Optimal Nanoparticle Forces, Torques, and Illumination Fields

A universal property of resonant subwavelength scatterers is that their optical cross-sections are proportional to a square wavelength, λ2, regardless of whether they are plasmonic nanoparticles, two-level quantum systems, or RF antennas. The maximum cross-section is an intrinsic property of the incident field: plane waves, with infinite power, can be decomposed into multipolar orders with finite powers proportional to λ2. In this article, we identify λ2/c and λ3/c as analogous force and torque constants, derived within a more general quadratic scattering-channel framework for upper bounds to optical force and torque for any illumination field. This framework also solves the reverse problem: computing globally optimal “holographic” incident beams, for a fixed collection of scatterers. We analyze structures and incident fields that approach the bounds, which for wavelength-scale bodies show a rich interplay between scattering channels, and we show that spherically symmetric structures are forbidden from reaching the plane-wave force/torque bounds. This framework should enable optimal mechanical control of nanoparticles with light.

Liu, Y., Fan, L., Lee, Y. E., Fang, N. X., Johnson, S. G., & Miller, O. D. (2018). Optimal Nanoparticle Forces, Torques, and Illumination Fields. ACS Photonics, 6(2), 395–402. doi:10.1021/acsphotonics.8b01263

The authors thank Chia -Wei Hsu and Ognjen Ilic for helpful discussions. Y.L. and O.D.M. were supported by the Air Force Office of Scientific Research under award number FA9550-17-1-0093. L.F. was supported by a Shanyuan Overseas scholarship from the Hong Kong Shanyuan Foundation at Nanjing University. S.G.J. was supported in part by the Army Research Office under contract number W911NF-13-D-0001. N.F. was supported by the Air Force Office of Scientific Research (AFOSR) Multidisciplinary Research Program of the University Research Initiative (MURI) and from KAUST-MIT agreement #2950.




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