A Raman microlaser sensor can detect and count individual particles — including viruses — as small as 10 nm. Researchers led by Dr. Lan Yang of Washington University achieved Raman lasing in a silica microcavity or "whispering gallery". Within the microcavity, two modes of the beam emitted by the Raman laser circulate in opposite directions. When a particle lands on the ring and scatters energy between these modes, the single Raman lasing line splits into two lasing lines with different frequencies, which can be analyzed to confirm detection. When a nanoparticle lands on a resonator, the Raman laser circulating inside undergoes mode splitting, leading to two new lasing modes in different colors. Monitoring the frequency difference enables detecting and measuring of nanoparticles with single-particle resolution. Courtesy of J. Zhu, B. Peng, S.K. Ozdemir, L. Yang/Washington University in St. Louis. “Our new sensor differs from the earlier whispering gallery sensors in that it relies on Raman gain, which is inherent in silica, thereby eliminating the need for doping the microcavity with gain media to boost detection capability,” said Dr. Sahin Kaya Ozdemir, first author of the study. “This new sensor retains the biocompatibility of silica and could find widespread use for sensing in biological media.” The technology could benefit the electronics, acoustics, biomedical, plasmonics, security and metamaterials fields, the researchers said. The work also shows the possibility of using intrinsic gain mechanisms — such as Raman and parametric gain — instead of optical dyes, rare-earth ions or quantum dots to compensate for loss in optical and plasmonic systems where dissipation hinders progress and limits applications. The research was funded by the National Science Foundation and the U.S. Army Research Office. The research was published in the Proceedings of the National Academy of Sciences (doi: 10.1073/pnas.1408283111). For more information, visit www.wustl.edu.