Improved theory for the polarization-dependent transverse shift of a paraxial light beam in free space

Spatial distribution of the longitudinal field component of a circularly polarized optical beam depends on the polarization handedness, which causes the lateral shift of the beam "center of gravity" when its polarization toggles. We present the generalized theory of this effect, which demonstrates its relation with the angular irradiance moments of the beam. The theory is applicable to arbitrary paraxial beams and shows that the lateral shift is the same for all cross sections of the beam.Comment: 9 pages, 2 figures. The paper is submitted to Ukr. J. Phys. Opt. It contains the refined and generalized theory of the effect first observed and explained by B. Zel'dovich et al. in 1994: polarizxation-dependent transverse shift of the focal spot obtained by focusing an asymmetric light bea

Subwavelength particles in an inhomogeneous light field: Optical forces associated with the spin and orbital energy flows

We analyze the ponderomotive action experienced by a small spherical particle immersed in an optical field, in relation to the internal energy flows (optical currents) and their spin and orbital constituents. The problem is studied analytically, based on the dipole model, and numerically. Three sources of the field mechanical action - energy density gradient and the orbital and spin parts of the energy flow - differ by the ponderomotive mechanism, and their physical nature manifests itself in the optical force dependence on the particle radius a. If a is much less than the radiation wavelength, the optical force behaves as a^n and integer n can be used to classify the sources of the mechanical action. This classification correlates with the multipole representation of the field-particle interaction: The gradient force and the orbital-momentum force appear due to the electric or magnetic dipole moments per se, the spin-momentum force emerges due to interaction between the electric and magnetic dipoles or between the dipole and quadruple moments (if the particle is polarisable electrically but not magnetically or vice versa). In principle, the spin and orbital currents can be measured separately by the probe particle motion, employing the special choice of particles with necessary magnetic and/or electric properties.Comment: 16 pages, 2 figures. The paper represents materials of the talk at conference "Singular Optics'12" in Sevastopol, 19 September 2012, and is designed to "Journal of Optics

Arrays of optical vortices formed by "fork" holograms

Singular light beams with optical vortices (OV) are often generated by means of thin binary gratings with groove bifurcation ("fork holograms") that produce a set of diffracted beams with different OV charges. Usually, only single separate beams are used and investigated; here we consider the whole set of diffracted OV beams that, at certain conditions, are involved in efficient mutual interference to form a characteristic pattern where the ring-like structure of separate OV beams is replaced by series of bright and dark lines between adjacent diffraction orders. This pattern, well developed for high diffraction orders, reflects the main spatial properties of the diffracted beams as well as of the fork grating used for their generation. In particular, it confirms the theoretical model for the diffracted beams (Kummer beam model) and enables to determine the sign and the absolute value of the phase singularity embedded in the hologram.Comment: 9 pages, 8 figure

Effects of misalignments in the optical vortex transformation performed by holograms with embedded phase singularity

Spatial characteristics of diffracted beams produced by a "fork" hologram from an incident circular Laguerre-Gaussian beam whose axis differ from the hologram optical axis are studied theoretically. General analytical representations for the complex amplitude distribution of a diffracted beam are derived in terms of superposition of Kummer beams or hypergeometric-Gaussian beams. The diffracted beam structure is determined by combination of the "proper" topological charge m of the incident vortex beam and the topological charge l of the singularity "imparted" by the hologram. Evolution of the diffracted beam structure is studied in detail for several combinations of m and l and for various incident beam displacements with respect to the optical axis of the hologram. Variations of the intensity and phase distribution due to the incident beam misalignment are investigated and possible applications for the purposeful optical-vortex beam generation and optical measurements are discussed.Comment: 23 pages, 8 figure