Researchers from the Molecular Foundry at Lawrence Berkeley National Laboratory have found a way to convert nanoparticle-coated microscopic plastic beads into lasers smaller than red blood cells. These microlasers convert infrared light into light at higher frequencies, and are among the smallest, continuously emitting lasers of their kind ever reported.
“Reducing the size of lasers to microscale dimensions enables new technologies that are specifically tailored for operation in confined spaces ranging from ultra-high-speed microprocessors to live brain tissue,” said Molecular Foundry researchers Bruce Cohen, Emory Chan, James Schuck and their colleagues.
“However, reduced cavity sizes increase optical losses and require greater input powers to reach lasing thresholds.”
In their research, the scientists found that when an infrared laser excites thulium-doped nanoparticles coated on the surface of the beads, the light emitted by the nanoparticles bounces around the inner surface of the bead just like whispers bouncing along the walls of a whispering gallery.
Light can make thousands of trips around the circumference of the microsphere in a fraction of a second, causing some frequencies of light to interact with themselves to produce brighter light while other frequencies cancel themselves out.
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 positive feedback loop — the basis for all lasers — produces intense light at a very narrow range of wavelengths in the beads.
When the researchers exposed the beads to an infrared laser with enough power, the beads turned into upconverting lasers, with higher frequencies than the original laser.
The beads also produce laser light at the lowest powers ever recorded for upconverting nanoparticle-based lasers.
Other upconverting nanoparticle lasers operate only intermittently; they are only exposed to short, powerful pulses of light because longer exposure would damage them.
In this case, the Molecular Foundry scientists found that their microlasers performed stably after five hours of continuous use, both in air and in biological media.
“The ability to produce continuous-wave lasing in microcavities immersed in blood serum highlights practical applications of these microscale lasers for sensing and illumination in complex biological environments,” they said.
The study was published in the journal Nature Nanotechnology.
A. Fernandez-Bravo et al. 2018. Continuous-wave upconverting nanoparticle microlasers. Nature Nanotech 13, 572-577; doi: 10.1038/s41565-018-0161-8
This article is based on text provided by Lawrence Berkeley National Laboratory.
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