Abstract
There is a strong demand for portable systems that can detect and characterize individual pathogens and other nanoscale objects without the use of labels, for applications in human health, homeland security, environmental monitoring and diagnostics1,2,3,4,5,6. However, most nanoscale objects of interest have low polarizabilities due to their small size and low refractive index contrast with the surrounding medium. This leads to weak light–matter interactions, and thus makes the label-free detection of single nanoparticles very difficult. Micro- and nano-photonic devices have emerged as highly sensitive platforms for such applications, because the combination of high quality factor Q and small mode volume V leads to significantly enhanced light–matter interactions7,8,9,10,11,12,13,14,15. For example, whispering gallery mode microresonators have been used to detect and characterize single influenza virions10 and polystyrene nanoparticles with a radius of 30 nm (ref. 12) by measuring in the transmission spectrum either the resonance shift10 or mode splitting12 induced by the nanoscale objects. Increasing Q leads to a narrower resonance linewidth, which makes it possible to resolve smaller changes in the transmission spectrum, and thus leads to improved performance. Here, we report a whispering gallery mode microlaser-based real-time and label-free detection method that can detect individual 15-nm-radius polystyrene nanoparticles, 10-nm gold nanoparticles and influenza A virions in air, and 30 nm polystyrene nanoparticles in water. Our approach relies on measuring changes in the beat note that is produced when an ultra-narrow emission line from a whispering gallery mode microlaser is split into two modes by a nanoscale object, and these two modes then interfere. The ultimate detection limit is set by the laser linewidth, which can be made much narrower than the resonance linewidth of any passive resonator16,17. This means that microlaser sensors have the potential to detect objects that are too small to be detected by passive resonator sensors18.
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Acknowledgements
The authors acknowledge support from the National Science Foundation (grant no. 0907467 and 0954941). This work was performed in part at the NRF/NNIN (NSF award no. ECS-0335765) of Washington University in St. Louis. We thank D. Chen for providing instrumentation for nanoparticle deposition, and F. Monifi and B. Peng for stimulating discussions.
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L.H., S.K.O., J.Z. and L.Y. designed the experimental concept. L.H. and J.Z. performed the experiments in air. L.H. and W.K. performed the experiments in water. L.H. and S.K.O. contributed to the theoretical work. L.Y. supervised the project. All authors contributed to the discussion of the results and the preparation of the manuscript.
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He, L., Özdemir, Ş., Zhu, J. et al. Detecting single viruses and nanoparticles using whispering gallery microlasers. Nature Nanotech 6, 428–432 (2011). https://doi.org/10.1038/nnano.2011.99
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DOI: https://doi.org/10.1038/nnano.2011.99
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