Tiny Lasers from Nanoparticles and Plastic Beads

Researchers found a way to create lasers smaller than red blood cells. These microlasers convert infrared light into light at higher frequencies. Made from nanoparticles, these are among the smallest, continuously emitting lasers of their kind ever reported. The lasers constantly and stably emit light for hours at a time. They emit the light even when submerged in blood serum or other biological fluids.

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 (5 millionths of a meter) bead is distributed. Image courtesy of Berkeley Lab.

The tiny lasers are stable enough to operate continuously for days. This breakthrough will enable technologies for imaging or controlling biological activity with infrared light. In addition, these tiny lasers open doors to make light-based computer chips.

An international team of researchers led by scientists at the Molecular Foundry has found a way to convert nanoparticle-coated microscopic plastic 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 constantly and stably emit light for hours at a time, even when submerged in biological fluids such as blood serum.The innovation opens up the possibilities for imaging or controlling biological activity with infrared light and for the fabrication of light-based computer chips.

In this study, the researchers 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 (or “interfere”) 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 team 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, researchers found that their microlasers performed stably after five hours of continuous use, both in air and in biological media.

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