Novel Method for Converting Nanoparticle-Coated Microscopic Beads into Microlasers

Scientists have devised a technique for converting nanoparticle-coated microscopic beads into lasers with a size less than that of red blood cells. These microlasers, with the ability to convert infrared light into light at higher frequencies, are one of the smallest continuously emitting lasers of their type ever developed. They have the potential to constantly and stably emit light for many hours at a stretch, even while being submerged in biological fluids such as blood serum.

At left, a tiny bead struck by a laser (at the yellowish spot shown at the top of the image) produces optical modes that circulate around the interior of the bead (pinkish ring). At right, a simulation of how the optical field inside a 5-micron (5 millionths of a meter) bead is distributed. (Image credit: Angel Fernandez-Bravo/Berkeley Lab, Kaiyuan Yao)

The novel technique, developed by an international team of researchers at the U.S. Department of Energy’s Lawrence Berkeley Laboratory (Berkeley Lab), paves the way for imaging or controlling biological activity using infrared light, and for developing light-based computer chips. The outcomes of the research have been reported in a paper published online in the Nature Nanotechnology journal on June 18, 2018.

The distinctive properties of these lasers, with dimensions of just 5 μm (one-millionth of 1 m) across, were accidentally discovered when scientists were analyzing the potential for using polymer (plastic) beads, formed of a translucent substance called colloid, in brain imaging.

Angel Fernandez-Bravo, the lead author of the study and a postdoctoral researcher at Berkeley Lab’s Molecular Foundry, mixed the beads with sodium yttrium fluoride nanoparticles “doped” (or embedded) with thulium, an element that belongs to a class of metals called lanthanides. The Molecular Foundry is a nanoscience research center accessible to scientists across the world.

In 2016, Emory Chan, a Staff Scientist at the Molecular Foundry, used computational models to hypothesize that when thulium-doped nanoparticles are exposed to infrared laser light at a particular frequency, they have the ability to emit light at a higher frequency when compared to the infrared light in a counterintuitive process called “upconversion.”

Moreover, back then, Elizabeth Levy, who was at the time a participant in the Lab’s Summer Undergraduate Laboratory Internship (SULI) program, observed that when coated with these “upconverting nanoparticles,” the beads emitted surprisingly bright light at specified wavelengths, or colors.

These spikes were clearly periodic and clearly reproducible,” stated Emory Chan, who co-headed the research together with Foundry Staff Scientists Jim Schuck (now at Columbia University) and Bruce Cohen.

The periodic spikes observed by Chan and Levy are a light-based equivalent to the familiar “whispering gallery” acoustics that makes sound waves to bounce along the walls of a circular room such that even a whisper is heard on the opposite side of the room. In the late 1800s, this whispering-gallery effect was noticed in the dome of St. Paul’s Cathedral in London, for instance.

In the recent study, Fernandez-Bravo and Schuck discovered that excitation of the thulium-doped nanoparticles with an infrared laser along the outer surface of the beads makes the light emitted by the nanoparticles to bounce around the inner surface of the bead quite similar to whispers bouncing along the walls of the cathedral.

Within a fraction of a second, light has the ability to travel a thousand times around the circumference of the microsphere, resulting in the interaction (or “interference”) of specific frequencies of light with themselves to produce brighter light while other frequencies are canceled out by themselves. This process elucidates the unusual spikes observed by Chan and Levy.

When the intensity of light moving around these beads attains a specific threshold, the light can trigger the emission of more light with precisely the same color, and that light, in turn, can trigger even more light emission. Such an amplification of light, which is fundamental for all lasers, generates intense light at very narrow wavelength ranges in the beads.

Schuck had suggested the use of lanthanide-doped nanoparticles as prospective materials for microlasers, and he became confident about this when the periodic whispering-gallery data was shared by Chan.

Fernandez-Bravo discovered that when the beads were exposed to an infrared laser with adequate power, the beads transformed into upconverting lasers, with higher frequencies when compared to the original laser. He also discovered that the beads had the ability to produce laser light at the lowest powers ever observed for upconverting nanoparticle-based lasers.

The low thresholds allow these lasers to operate continuously for hours at much lower powers than previous lasers,” stated Fernandez-Bravo.

Other upconverting nanoparticle lasers function only discontinuously; they are exposed only to short, powerful light pulses since longer exposure could damage them.

Most nanoparticle-based lasers heat up very quickly and die within minutes,” stated Schuck. “Our lasers are always on, which allows us to adjust their signals for different applications.”

In this instance, the scientists discovered that their microlasers functioned in a stable manner after being used continuously for five hours. “We can take the beads off the shelf months or years later, and they still lase,” stated Fernandez-Bravo.

Scientists are also investigating to find ways to cautiously tune the output light from the constantly emitting microlasers by just varying the size and composition of the beads. And they have used a robotic system at the Molecular Foundry known as WANDA (Workstation for Automated Nanomaterial Discovery and Analysis) to combine different dopant elements and tune the nanoparticles’ performance.

The researchers also noted that there are many prospective applications for the microlasers, for instance, sensing chemicals, tweaking the activity of optical microchips or neurons, and detecting variations in the environment and temperature.

At first these microlasers only worked in air, which was frustrating because we wanted to introduce them into living systems. 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.

Bruce Cohen

The recent study, and the new doors of research it has opened, demonstrates the favorable nature of an unexpected outcome, he stated. “We just happened to have the right nanoparticles and coating process to produce these lasers,” stated Schuck.

Scientists from UC Berkeley, the National Laboratory of Astana in Kazakhstan, the Polytechnic University of Milan, and Columbia University in New York were also involved in this project. This study was supported by the DOE Office of Science, and by the Ministry of Education and Science of the Republic of Kazakhstan.

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