Whispering gallery mode single nanoparticle detection and sizing: The validity of the dipole approximation

Matthew R. Foreman, David Keng, Eshan Treasurer, Jehovani R. Lopez, Stephen Arnold

Research output: Contribution to journalArticle


Interactions between whispering gallery modes (WGMs) and small nanoparticles are commonly modeled by treating the particle as a point dipole scatterer. This approach is assumed to be accurate as long as the nanoparticle radius, a, is small compared to the WGM wavelength λ. In this Letter, however, we show that the large field gradients associated with the evanescent decay of a WGM causes the dipole theory to significantly underestimate the interaction strength and, hence, the induced WGM resonance shift, even for particles as small as a ∼ λ/10. To mitigate this issue, we employ a renormalized Born approximation to more accurately determine nanoparticle-induced resonance shifts and, hence, enable improved particle sizing. The domain of validity of this approximation is investigated, and supporting experimental results are presented.

Original languageEnglish (US)
Pages (from-to)963-966
Number of pages4
JournalOptics Letters
Issue number5
StatePublished - Mar 1 2017


ASJC Scopus subject areas

  • Atomic and Molecular Physics, and Optics

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