Svetlana P. Kotova

Light modulation in planar aligned short-pitch deformed-helix ferroelectric liquid crystals

We study both experimentally and theoretically modulation of light in a planar aligned deformed-helix ferroelectric liquid crystal (DHFLC) cell with subwavelength helix pitch, which is also known as a short-pitch DHFLC. In our experiments, the azimuthal angle of the in-plane optical axis and electrically controlled parts of the principal in-plane refractive indices are measured as a function of voltage applied across the cell. Theoretical results giving the effective optical tensor of a short-pitch DHFLC expressed in terms of the smectic tilt angle and the refractive indices of the ferroelectric liquid crystal (FLC) are used to fit the experimental data. The optical anisotropy of the FLC material is found to be weakly biaxial. For both the transmissive and reflective modes, the results of fitting are applied to model the phase and amplitude modulation of light in the DHFLC cell. We demonstrate that if the thickness of the DHFLC layer is about 50μm, the detrimental effect of field-induced rotation of the in-plane optical axes on the characteristics of an axicon designed using the DHFLC spatial light modulator in the reflective mode is negligible.

Experiments on microscopic object movement along various fixed trajectories caused by spiral beams (Invited Paper)

The range of possibilities ofthe laser manipulation with microscopic objects could be sufficiently expanded by using of the beams with predetermined spatial intensity and orbital momentum density distributions in the focusing plane. Such beams permit to realize rotation and fixed trace movement of absorbing particles. The spiral beams having intensity in the shape of triangular boundary, the line with self-intersection and Archimedes spiral were formed by composition of amplitude and phase masks produced on the base of bichromated gelatin. The spiral beams keep their intensity structure unchanged under propagation except scale and rotation. The Ar-laser and microscope MIN-8 with immersion micro objective (60x, NA=O.85) were used in experimental set-up. Particles of the cetylpiridiniumbromide and colored latex spheres were chosen as an objects for manipulation. Experimental results are presented on microobjects movement effectuated with spiral beams along different fixed trajectories. The motion direction is determined by the direction of the beams orbital momentums.

Modeling of spiral-type beam generation by the one-dimensional light modulators

Spiral beams while propagating and focusing, keep their intensity structure unchanged neglecting scale and rotation. One of the experimental ways to obtain spiral beams is the astigmatic transformation method. It allows producing spiral beams by means of structurally one-dimensional amplitude-phase elements and simple cylindrical optics. The paper is dedicated to modeling of spiral light fields formed with one-dimensional spatial light modulators. The effect of the ultimate resolution of the modulator and ultimate accuracy of the intensity and phase choice on the quality of the synthesized field in a far-zone of diffraction has been estimated in this work. The results can be of interest for various laser applications including the laser manipulation with micro-objects.

Micro-object manipulation by laser beams with nonzero orbital momentum

Possibilities of the laser manipulation with microscopic objects could be sufficiently expanded by using of the beams with predetermined spatial intensity and orbital momentum density distributions in the focusing plane. The experiments on the visualization of the wave front of the beams with nonzero orbital momentum by using controllable liquid crystal plate are described. The experimental results presented on trapping, rotation and motion along a specified trajecotry of absorbing particles by means of the beams.

Multichannel modal liquid crystal wavefront corrector

37-channel modal liquid crystal wavefront correctors with a 30 and 80 mm diameter aperture are developed. Optical response, voltage-phase and dynamic properties of the devices have been studied. The possibility of synthesis of low order aberrations was experimentally demonstrated.

Optical traps formed by different laser modes

The most promising sphere of the application of the laser trapping of neutral particles is microbiology. To determine the optimum parameters of the laser trap for transparent neutral particles, a computer model of the interaction of tightly focused laser beams of various modes with a transparent dielectric sphere was created based on the laws of the ray and wave optics. In modeling the Laguerre-Gaussian laser modes L-G00, L-G01, L-G02, L-G03, L-G04, L-G10, L-G20 and L- G11 were used and latex spheres suspended in water were simulated to serve as transparent dielectric particles. The reason why transparent latex spheres had been selected is that many bio-objects are transparent for red and near IR of the light ranges. For all applied laser modes their forces of action on the sphere are calculated both in axial and transversal directions of the beam propagation. It was revealed that the Laguerre-Gaussian beams generate axial reverse forces exceeding those of the zero Gaussian mode. At the same time, the transversal forces of trapping are stronger at the zero Gaussian mode. Besides, the forces of interaction of the Gaussian mode (TEM00), focused with different micro- objectives for latex spheres of various diameters, are calculated. It was found that for the three simulated micro- objectives in the transversal direction of the largest force of trapping is provided by the micro-objective with the smallest numerical aperture (NA equals 0,4) while and in the longitudinal direction -- by the micro-objective with the maximum numerical aperture (NA equals 1,25).

Analysis of contour images using optics of spiral beams

An approach is outlined to the recognition of contour images using computer technology based on coherent optics principles. A mathematical description of the recognition process algorithm and the results of numerical modelling are presented. The developed approach to the recognition of contour images using optics of spiral beams is described and justified.

Aberration influenced generation of rotating two-lobe light fields

The influence of aberrations on light fields with a rotating intensity distribution is considered. Light fields were generated with the phase masks developed using the theory of spiral beam optics. The effects of basic aberrations, such as spherical aberration, astigmatism and coma are studied. The experimental implementation of the fields was achieved with the assistance of a liquid crystal spatial light modulator HOLOEYE HEO-1080P, operating in reflection mode. The results of mathematical modelling and experiments have been qualitatively compared.

Diffractive elements based on spiral beams as devices for determining the depth of bedding of radiation sources

A modified iteration method is proposed for calculations in developing diffraction optical elements for the generation of rotating light fields. The main feature of the proposed approach is the use of the spiral light beam theory for selecting the initial phase approximation. The diffractive optical elements under study could be used in microscopic analysis of the spatial location of different objects.

Light fields generated by LC focusing device in different operational regimes

The types of light fields that can be generated with the use of proposed by the authors 4-channel modulator (LC focusing device) in different modes depending on the value of χl (where χ is the modal parameter, l – the characteristic size of the aperture and the square of χl is the ratio of the resistance of the high resistance layer and the impedance of the LC layer) are analyzed. By means of numerical simulation it was shown that χl value determines reshaping of equipotential lines of voltage from ring/ellipses to square/parallelogram contours or octagon contours. The capability of LC focusing device to generate light fields with intensity distribution in the shape of intersections, sets of points, boundaries of squares, diamonds, parallelograms, octagons is demonstrated. Obtained light fields can be useful in the problems of optical manipulation.