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.
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.