St. Louis, MO--An erbium-doped glass-on-silicon microring-resonator laser with a high Q factor developed at Washington University in St. Louis can directly detect 15 nm dielectric (polystyrene) particles, 10 nm metal (gold) particles, and single influenza virus particles in air, and 30 nm dielectric particles in water.1
The whispering-gallery ring laser has two counterrotating modes at the same frequency. When a nanoparticle lands on the laser, the modes take different frequencies, splitting the emission line in two. The resulting beat frequency is what is sensed. Because the resonator is active (lasing) rather than passive, its linewidth can be very narrow, allowing much-more-sensitive detection than for passive microring resonators.
Because the resonator field traps particles on the resonator, once landed, they rarely drop off. But the team found they were able to count many particles before the losses induced by the particles made the laser linewidths so broad they couldn't detect changes in frequency splitting due to the latest arrival.
Nanoparticle counting
As particles enter the mode volume of the laser one by one, the scientists see a discrete upward or downward jump in the beat frequency. Each discrete jump signals the binding of a particle on the ring; the number of the jumps reflects the number of particles. They were able to detect and count as many as 816 gold nanoparticles using the same laser mode.
"When the line broadening is comparable to the change in splitting, then you're done," says Lan Yang, one of the researchers. "However, the whole resonator is fabricated on the chip, so you could just move on to the next resonator if necessary."
The sensors are mass produced by a sol-gel method on a silicon wafer; it is easy to switch the gain medium, notes Lina He, another researcher, graduate student and first author of the paper. "The resonators are made by mixing the rare-earth ions of choice into a solution of tetraethoxysilane, water, and hydrochloric acid," she says. "The solution is heated until it becomes viscous and then spin-coated on a silicon wafer and annealed to remove solvents and complete the transition to amorphous glass. The thin film of glass is then etched to create silica disks supported underneath by silicon pillars. As a final step, the rough silica disks are reflowed into smooth toroids by laser annealing."
The next step will be to engineer the surface of the microlasers to detect DNA and individual biological molecules. If DNA is tagged with engineered nanoparticles, the sensor could count individual DNA molecules or fragments of molecules.
REFERENCE:
1. Lina He et al., Nature Nanotechnology, published online 26 June 2011; doi:10.1038/nnano.2011.99.
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