ASTANA – The National Laboratory of Astana in the Muslim country of Kazakhstan and the Kazakh Ministry of Education and Science partially funded a successful project that discovered the right mix of nanocrystals and light to activate unique laser properties in extremely small spheres.
The crew have found a way to convert nanoparticle-coated microscopic beads into lasers smaller than red blood cells at a diameter of just five micrometres. Their findings are detailed in a report published online June 18 in Nature Nanotechnology.
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 opens up the possibility for imaging or controlling biological activity with infrared light, and for the fabrication of light-based computer chips.
The Kazakh scientist and his colleagues from Berkeley Lab at the University of California, the Polytechnic University of Milan in Italy, and Columbia University in New York were also supported by the Office of Science at USA’s Department of Energy.
Luck & Hard Work
The unique properties of these lasers were discovered by accident as researchers were studying the potential for the polymer (plastic) beads, composed of a translucent substance known as a colloid, to be used in brain imaging.
The study’s author, Angel Fernandez-Bravo, a postdoctoral researcher at Berkeley Lab’s Molecular Foundry, mixed the beads with sodium yttrium fluoride nanoparticles doped with thulium.
In 2016, Bravo’s colleague, Emory Chan, had 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 process known as “upconversion.”
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, or colors.
The observed periodic spikes are a light-based analog to so-called “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.
This whispering-gallery effect was observed in the dome of St. Paul’s Cathedral in London in the late 1800s, for example.
In the latest study, Bravo and his colleague, Jim 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 just like whispers bouncing along the walls of the cathedral.
Basis of Laser
In fact, light can make thousands of trips around the circumference of the microsphere in a fraction of a second.
This causes 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 amplification of light, the basis for all lasers, 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.
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.
Moreover, he found that beads could produce laser light at the lowest powers ever recorded for upconverting nanoparticle-based lasers.
The team 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.
To achieve this researchers used the WANDA robotic system or ‘Workstation for Automated Nanomaterial Discovery and Analysis’ to combine different dopant elements and tune the nanoparticles’ performance.
The researchers 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.