Low-temperature laser-stimulated controllable generation of micro-bubbles in a water suspension of absorptive colloid particles.

A method is described for the generation of micrometer-sized vapor-gas bubbles in a water suspension containing absorptive pigment nanoparticles. The diluted suspension (mean interparticle distance 20 µm) absorbs the continuous laser radiation (wavelength 808 nm), and each particle in the best illuminated volume (~10 × 10 × 200 µm3) serves as a bubble-nucleation center. The suspension heating is inessential (several degrees above the room temperature) and the bubbles are formed mainly of the air gases dissolved in water. The bubbles can stably exist within or near the illuminated area where their location is governed by the competition between thermal and optical forces and can be controlled via the laser beam parameters. The method enables controllable creation, support, prescribed transportation, and destruction of the bubbles. This can be useful in applications aimed at precise sorting, transportation, and delivery of species in nano- and micro-engineering as well as for biomedical studies.

Controllable generation and manipulation of micro-bubbles in water with absorptive colloid particles by CW laser radiation.

Micrometer-sized vapor-gas bubbles are formed due to local heating of a water suspension containing absorptive pigment particles of 100 nm diameter. The heating is performed by CW near-infrared (980 nm) laser radiation with controllable power, focused into a 100 µm spot within a 2 mm suspension layer. By changing the laser power, four regimes are realized: (1) bubble generation; (2) stable growth of the existing bubbles; (3) stationary existence of the bubbles and (4) the bubbles' shrinkage and collapse. This behavior is interpreted based on the temperature conditions. The generation and evolution of single bubbles and ensembles of bubbles with controllable sizes and numbers is demonstrated. The bubbles are grouped within the laser-illuminated region and form quasi-ordered structures. They can easily be moved and transported controlled by the focal spot. The results are useful for applications associated with the precise manipulation, sorting and specific delivery in nano- and micro-engineering problems.

Influence of evanescent wave on birefringent microplates.

Mechanical action caused by the optical forces connected with the canonical momentum density associated with the local wavevector or Belinfante's spin angular momentum is experimentally verified. The helicity-dependent and the helicity-independent forces determined by spin momenta of different nature open attractive prospects for the use of optical structures for manipulating minute quantities of matter of importance in nanophysics, nanooptics and nanotechnologies, precision chemistry and pharmacology and in numerous other areas. Investigations in this area reveal new, extraordinary manifestations of optical forces, including the helicity-independent force caused by the transverse helicity-independent spin or vertical spin of a diagonally polarized wave, which was not observed and exploited up to recently. The main finding of our study consists in a direct experimental demonstration of the physical existence and mechanical action of this recently discovered extraordinary transverse component of the spin here arising in an evanescent light wave due to the total internal reflection of a linearly polarized probing beam with azimuthal angle 45° at the interface between the birefringent plate and air, which is oriented perpendicularly to the wavevector of an evanescent wave and localized over the boundary of the transparent media with polarization-dependent refraction indices.

Some current views on the origins and prospects of correlation optics.

The state of the art modern branch of optics and photonics now referred to as correlation optics is discussed in connection with both its origins and promising prospects. We use here the term "correlation" not only as a synonym of the term "coherence," but also for emphasizing the necessity of taking into account fine, sometimes enigmatic, phase relations among the components of complex optical fields, even if such fields are conventionally defined as completely coherent. Selection of topics for this brief review of correlation optics outlooks was not dictated by intention of comprehensive representation of this field of research, but rather by the scientific interests of the authors, ranging from classical theory of diffraction, holography, and light-scattering to modern singular optics.

Measurement of small light absorption in microparticles by means of optically induced rotation.

The absorption parameters of micro-particles have been associated with the induced spin exerted upon the particle, when embedded in a circularly polarized coherent field. The induced rotational speed is theoretically analyzed, showing the influence of the beam parameters, the parameters of the particle and the tribological parameters of the surrounding fluid. The theoretical findings have been adequately confirmed in experiments.

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.

Optical correlation algorithm for reconstructing phase skeleton of complex optical fields for solving the phase problem.

We propose an optical correlation algorithm illustrating a new general method for reconstructing the phase skeleton of complex optical fields from the measured two-dimensional intensity distribution. The core of the algorithm consists in locating the saddle points of the intensity distribution and connecting such points into nets by the lines of intensity gradient that are closely associated with the equi-phase lines of the field. This algorithm provides a new partial solution to the inverse problem in optics commonly referred to as the phase problem.

Self-diffraction of continuous laser radiation in a disperse medium with absorbing particles.

We study the self-action of light in a water suspension of absorbing subwavelength particles. Due to efficient accumulation of the light energy, this medium shows distinct non-linear properties even at moderate radiation power. In particular, by means of interference of two obliquely incident beams, it is possible to create controllable phase and amplitude gratings whose contrast, spatial and temporal parameters depend on the beams' coherence and power as well as the interference geometry. The grating characteristics are investigated via the beams' self-diffraction. The main mechanism of the grating formation is shown to be thermal, which leads to the phase grating; a weak amplitude grating also emerges due to the particles' displacements caused by the light-induced gradient and photophoretic forces. These forces, together with the Brownian motion of the particles, are responsible for the grating dynamics and degradation. The results and approaches can be used for investigation of the thermal relaxation and kinetic processes in liquid suspensions.

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

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)].

Orbital rotation without orbital angular momentum: mechanical action of the spin part of the internal energy flow in light beams.

The internal energy flow in a light beam can be divided into the "orbital" and "spin" parts, associated with the spatial and polarization degrees of freedom of light. In contrast to the orbital one, experimental observation of the spin flow seems problematic because it is converted into an orbital flow upon tight focusing of the beam, usually applied for energy flow detection by means of the mechanical action upon probe particles. We propose a two-beam interference technique that results in an appreciable level of spin flow in moderately focused beams and detection of the orbital motion of probe particles within a field where the transverse energy circulation is associated exclusively with the spin flow. This result can be treated as the first demonstration of mechanical action of the spin flow of a light field.

Investigation of optical currents in coherent and partially coherent vector fields.

We present the computer simulation results of the spatial distribution of the Poynting vector and illustrate motion of micro and nanoparticles in spatially inhomogeneously polarized fields. The influence of phase relations and the degree of mutual coherence of superimposing waves in the arrangements of two-wave and four-wave superposition on the characteristics of the microparticle's motion has been analyzed. The prospects of studying temporal coherence using the proposed approach are made. For the first time, the possibility of diagnostics of optical currents in liquids caused by polarization characteristics of an optical field alone, using nanoscale metallic particles has been shown experimentally.

On polarization metrology (estimation) of the degree of coherence of optical waves.

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

Polarization singularities of biological tissues images.

The ways to the polarization singularities of the biological tissues images of various morphological structures have been theoretically analyzed. The coordinate distributions of singly (linear states of polarization) and doubly degenerated (circular states of polarization) polarization singularities of the physiologically normal and pathologically changed biological tissues have been experimentally examined. The statistical criteria of diagnostics of the kidney tissue collagenous disease (the third and the fourth statistical moments of the linear density singularity points) have been determined. It was discovered that the process of the pathological change of the kidney tissue morphology leads to the formation of the self-similar (fractal) coordinate distribution of the polarization singularities of its image.

Investigation of the correlation structure of biological tissue polarization images during the diagnostics of their oncological changes.

We have considered the theoretical background of biological tissue complex polarization analysis: (1) polarization mapping; (2) polarization-correlation mapping; (3) polarization correlometry. The technique of interferometric measurements of 2D distributions of the complex degree of mutual polarization of biological tissue images has been proposed. The totality of diagnostically urgent interconnections between the statistical and correlation structure of biological tissue polarization images and the optical-geometrical structure of their architectonics has been determined.

Polarization visualization and selection of biotissue image two-layer scattering medium.

We analyze and experimentally test the concept of laser polarization biotissue probing. The methods of increasing the SNR in coherent images of the optically anisotropic architectonics of the morphological biotissue structure are considered. The possibilities of polarization selection and contrasting of such images screened by other biotissues are examined. The influence of the depolarization degree of the scattered background on the SNR is investigated. The possibilities of polarization correction of the probing beam for contrasting biotissue images are analyzed.

Polarization-correlation mapping of biological tissue coherent images.

We investigate the statistical polarization parameters of biological tissue histological section images with different morphological structure. First we outline the results of polarization coordinate mapping and analysis of the statistics of the first to fourth orders of biological tissue image polarization azimuth and ellipticities. Second, we study the statistics of the first to fourth orders of coordinate distributions of the complex degree of mutual polarization (CDMP) of biological tissue images. Finally, we consider the diagnostic possibilities of investigating 2-D distributions of CDMP of images that correspond to physiologically normal and degeneratively and/or dystrophycally changed biological tissues that are being analyzed.

Stokes singularity relations.

Polarization singularities in paraxial vector optical fields are analyzed in terms of the phase singularities of complex Stokes scalar fields. Six independent relationships are obtained that connect the topological charges of these singularities on special closed contours with the charges of singularities that are enclosed by these contours. These relationships, which have been confirmed by experimental data and computer simulations, imply topological polarization correlations of an infinite range.

New feasibilities for characterizing rough surfaces by optical-correlation techniques.

New feasibilities are considered for the optical-correlation diagnostics of rough surfaces with different distributions of irregularities. The influence of deviations of the height surface roughness distribution from a Gaussian probability distribution on the accuracy of optical analysis is discussed. Possibilities for the optical diagnostics of fractal surface structures are shown, and a set of statistical and dimensional parameters of the scattered fields for surface roughness diagnostics is determined. Finally, a multifunctional measuring device for estimating these parameters is proposed.

Experimental revealing of polarization waves.

Stationary and traveling waves of the states of optical polarization are considered in the framework of Jones vector formalism. The feasibility of revealing these waves in holographic and interference arrangements is substantiated and demonstrated.

Dimensionality in optical fields and signals.

The spatial chaos in optical fields that result from diffraction of plane waves by random-phase objects with a larger-than-unity phase dispersion is studied. An analog method for evaluating the dimension of chaos in the field is described, and a real-time measuring device that uses this method is proposed. A new method for evaluating the signal-to-noise ratios in optical signals is also proposed.