supports surface waves. 3, 4 By applying
a rigorous treatment of Maxwell’s equations, we established the balance of
power and momentum in the system.
Because the forward and backward
scattering by the cylinder are almost
equal, the momentum flow carried by
the surface wave is imparted to the
object, giving it a propulsion force (Fx )
which, under resonant conditions,
reaches the total absorption limit. We
show that the transverse optical force
can be either attractive or repulsive
depending on the particle-to-surface
distance in the vicinity of WGM resonances. These results demonstrate that
the spheres with the desired positions
of high-quality (Q > 104) WGM peaks
can be sorted by using a tunable laser
In 1977, Arthur Ashkin and Joseph M. Dziedzic demonstrated that optical
forces possess narrow spectral peaks
determined by the internal resonances
in microdroplets. 1 These resonant
optical forces open up a unique way of
high-volume sorting of nearly identical
“photonic atoms,” which may be used
as building blocks for coupled-cavity
structures and devices. 2 However, to
date, only relatively subtle resonant
effects have been observed.
Recently, we proved the existence of
strong optical force peaks by studying
the propulsion of dielectric microspheres in evanescent fields created by
tapered microfibers. 3 We used spheres
with a standard diameter deviation of
approximately 1 percent and showed
that the small fraction of microspheres
in resonance with a tunable laser beam
traveled through water up to 10 times
faster than those of other sizes, based
on their enhanced ability to trap and
subsequently scatter light.
We observed ultra-high propelling
velocities vmax approximately 0.45
mm/s for some of the 20 µm polystyrene microspheres for guided powers
limited at P0 = 43 m W. That exceeds the
previous measurements with various
evanescent couplers by more than an
order of magnitude. Variations in size
explain the broad distributions of the
velocities observed for spheres with 15
and 20 µm mean diameters, leading to
random detuning of whispering gallery
mode (WGM) peaks in individual
spheres relative to the laser frequency.
We studied the mechanism of ultra-high propulsion forces for a cylindrical
resonator located near to a surface that
(a) Microfluidic fiber-integrated platform with optical tweezers. (b) Maximal propelling
velocity as a function of mean sphere diameters. Dashed blue area represents multiple
measurements on spheres with 1 percent size deviations. (Inset) Snapshots of resonant
propelling of 20 µm spheres. (c) Theoretical model. (d) Calculated propulsion force.
Vasily N. Astratov
Yangcheng Li and
Oleksiy V. Svitelskiy
University of North
Carolina at Charlotte, U.S.A.
Alexey V. Maslov and
Michael I. Bakunov
University of Nizhny Novgorod,
and Edik Rafailov
University of Dundee,
Scotland, United Kingdom
1. A. Ashkin and J.M. Dziedzic.
Phys. Rev. Lett. 38, 1351
2. V.N. Astratov. Photonic
and Applications, Ch. 17,
Springer, New York (2010).
3. Y. Li. et al. Light: Sci.
& Appl. 2, e64 (2013).
4. A.V. Maslov et al. Phys.
Rev. A 87, 053848 (2013).
Microspherical Photonics: Ultra-High
Resonant Propulsion Forces
Diameter, D (mm)
y , vmax