Trapping and transportation of large microparticles with singular IR beams

The combined singular beams generated by saddle-like imperfections of the cover glass surface in IR spectral range is subjected to the unfolding process near the focal plane of a microobjective. As far as such beams have their intensity profile similar to those of the optical vortices with the quaternary topological charge they can be exploited for the trapping and transportation of large micro particles. In contrast to the ordinary high-order optical vortices being unstable against the slight perturbations of the beam's shape, the given singular beams conserve their topological structure to trap and carry over the particles up to 200 <i>&mu;m </i>in sizes. This fact is discussed in detail in the given work.

Sensitivity of singular beams in the presence of Zernike aberrations

The process of generation and transformation of the structure of singular beams both coherent and incoherent, in a crystal-polarization compensator-polarizer system is studied. It is shown that this system can transform the topological charge of the initial beam. The value of the transformed charge depends both on the structure of the initial beam and on its polarization. The initial singular beams, transferring topological multipoles, are shown to acquire unique properties after passing through the system. This system, in particular, makes it possible to control the position of bound optical vortices and the magnitudes of their angular momenta, which may find practical application in devices for trapping, transportation, and mutual arrangement of microparticles.

We investigate the statistical properties of partially coherent polarization singular beams embedded with a V-point polarization singularity. An analytical formula for the cross-spectral density matrix is derived for the family of partially coherent polarization singular vector beams (PSVBs) propagating through a paraxial ABCD optical system. It is observed that the far-field intensity profiles and the coherence-induced depolarization effect in partially coherent PSVBs depend on both the input spatial coherence length and the Poincar\'e-Hopf index (PHI) of the beam. Interestingly, it is found that in this process of coherence degradation, the polarization (Stokes, ${S}_{12}$) vortices are preserved. The depolarization is due to an enhanced unpolarized light field that in turn modulates the beam profile, the transverse distribution of the degree of polarization (DOP) and the degree of coherence (DOC). Furthermore, the Gaussian distribution of the DOC evolves into a non-Gaussian profile in the far-field with the number of ring dislocations equal to the magnitude of PHI of the beam. The degeneracy associated with the intensity profile, the Stokes intensity distribution, the DOP, and DOC profiles of these partially coherent PSVBs carrying opposite polarity of PHI are also discussed to complete this study. Subsequently, all of these findings are experimentally verified by generating a family of partially coherent PSVBs with controllable spatial coherence. The modulation of the spatial coherence length in the source plane leads to efficient control of its intensity, the DOC and DOP profiles on propagation, which are of importance in particle trapping, material thermal processing, free-space optical communications, and detection of a phase object.

Earlier investigations have shown that ordinary cover glasses for microscopes have small areas near a plate edge with a sharp surface profile that transform an initial fundamental Gaussian beam into a steady singular beam with the central zero at its axis. We have called such a glass area an optical vortex trap. In this paper, we study experimentally these singular beams in view of the optical traps of large microparticles in the near-IR spectrum area and discuss capture-particle theory in relation to such singular beams.

The aim of the study was a comparative histomorphological evaluation of the results of laser-technological treatment of patients with varicose veins of the lower extremities. The study planned to examine the effectiveness of the combined use of endovascular laser ablation (EVLA), Venocoryl ointment and low-intensity laser beams in the treatment of trophic wounds caused by varicose veins of the lower extremities, as well as morphological changes in the skin and subcutaneous tissue in the ulcer area. Patients were randomized into 3 groups of 25 patients each: Group I – EVLA and mini-phlebectomy were performed together. Group II – EVLA, Venocoryl ointment and low-intensity laser beams were used in complex treatment. Group III – EVLA, miniphlebectomy and Venocoryl ointment were prescribed together for treatment. During the experiment, the morphological changes recorded in different groups were mostly identical and their dynamics were parallel. During this period, many morphological parameters representing tissue damage disappeared, especially in biopsies taken on the 7th day. Based on the above indicators, it can be said that in dynamic observation, the combined application of EVLA, Venocoryl ointment and low-intensity laser beams in the main group has almost the same effect as other treatment methods, but the faster reduction of thrombi in the microcirculation in the main group allows us to say that this organization of treatment provides earlier restoration of microcirculatory circulation. Therefore, it can be successfully applied in the treatment of varicose veins as an alternative treatment plan.

The aim of the study was a comparative histomorphological evaluation of the results of laser-technological treatment of patients with varicose veins of the lower extremities. The study planned to examine the effectiveness of the combined use of endovascular laser ablation (EVLA), Venocoryl ointment and low-intensity laser beams in the treatment of trophic wounds caused by varicose veins of the lower extremities, as well as morphological changes in the skin and subcutaneous tissue in the ulcer area. Patients were randomized into 3 groups of 25 patients each: Group I – EVLA and mini-phlebectomy were performed together. Group II – EVLA, Venocoryl ointment and low-intensity laser beams were used in complex treatment. Group III – EVLA, miniphlebectomy and Venocoryl ointment were prescribed together for treatment. During the experiment, the morphological changes recorded in different groups were mostly identical and their dynamics were parallel. During this period, many morphological parameters representing tissue damage disappeared, especially in biopsies taken on the 7th day. Based on the above indicators, it can be said that in dynamic observation, the combined application of EVLA, Venocoryl ointment and low-intensity laser beams in the main group has almost the same effect as other treatment methods, but the faster reduction of thrombi in the microcirculation in the main group allows us to say that this organization of treatment provides earlier restoration of microcirculatory circulation. Therefore, it can be successfully applied in the treatment of varicose veins as an alternative treatment plan.

Over the past decade, the field of optical manipulation has been “shaped” by intelligent design of nonconventional optical beams. In particular, optical beams carrying angular momentum have provided a new twist to optical tweezers, enabling dynamical spin and rotation of trapped particles. Although optical vortex beams, nondiffracting Bessel beams, and recently, self-accelerating Airy beams have each played a unique role in the arena of particle trapping and manipulation, combining their features would certainly lead to a more powerful tool.A few years ago, we designed and demonstrated diffraction-resisting Bessel-like beams that travel along arbitrary trajectories. With the similar method, the singular beam was shaped in this paper, which owns the form of a higher-order Bessel function with a preserving OAM and a nonexpanding dark “hole” in the main lobe of the beam. The beam can propagate along an arbitrary trajectory, including parabolic, hyperbolic and even three-dimensional (3D) spiraling trajectories. Experimentally, not only we observe such a curved singular beam, but also we employ it to optically trap and rotate microparticles in a 3D spiral motion under the combined action of radiation pressure, gradient force, and the OAM. Our findings may open up new avenues for shaped light in various applications.Over the past decade, the field of optical manipulation has been “shaped” by intelligent design of nonconventional optical beams. In particular, optical beams carrying angular momentum have provided a new twist to optical tweezers, enabling dynamical spin and rotation of trapped particles. Although optical vortex beams, nondiffracting Bessel beams, and recently, self-accelerating Airy beams have each played a unique role in the arena of particle trapping and manipulation, combining their features would certainly lead to a more powerful tool.A few years ago, we designed and demonstrated diffraction-resisting Bessel-like beams that travel along arbitrary trajectories. With the similar method, the singular beam was shaped in this paper, which owns the form of a higher-order Bessel function with a preserving OAM and a nonexpanding dark “hole” in the main lobe of the beam. The beam can propagate along an arbitrary trajectory, including parabolic, hyperbolic and even three-dimensional (3D) spiraling trajectorie...

Recent developments in solving a paraxial wave equation¹ open new perspectives in theoretical analysis of different types of singular beams stimulating in turn a great series of experimental investigations^2,3. In particular, authors of Ref.⁴ propose to use pure phase masks for creating a structurally stable helico-conical singular beam with spiral-like intensity distribution. On the other hand, artificial phase masks need a great precision in their manufacturing connected with a large industrial outlay. Because of a great interest is to use natural objects for generating singularities in beams. Such objects are anisotropic crystals. As is well-known, uniaxial and biaxial crystals serve as basic elements for generating optical vortices nested in different types of singular beams⁵. The most amazing feature of the crystal is ability to create stable polychromatic vortices with high energy effectiveness. In contrast to the method of computer-generated holograms^6'7 the crystal forms a white vortex-bearing beam without any additional gadgets. The aim of the present article is to consider one more way permitting to generate singular beams bearing spiral edge dislocations and optical vortices with the help of two gyrotropic crystals.

It is developed the theory of diffraction of a singular light beam with axial optical vortex on a half-plane screen, which cuts off the vortex zero-amplitude center ('severe screening'). It is shown that such diffraction features differ strongly from well known diffraction of a light beam with smooth wave front. It was founded that the beam singular properties are restored on some distance behind the screen. It is happened through the complicated space dynamics due to generation of secondary vortices. The main features of severe screened singular beam 'self-treatment' are established.

We propose the optical trapping of Rayleigh particles using tailored anisotropic and hyperbolic metasurfaces illuminated with a linearly polarized Gaussian beam. This platform permits to engineer optical traps at the beam axis with a response governed by nonconservative and giant recoil forces coming from the directional excitation of ultra-confined surface plasmons during the light scattering process. Compared to optical traps set over bulk metals, the proposed traps are broadband in the sense that can be set with beams oscillating at any frequency within the wide range in which the metasurface supports surface plasmons. Over that range, the metasurface evolves from an anisotropic elliptic to a hyperbolic regime through a topological transition and enables optical traps with distinctive spatially asymmetric potential distribution, local potential barriers arising from the momentum imbalance of the excited plasmons, and an enhanced potential depth that permits the stable trapping of nanoparticles using low-intensity laser beams. To investigate the performance of this platform, we develop a rigorous formalism based on the Lorentz force within the Rayleigh approximation combined with anisotropic Green's functions and calculate the trapping potential of nonconservative forces using the Helmholtz-Hodge decomposition method. Tailored anisotropic and hyperbolic metasurfaces, commonly implemented by nanostructuring thin metallic layers, enables using low-intensity laser sources operating in the visible or the IR to trap and manipulate particles at the nanoscale, and may enable a wide range of applications in bioengineering, physics, and chemistry.

Waveguide trapping has emerged as a useful technique for parallel and planar transport of particles and biological cells and can be integrated with lab-on-a-chip applications. However, particles trapped on waveguides are continuously propelled forward along the surface of the waveguide. This limits the practical usability of the waveguide trapping technique with other functions (e.g. analysis, imaging) that require particles to be stationary during diagnosis. In this paper, an optical waveguide loop with an intentional gap at the centre is proposed to hold propelled particles and cells. The waveguide acts as a conveyor belt to transport and deliver the particles/cells towards the gap. At the gap, the diverging light fields hold the particles at a fixed position. The proposed waveguide design is numerically studied and experimentally implemented. The optical forces on the particle at the gap are calculated using the finite element method. Experimentally, the method is used to transport and trap micro-particles and red blood cells at the gap with varying separations. The waveguides are only 180 nm thick and thus could be integrated with other functions on the chip, e.g. microfluidics or optical detection, to make an on-chip system for single cell analysis and to study the interaction between cells.

In general, singular acoustic beams travel in a straight line and expand during propagation, which limits the scope of their applications. We present a method for generating paraxial zero-order Bessel-like acoustic beams with arbitrary trajectories in water. We then add a vortex phase to obtain a first-order Bessel-like acoustic beam. This beam exhibits a combination of features of vortex, Bessel, and Airy beams. Moreover, it maintains a dark ``hole'' in the center, preserving its angular momentum, and displays resistance to diffraction, self-healing, and self-bending during propagation. Furthermore, we experimentally realize the beam using a three-dimensionally printed phase mask and observe the acoustic field distribution using the Schlieren imaging method. Finally, we realize particle trapping and transport in a curved trajectory in water.

We investigate the formation of optical dynamic waveguide structures in media with thermal and resonant nonlinearities. An optical waveguide is formed experimentally in absorbing a solution of organic dye with negative thermo-optical coefficient under a powerful singular light beam (an optical vortex).

The plasmonic optical tweezer has been developed to overcome the diffraction limits of the conventional far field optical tweezer. Plasmonic optical lattice consists of an array of nanostructures, which exhibit a variety of trapping and transport behaviors. We report the experimental procedures to trap micro-particles in a simple square nanoplasmonic optical lattice. We also describe the optical setup and the nanofabrication of a nanoplasmonic array. The optical potential is created by illuminating an array of gold nanodiscs with a Gaussian beam of 980 nm wavelength, and exciting plasmon resonance. The motion of particles is monitored by fluorescence imaging. A scheme to suppress photothermal convection is also described to increase usable optical power for optimal trapping. Suppression of convection is achieved by cooling the sample to a low temperature, and utilizing the near-zero thermal expansion coefficient of a water medium. Both single particle transport and multiple particle trapping are reported here.