Spin momentum-dependent orbital motion

Abstract

We present a theoretic analysis on (azimuthal) spin momentum-dependent orbital motion experienced by particles in a circularly-polarized annular focused field. Unlike vortex phase-relevant (azimuthal) orbital momentum flow whose direction is specified by the sign of topological charge, the direction of (azimuthal) spin momentum flow is determined by the product of the field's polarization ellipticity and radial derivative of field intensity. For an annular focused field with a definite polarization ellipticity, the intensity's radial derivative has opposite signs on two sides of the central ring (intensity maximum), causing the spin momentum flow to reverse its direction when crossing the central ring. When placed in such a spin momentum flow, a probe particle is expected to response to this flow configuration by changing the direction of orbital motion as it traversing from one side to the other. The reversal of the particle's orbital motion is a clear sign that spin momentum flow can affect particles' orbital motion alone even without orbital momentum flow. More interestingly, for dielectric particles the spin momentum-dependent orbital motion tends to be 'negative', i.e., in the opposite direction of the spin momentum flow. This arises mainly because of spin-orbit interaction during the scattering process. For the purpose of experimental observation, we suggest the introduction of an auxiliary radially-polarized illumination to adjust the particle's radial equilibrium position, for the radial gradient force of the circularly-polarized annular focused field tends to constrain the particle at the ring of intensity maximum.

Theory of optical tweezing of dielectric microspheres in chiral host media and its applications

“…In addition to linear momentum, a circularly-polarised (CP) light beam also carries spin angular momentum (SAM) that can be transferred to trapped particles [22][23][24][25] . The resulting optical torque leads to a rotation of the equilibrium position when a Stokes drag force is simultaneously applied 26 .…”

“…The upper bound for the integration in (25) and (26) represents the angular semi-aperture in the glass medium:…”

Reactive helicity and reactive power in nanoscale optics: Evanescent waves. Kerker conditions. Optical theorems and reactive dichroism

“…Reactive quantities of electromagnetic fields, such as reactive and stored energy or the imaginary Poynting vector (IPV), are associated to physical entities that do not propagate in the environment, like evanescent waves in their varied forms as surface waves [1][2][3], standing waves [3], along with near-fields of RF-antennas [3-9, 11, 12]. In recent years, advances in photonics and nano-optics led to developing concepts such as chirality, helicity [13][14][15][16][17][18][19], magnetoelectric effects associated with the imaginary Poynting vector (IPV) [20], and its consequent transverse spin momentum and magnetoelectric energy density [21][22][23], as well as related to the azimuthal imaginary Poynting momentum [24] and the Kerker-type non-conservative intensity gradient force [25].…”

Theory of optical tweezing of dielectric microspheres in chiral host media and its applications

“…21 In addition to linear momentum, a circularly-polarised (CP) light beam also carries spin angular momentum (SAM) that can be transferred to trapped particles. [22][23][24][25] The resulting optical torque leads to a rotation of the equilibrium position when a Stokes drag force is simultaneously applied. 26 The optical force on chiral particles has been employed for chirality sorting and recognition in optical traps.…”