Artist’s rendering of an array of WGM microresonators with Raman microlasers for single nanoparticle detection. (Credit: J. Zhu, B. Peng, S.K. Ozdemir, L. Yang)
The growing awareness of the health effects of nanoparticles has stoked a demand for technologies to detect these tiny molecules. But one promising optical approach, involving the use of on-chip whispering gallery mode (WGM) microresonators, has been stymied by the cost and potential biocompatibility problems of using rare-earth dopants to boost optical gain. Now, scientists from the United States and China have developed a dopant-free technology, using Raman microlasers, that can detect and count single particles as small as 10 nm across (Proc. Natl. Acad. Sci. USA, doi: 10.1073/pnas.1408283111).
WGM microresonators have generated interest as potential nanoparticle sensors, as their high quality factor and small mode volume make them extremely sensitive to local changes and perturbations. To investigate a dopant-free, biocompatible approach to the technology, a research team headed by Lan Yang at the Washington University of St. Louis, along with colleagues at Tsinghua University in China, integrated tiny Raman microlasers in silica microcavities. The microlasers, rather than relying on rare earths for optical gain and loss compensation, use the intrinsic Raman gain in the silica itself.
Packed into a tight array on a silicon chip, the individual microresonators are activated using a pump laser, which spurs a single Raman lasing mode inside the resonator. When a nanoparticle lands on one of the microresonators, it causes the single Raman mode to split into two new lasing modes at different frequencies. The nanoparticle’s presence is read from the color change resulting from the mode splitting.
Because the system retains the known biocompatibility of silica, without rare-earth additions, the researchers believe it could find widespread use in sensing in biological media. They also believe that the more general concept of using Raman gain for loss compensation in silica can be extended to other materials and systems, opening up possible applications in plasmonics, security and metamaterials.