A tiny donut-shaped laser is the latest marvel of silicon microminiaturization; but instead of manipulating bits, it detects very small particles, which play a big—largely unnoticed—role in our everyday lives. Virus particles make us sick, salt particles trigger cloud formation, etc.
The sensor belongs to the whispering gallery resonator (WGRs) category and works like the whispering gallery in St. Paul's Cathedral, where someone on the one side of the dome can hear a message spoken to the wall by someone on the other side. Unlike the dome, which has audible resonances, the sensor resonates at light frequencies.
The new sensor is also glass, but it's doped with rare earth element (REE) atoms that, driven by an external light source, reach an excited state that causes the ring to start lasing at its own preferred frequency.
Graduate student Lina He said the resonators are made by mixing the REE ions into a solution of tetraethoxysilane, water and hydrochloric acid. The following image shows erbium, excited by the pump light, beginning to fluoresce.
Microlasers intended to sense particles in air were doped with erbium, an REE whose optical properties are well matched with those of air.
To see whether this technique could be used to sense particles in water or blood, the team fabricated sensors that were doped with ytterbium rather than erbium. Ytterbium lases at wavelengths with low absorption by water. The near-term use will be monitoring the dynamic behaviors of particles in response to environmental and chemical changes at single particle resolution.
The next step for this National Science Foundation-funded work is to engineer the surface of these tiny microlasers to detect DNA and individual biological molecules. If the DNA is tagged with engineered nanoparticles, the microlaser sensor can count individual DNA molecules or fragments thereof.
