Circular motion of particles suspended in a Gaussian beam with circular polarization validates the spin part of the internal energy flow

O. V. Angelsky,A. V. Tyurin,A. Ya. Bekshaev,A. P. Maksimyak,C. Yu. Zenkova,S. G. Hanson,I. I. Mokhun,P. P. Maksimyak

doi:10.1364/oe.20.011351

Abstract

Non-spherical dielectric microparticles were suspended in a water-filled cell and exposed to a coherent Gaussian light beam with controlled state of polarization. When the beam polarization is linear, the particles were trapped at certain off-axial position within the beam cross section. After switching to the right (left) circular polarization, the particles performed spinning motion in agreement with the angular momentum imparted by the field, but they were involved in an orbital rotation around the beam axis as well, which in previous works [Y. Zhao et al, Phys. Rev. Lett. 99, 073901 (2007)] was treated as evidence for the spin-to orbital angular momentum conversion. Since in our realization the moderate focusing of the beam excluded the possibility for such a conversion, we consider the observed particle behavior as a demonstration of the macroscopic "spin energy flow" predicted by the theory of inhomogeneously polarized paraxial beams [A. Bekshaev et al, J. Opt. 13, 053001 (2011)].

Highlights

During the past few years, internal energy flows have become a rather appealing and promising topic of physical optics [1,2,3,4,5,6,7]
Non-spherical dielectric microparticles were suspended in a water-filled cell and exposed to a coherent Gaussian light beam with controlled state of polarization
When the beam polarization is linear, the particles were trapped at certain off-axial position within the beam cross section

Summary

Introduction

During the past few years, internal energy flows (optical currents) have become a rather appealing and promising topic of physical optics [1,2,3,4,5,6,7]. In Eq (1), the connection between the Poynting vector S and the field momentum density p is explicitly stated, which permits us, in what follows, to use both quantities S and p on equal terms This correspondence suggests an alternative way for the energy flow evaluation: since the electromagnetic momentum can be imparted to particles and trigger their motion, the optical currents can be measured by the mechanical action exerted on the probe particles deliberately localized (trapped) within the optical field [18]. Even in situations where all nonPoynting sources are isolated (e.g., due to special geometry of the field and the measuring equipment [19]), it is rather difficult to establish an exact numerical correspondence between the probe particle motion and the local value of the field momentum: at best, the particles’ motion provides only a qualitative picture of the internal energy flows This approach appears to be rather suitable in cases where this qualitative picture is sufficient. We present a more direct approach in which, due to the improved radiation source, a moderately focused beam itself contains sufficient spin flow to perform the orbital or (locally) translational motion of the probe particles

Measurements

Conclusions