beads.

Schuck had considered lan­thanide-doped nano­particles as potential candidates for microlasers, and he became convinced of this when Chan shared with him the periodic whis­pering-gallery data. Fernandez-Bravo found that when he exposed the beads to an infrared laser with enough power the beads turned into upcon­verting lasers, with higher frequencies than the original laser. He also found that beads could produce laser light at the lowest powers ever recorded for upcon­verting nano­particle-based lasers. “The low thresholds allow these lasers to operate conti­nuously for hours at much lower powers than previous lasers,” said Fernandez-Bravo.

Other upcon­verting nano­particle lasers operate only inter­mittently; they are only exposed to short, powerful pulses of light because longer exposure would damage them. “Most nano­particle-based lasers heat up very quickly and die within minutes,” Schuck said. “Our lasers are always on, which allows us to adjust their signals for different appli­cations.” In this case, researchers found that their micro­lasers performed stably after five hours of continuous use. “We can take the beads off the shelf months or years later, and they still lase,” Fernandez-Bravo said.

The researchers are also exploring how to carefully tune the output light from the conti­nuously emitting micro­lasers by simply changing the size and compo­sition of the beads. And they have used a robotic system to combine different dopant elements and tune the nano­particles’ perfor­mance. They also noted that there are many potential appli­cations for the micro­lasers, such as in control­ling the activity of neurons or optical micro­chips, sensing chemicals, and detecting environ­mental and tempera­ture changes.

“At first these micro­lasers only worked in air, which was frustra­ting because we wanted to intro­duce them into living systems,” Cohen said. “But we found a simple trick of dipping them in blood serum, which coats the beads with proteins that allow them to lase in water. We’ve now seen that these beads can be trapped along with cells in laser beams and steered with the same lasers we use to excite them.” The latest study, and the new paths of study it has opened up, shows how for­tuitous an unex­pected result can be, he said. “We just happened to have the right nano­particles and coating process to produce these lasers,” Schuck said. (Source: LBNL)

Reference: A. Fernandez-Bravo et al.: Continuous-wave upconverting nanoparticle microlasers, Nat. Nano., online 18 June 2018; DOI: 10.1038/s41565-018-0161-8

Link: The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, USA

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