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.
In vivo experimental results of plant tissue properties kinetics are presented. It is shown that the method of differential backscattering can serve for estimation of the physiological state of the plants, as evidenced by results of mathematical modeling.
The high efficiency of the perturbation theory for the calculation of electromagnetic field in the non-paraxial laser
radiation beam is revealed. The perturbation method is compared with the exact solution of Maxwell's equations for a
non-paraxial Gaussian wave. Calculations on the microscopic particles manipulation in the laser radiation field are
performed under the actual experiments conditions.
The intensity of spiral beams remains unchanged under propagation and focusing neglecting scaling and rotation. The spiral beam with predetermined intensity in the shape of any planar curve can be generated by use of amplitude and phase elements concurrently. We introduce the new method of singular laser fields formation, close to spiral type, by means of pure phase modulation. Our algorithm is based on the well-known Gerchberg-Saxton phase retrieval algorithm and spiral beams optics. It demonstrates fast convergence and some other advantages: phase distributions obtained are stable to spatial resolution changing (it is enough 128 x 128 pixels for some patterns), theoretical energy efficiency is about 85 % with acceptable intensity homogeneity. We demonstrate theoretical results on fields formation in the shape of closed-curves (triangular, square, "snowflake") and open-ended curve (Archimedes spiral) by means of elements on dichromate gelatin. Besides, the example of experiment on micromanipulation with the use of the square-shaped field is presented.
In vivo experiment results and mathematical simulation of plant optical properties are presented. The changes and
modification of plant optical parameters are investigated in conditions of external factor action, which results in
regenerations of the internal structures of the leaf. It is shown that differential backscattering method may serve as
effective method of plant physiological status determination.
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.
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.
Development of methods for generating of laser beams with predetermined values of intensity and angular momentum distributions is a challenge of great interest for various laser technologies including laser manipulation by microscopic objects. The suggested method oftransformation of laser radiation to complex structure modes in build-up beam rotator interferometer has been theoretically evolved and experimentally tested. Its main advantage over the others is that the method doesn't require complex diffractive optical elements to be used. Experiments were performed using tunable (adjustable) diode laser and interferometer formed by three mirrors. The beam rotation has been achieved by Dove prism inserted into the interferometer. The evolution of the transformed beam was observed with alternation of the prism rotation angle and the injection current of the laser diode.
Liquid crystal (LC) wavefront correctors with modal addressing are described. Three different approaches are considered. The first one is based on a continuous thin-film resistive layer. This layer is used for forming of the local voltage profile that controls the phase distribution across the corrector’s aperture. The second approach is a modification of the first one, where the continuous resistive coating is replaced by a network of discrete resistors. It is based on silicon technology. The third approach makes use of distributed electric field in thick dielectric layers for forming of the modal response of an actuator. Technologies, methods of control and experimental results are discussed for each case.
In this paper we review progress towards making a liquid crystal spatial light modulator (LC-SLM) which has all the desired specifications required for (astronomical) adaptive optics (AO). Our work at Durham is currently focused on developing modal LCs, as they have some key advantages over conventional LC-SLMs. A modal LC-SLM is a device whose optical properties more closely resemble a deformable facesheet mirror than a conventional LC-SLM which are pixelated. Therefore they have a Gaussian, rather than a piston-only, influence function.
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.
The outcomes of experiments on a research of in vivo human organism response for the influence of the low intensity optical radiation are presented. The dynamics of spectral intensity of backscattering diagnostic radiance was registered. It was shown, that the biotissues optical properties alteration process has similar character in a whole investigated optical range from 430 up to 860 nm and its kinetics was explained by the field model mechanism of interaction.
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.
A general phenomenological approach - a Flicker Noise Spectroscopy (FNS)- to revelation of information valuable parameters characterizing the arbitrary chaotic surfaces was develop to distinguish their patterns and describe quantitatively their functional properties. The consideration was carried out in terms of correlation lengths and additional parameters characterizing the rate of correlation links lost in the sequences of surface irregularities. The parameters are obtained by fitting the Fourier spectra and structural functions (difference moments of different orders) calculated for the digitized surface profiles using the approximations derived on the base of model representation of the profiles as the sequences of irregularities of different types (“bursts”, “jumps”, etc.). The method developed was applied to revelation of effects of a shungit filling agent in polypropylen matrix on the composite properties, revelation of hydrogen treatment effects on the cleavage surfaces of LiF monocrystals after their dissolution in water with quantitative evaluations of their anisotropy, analysis of activity of vacuum deposited porphyrins layers in a photosensibilized gnenration of singlet oxygen into gaseous phase. The approach elaborated can be used for developing the new control tools in nano-technologies, microelectronics, production of polymeric material with the specific surface properties, and others.
The suppression of chaos in a laser diode with an external optical feedback is analyzed and experimental results are presented. The intensity noise suppression and the emission line reduction by a small perturbation in the laser current are shown. This technique can be applied as a low cost replacement for an optical isolator in spectroscopy, interferometry and other areas.
The major directions of manipulations with micro objects by laser beams are considered. Using spiral beams can essentially extend the potential of manipulation. In ray optics approximation the calculation of the torque transmitted from a spiral beam to an absorbing particle is made in accordance with the wave optics. The experimental observation of torque transfer from non zero orbital moment beams to particles trapped into the laser beam focus is described.
The computer model of interaction of a tightly focused laser beam with transparent dielectric particles in the shape of ellipsoid of revolution of the sizes > 10(lambda) was created. For a zero Gaussian mode the interaction with latex particles weighed in water was simulated. For these particular particles the forces exerted by Gaussian beam in a longitudinal direction were calculated. It was revealed that the particles flattened out along the axis of the beam propagation were better trapped, than the longitudinal ones. It was found, that the behavior of the trapping force acting on the longitudinal particles is influenced by the total reflection arising at the light propagation inside a particle.
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).
The real time control of the rotation rate of a microscopic particle trapped in a focused laser beam by mean of polarization ellipticity of laser radiation has been studied. The ellipticity change was carried out with the modulator using orientational S-effect in nematic liquid crystals. The dependence of the particle rotation speed on the state of polarization ellipticity was determined. The calculation of an angular momentum of rotation per a unit of energy of the light field is shown.
The delayed feedback control of the chaotic dynamics of the optoelectronic system with a laser diode is analyzed and experimental results are presented. The degree of the chaos suppression and suppression time were measured for three cases differing in the functional dependence of the control signal on the change of the laser output power: proportional, exponential or combined. It was found that the degree of the chaos suppression did not depend much on the control technique. The suppression time was different in all cases and in the third one it was minimal.
Structure and optical properties of vacuum deposited poly- para-phenylene (PPP) films were investigated by UV-Vis absorption, luminescence, polarized luminescence, FTIR, and electron diffraction methods. It was shown that molecular structure, morphology and spectral parameters of vacuum deposited PPP films drastically depend on evaporation temperature, substrate temperature and distance of substrate from PPP powder. The increase of the evaporation temperature results both in red shift of the luminescence peaks from 480 nm at 650 degrees C to 540 nm at 800 degrees C and in considerable decrease of the photoluminescence intensity. Electron diffraction studies show that the highest degree of crystallinity of PPP films is observed for evaporation temperature 680 degrees C and support temperature 300 degrees C. These conditions yield optically anisotropic films with high luminescence intensity on various substrates. Electron diffraction and polarized optical measurements of highly oriented PPP films indicate that PPP molecules are essentially arrange perpendicularly to the surface of the support. It has been demonstrated that in FTIR spectra of highly oriented PPP films an extremely strong band at 1375 cm-1 is observed. That band is not typical for currently known PPP modifications and could be associated with change in the order of the 'defect' C-C bond between adjacent quinoid and benzenoid units.
The dependence of intensity distribution of diffusely reflected light during medium irradiation with narrow laser beam on optical parameters of multiple scattering media such as tissue has been investigated. The dependence of backscattered spot size on penetration depth as well as dependence of sensitivity on absorption and scattering coefficients have been analyzed by Monte Carlo simulation. Experiments in model multiple scattering media which determine the dependence between the spot size of backscattered radiation and optical characteristics have been carried out. Scanning with optical fibers have been used for measurements of intensity distribution.
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