C. Yu. Zenkova

Self-action of continuous laser radiation and Pearcey diffraction in a water suspension with light-absorbing particles

Water suspension of light-absorbing nano-sized particles is an example of a medium in which non-linear effects are present at moderate light intensities favorable for optical treatment of organic and biological objects. We study experimentally the phenomena emerging in a thin layer of such a medium under the action of inhomogeneous light field formed due to the Pearcey diffraction pattern near a microlens focus. In this high-gradient field, the light energy absorbed by the particles induces inhomogeneous distribution of the medium refraction index, which results in observable self-diffraction of the incident light, here being strongly sensitive to the medium position with respect to the focus. This technique, based on the complex spatial structure of both the incident and the diffracted fields, can be employed for the detection and measurement of weak non-linearities.

Scattering of inhomogeneous circularly polarized optical field and mechanical manifestation of the internal energy flows

Based on the Mie theory and on the incident beam model via superposition of two plane waves, we analyze numerically the momentum flux of the field scattered by a spherical microparticle placed within the spatially inhomogeneous circularly polarized paraxial light beam. The asymmetry between the forward- and backward-scattered momentum fluxes in the Rayleigh scattering regime appears due to the spin part of the internal energy flow in the incident beam. The transverse ponderomotive forces exerted on dielectric and conducting particles of different sizes are calculated and special features of the mechanical actions produced by the spin and orbital parts of the internal energy flow are recognized. In particular, the transverse orbital flow exerts the transverse force that grows as a^3 for conducting and as a^6 for dielectric subwavelength particle with radius a, in compliance with the dipole mechanism of the field-particle interaction; the force associated with the spin flow behaves as a^8 in both cases, which testifies for the non-dipole mechanism. The results can be used for experimental identification and separate investigation of the spin and orbital parts of the internal energy flow in light fields.

Mechanical Action of Inhomogeneously Polarized Optical Fields and Detection of the Internal Energy Flows

We analyze numerically correspondence between the mechanical action, experienced by a spherical microparticle, and the internal energy flows in the light field incident on the particle. The inhomogeneous incident field is modelled by superposition of two plane waves; the mechanical action is calculated via the Mie theory for dielectric and conducting particles of different sizes and optical properties. It is shown that both spin and orbital components of the field momentum can produce the mechanical action whose value and sign depend on many additional details of the field-particle interaction. Besides, forces that are not associated with any sort of the energy flow (e.g., the gradient force owing to the inhomogeneous intensity and the polarization-dependent dipole force emerging due to inhomogeneous polarization) can strongly modify the observed mechanical action. The polarization-dependent mechanical action on particles can be treated as a form of the spin-orbit interaction of light.

The electromagnetic degree of coherence in the near field

New approach is proposed for estimating the degree of coherence of optical waves. The possibility of transformation of spatial polarization distribution in measured intensity distribution for estimating the degree of correlation of superposing vector waves linearly polarized at the incidence plane is shown.

Self-action of continuous laser radiation in a water suspension with light-absorbing particles

Water suspension of light-absorbing nano-sized particles is an example of a medium in which non-linear effects are present at moderate light intensities favorable for optical treatment of organic and biological objects. We study experimentally the phenomena emerging in a thin layer of such a medium under the action of inhomogeneous light field formed due to the Pearcey diffraction pattern near a microlens focus. In this high-gradient field, the light energy absorbed by the particles induces inhomogeneous distribution of the medium refraction index, which results in observable self-diffraction of the incident light, here being strongly sensitive to the medium position with respect to the focus.

The use of the Rayleigh nanoparticles to diagnose optical currents and optical fields

Theoretical and experimental approaches to diagnosing of internal spin and orbital optical flows and corresponding optical forces caused by these flows are offered. These approaches are based on the investigation of the motion of tested particles in the formed optical field. The possibility of using kinematic values defining the motion dynamics of particles of the Rayleigh light scattering mechanism for making a quantitative assessment of the degree of coherence of mutually orthogonal linearly polarized in the incidence plane waves is demonstrated.

Circular motion of particles by the help of the spin part of the internal energy flow

Non-spherical dielectric microparticles were suspended in the water-filled cell and exposed to the coherent Gaussian light beam with switchable 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 also they were involved in the orbital rotation around the beam axis, which in previous works [Y. Zhao et al, Phys. Rev. Lett. 99, 073901 (2007)] was treated as an evidence for the spin-to orbital angular momentum conversion. Since in our situation the moderate focusing of the beam excluded possibility of such a conversion, we treat the observed particle behaviour 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)].

A new method for estimating the degree of coherence of mutually orthogonal linearly polarized interacting waves

An additional possibility for estimating the degree of coherence of interacting fields when classic methods of diagnostics cannot be applied is offered.

The investigation of the peculiarities of the nano- and micro-object motion in the inhomogenous optical field

The paper proposes for consideration an additional possibility to evaluate the degree of coherence of superposing mutually orthogonal linearly-polarized in the incidence plane waves. When analyzing the behavior of various-type particles (Mie and Rayleigh) in the inhomogeneously polarized optical field and in the inhomogeneous field of the averaged values of the Poynting vector, we obtain an additional tool for defining the degree of coherence of interacting fields. The spatial modulation of polarization in the observation plane shapes the spatial modulation of the energy density volume, which changes the velocity of the particle motion according to the coherence characteristics of the superposing fields and to the resulting optical force that causes the motion and trapping of the tested particles.

About the prospects of determining the degree of coherence

The situation when information on the degree of coherence of electromagnetic optical waves is contained both in intensity modulation and in spatial polarization modulation of the resulting distribution of superposing waves is considered. It is pointed out that such experimental situation is often realized in near-field optics. The possibility of experimental estimation of the degree of mutual coherence of waves polarized at the incidence plane is shown in this paper.