Nanoparticle-covered lasing microbeads upconvert IR light
The research stems from original theoretical findings at Berkeley Lab’s Molecular Foundry, using computational models that predicted thulium-doped nanoparticles exposed to infrared laser light at a specific frequency would emit light at a higher frequency, in effect a light “upconversion.”
This upconversion had already been proven experimentally and have now been fully documented as being a form of whispering mode laser in the paper “Continuous-wave upconverting nanoparticle microlasers” published in Nature Nanotechnology.
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 bead is distributed. Photo Credit: Angel
Fernandez-Bravo/Lawrence Berkeley National Laboratory.
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 circular walls. As the light makes thousands of trips around the circumference of the microsphere in a fraction of a second, it causes some frequencies of light to interfere with themselves, producing spikes of bright light at constructive interferences and dark spots as destructive interferences occur. Once a certain threshold is reached the light can stimulate the emission of more light in a cascading amplifying effect.
By harnessing the energy-looping excitation mechanism they had found in Tm3+-doped upconverting nanoparticles together with the right size of microbeads, the researchers achieved continuous-wave upconverted lasing with very low excitation levels.
Pumping the specially coated microparticles with infrared light, they induced stable lasing for more than 5h at blue and near-infrared wavelengths, simultaneously, the paper reports. This is in stark comparison with other reported upconverting nanoparticle lasers which only operate intermittently.
“Most nanoparticle-based lasers heat up very quickly and die within minutes,” explained Jim Schuck, one of the co-authors of the paper. “Our lasers are always on, which allows us to adjust their signals for different applications.”
Now the researchers are exploring how they could tune the output light from the continuously emitting microlasers by changing the size and composition of the beads. They are using 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 authors anticipate these microscale lasers could find applications for sensing and illumination in complex biological environments.
Berkeley Lab – www.lbl.gov
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