We have experimentally analyzed the behavior of Gaussian beam with elliptical cross-section passed through the uniaxial crystal in direction near perpendicular to the crystal’s optical axis. The projection of total angular momentum flux on the axis consists of projection of two components: the spin (SAM) and orbital (OAM) angular momentum. We have revealed that in anisotropic media the SAM describes the polarization state of the beam cross-section and OAM reflects the shape and topological charge of the singularity at ordinary and extraordinary beams. The sum of projections SAM and OAM remains constant. The oscillations of spin-orbital angular momentum projection were described experimentally during the crystal was rotated around z axis with small beam inclination. The low changes in the extraordinary beam cross section and their correlation with polarization state allow us to analyze the form of angular momentum conversion and possibility for generation of polarizing singularities. The feasibility of optical vortex generation in the system of uniaxial crystal with tilted Gaussian beam was theoretically and experimentally investigated.
We built theoretical model of evolution of intensity of asymmetric and symmetric arrays of singular beams passing along optical axis of uniaxial crystal (LiNbO3) for arrays consisting different number of beams. We compared theoretical and experimental evolution of intensity distribution and dependence of angular rotation of an asymmetric and symmetric singular beam arrays on the inclination angle of corresponding arrays of singular beams passing along optical axis of uniaxial crystal for arrays consisting different number of beams.
We have made a theoretical modeling of the evolution of a monochromatic Gaussian beam diffracted by the angle formed by three sides of the phase wedge different types. We have found that the edges of the phase wedge generate macroscopic chains of identical optical vortices that disappear at the far field zone. At the same time, the π -phase plate can reproduce a very complex wave field whose structure depends on the scale of observation. At large scales there appear two π -cuts resembling broken edge dislocations with perpendicular directions. At small (some microns) scales two short vortex chains consisting of alternating-sign optical vortices are nucleated near the corner of the wedge. The analysis shows that the sizes of the chains decrease quickly when approaching the wedge surface. This enables us to assume that the π -phase plate can create so-called optical quarks in the evanescent waves of the edge field. On the basis of theoretical considerations experiment was conducted in which were obtained fractions optical vortices at the edge of the phase wedge.
We have experimentally analyzed the topological reactions occurred in the elliptic vortex-beam transmitting
orthogonally to the optical axis of the SiO2 crystal. We have revealed that the oscillations of the polarization state when
propagating the beam are accompanied by reconstruction of the polarization singularities at the beam cross-section that, in
turn, entails the reconstruction of the wavefront in each circularly polarized beam component. Both synchronic oscillations of
the spin angular momentum and the sign of the vortex topological charge are expressing in a field structure as birth and
annihilation of topological dipoles. Also periodical conversion of the vortex ellipticity along the crystal length z and huge
splash of spin angular momentum were analysed. The run of the dislocation reactions in the beam component results in
converting the sign of the topological charge in the centered optical vortex, the distance of the vortex conversion being about
0.05 of the wavelength.
The splitting of a fundamental vortex beam into four separate beams is theoretically and experimentally described for the
propagation of light through a uniaxial crystal. This novel phenomenon opens new opportunities for higher order the
entanglement of and correlation of photons.
We consider experimentally the evolution of polarization singularities in singular beams transmitting at some angle
to the optical axis of the LiNb03 crystals. We focused our attention on the processes of birth and annihilation of coupled
singularities and compared them with those in a computer-simulation experiment. The conditions of the unfolding of the
polarization singularities were brought to light. A number of difficulties arising in the course of the experiment are
discussed.
Experimental showed that a monochromatic singular beam emitted from a crystal has three focal planes. The distances between additional focuses of the beam depends both on the focal distance of the lens, a lens position relative to the crystal and the beam and the crystal parameters.
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 investigations2,3. 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 holograms6'7 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.
Conversion of singular beams in a single-axis crystal is experimentally studied. It is shown that the given system is able to generate singular beams in a wide spectral range.
The high-intensity singular beam turns out to be created by means of the ordinary cover glass for a microscope. For this it is necessary to place the edge of the cover glass into the high-power fundamental laser beam. At the expense of the technical wedging of the cover glass beyond the transparent appears the high-quality singular beam bearing the optical vortex. The given paper represents the physical mechanism of such process illustrated with the vast experimental and theoretical results.
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