Residue orbital angular momentum in interferenced double vortex beams with unequal topological charges

All-optically phase-induced polarization modulation by means of holographic method

Phase-induced polarization modulation has been achieved experimentally by means of the all-optical holographic method. An extra spiral phase is added to a Gaussian beam and then a holographic grating is recorded through the interference of a Gaussian beam and the phase-vortex beam with the same linear polarization state in an azobenzene liquid-crystalline film. We report here that the polarization state of the diffraction light from the recorded grating is different from that of the incident light, while no polarization variation occurs for the holographic grating recorded by two Gaussian beams. The phase-induced polarization modulation is mainly attributed to the formation of birefringence in the film generated by phase vortex, which is investigated through the ripple patterns resulting from the competition between photoinduced torques and analysed by the Jones matrix. The experimental results could enrich the connotation between optical parameters and offer a method to realize polarization modulation through phase control.

Gouy phase induced polarization transition of focused vector vortex beams

We propose a common criterion for the effect of Gouy phase on the distinct polarization transition of focused vector vortex beams (VVBs). Such polarization transition is strongly dependent on the parity of the smaller modulus between VVB's polarization order and topological charge. Significantly, the cross polarization transitions are observed at areas where the two spin components with equi-intensity are exactly overlapping and the Gouy phase difference (GPD) between them equals to (2k + 1)π, k is an integer. As a whole, the focal field shows radially variant polarization distributions resulting from the unequal intensity proportion of the two spin components. This polarization transition holds potential in modifying the patterns of periodic surface structure induced by femtosecond vector beams.

Generation of photonic orbital angular momentum superposition states using vortex beam emitters with superimposed gratings

An integrated approach to produce photonic orbital angular momentum (OAM) superposition states with arbitrary OAM spectrum has been demonstrated. Superposition states between two vector OAM modes have been achieved by integrating a superimposed angular grating in one silicon micro-ring resonator, with each mode having near equal weight. The topological charge difference between the two compositional OAM modes is determined by the difference between the numbers of elements in the two original gratings being superimposed, while the absolute values of the topological charge can be changed synchronously by switching WGM resonant wavelengths. This novel approach provides a scalable and flexible source for the OAM-based quantum information and optical manipulation applications.

Helical wavefront and beam shape modulated by advanced liquid crystal q-plate fabricated via photoalignment and analyzed by Michelson's interference

In this study, electrically tunable advanced liquid crystal q-plates (ALCQPs) that combine two q values in one device to generate optical vortex beams were fabricated using a photoalignment method that involves the use of azo dye, a surfactant alignment material. The electrically tunable ALCQP device could be modulated to control the shape and polarization of a circularly polarized Gaussian laser beam that propagated through the device. A Gaussian beam modulated by an ALCQP under suitable applied voltage showed a variation beam shape with helical wavefront, as demonstrated by Michelson's interference. This helical wavefront beam carries an orbital angular momentum and can be used in an optical tweezers system to trap, move, and rotate particles simultaneously.

Modulation of shape and polarization of beam using a liquid crystal q-plate that is fabricated via photo-alignment

A liquid crystal (LC) device, called a "q-plate" (QP), which is based on axially symmetric photo-alignment was investigated. The electrically tunable LC QP device could be modulated to control the shape and polarization of a linearly polarized Gaussian laser beam that propagated through it. The intensity profile and polarization distribution were simulated by MATLAB and 1D-DIMOS. The results of the simulation were consistent with experimental findings. In the fabricated electrically tunable LC QP device, switching between different beam-profile configurations can be realized by applying a voltage. Moreover, the fabrication of an LC QP is relatively simple, and the device has potential for such practical applications as beam shape modulators and spatial polarization converters use in diffractive optics and imaging systems.

Polarization singularities in superposition of vector beams

We present a systematic study of the superposition of two vector Laguerre-Gaussian (LG) beams. Propagation depended field distribution obtained from the superposition of two vector LG beams has many interesting features of intensity and polarization. Characteristic inhomogeneous polarization distribution of the vector LG beam appears in the form of azimuthally modulated intensity and polarization distributions in the superposition of the beams. We found that the array of polarization singular points, whose number depends upon the azimuthal indices of the two beams, evolves during propagation of the field. The position and number of C-points generated in the field were analyzed using Stokes singularity relations. Novel intensity and polarization patterns obtained from the superposition of two vector LG beams may find applications in the field of molecular imaging, optical manipulation, atom optics, and optical lattices.

Quantitative measurement of the orbital angular momentum density of light

In this work we derive expressions for the orbital angular momentum (OAM) density of light, for both symmetric and nonsymmetric optical fields, that allow a direct comparison between theory and experiment. We present a simple method for measuring the OAM density in optical fields and test the approach on superimposed nondiffracting higher-order Bessel beams. The measurement technique makes use of a single spatial light modulator and a Fourier transforming lens to measure the OAM spectrum of the optical field. Quantitative values for the OAM density as a function of the radial position in the optical field are obtained for both symmetric and nonsymmetric superpositions, illustrating good agreement with the theoretical prediction.

Poynting vector and orbital angular momentum density of superpositions of Bessel beams

We study theoretically the orbital angular momentum (OAM) density in arbitrary scalar optical fields, and outline a simple approach using only a spatial light modulator to measure this density. We demonstrate the theory in the laboratory by creating superpositions of non-diffracting Bessel beams with digital holograms, and find that the OAM distribution in the superposition field matches the predicted values. Knowledge of the OAM distribution has relevance in optical trapping and tweezing, and quantum information processing.

Tight focusing of partially coherent and circularly polarized vortex beams

Based on the vectorial Debye theory, the tight focusing properties of partially coherent and circularly polarized vortex beams are investigated. The focused characteristics of right-circular and left-circular polarized partially coherent vortex beams in the focal region are presented and compared by some numerical calculation results. Furthermore, the influences of the source coherence and the numerical aperture of the focusing objective on the tight focusing properties are studied in great detail. It is shown that the coherence and polarization properties of the focused left-circular polarized beam is less influenced by the source coherence and the numerical aperture of the focusing objective than that of the focused right-circular polarized beam. By selecting certain parameters, the widely used flat top beam can be obtained.

Spatial correlation properties of focused partially coherent vortex beams

The spatial correlation properties in the geometrical focal region of a converging, partially coherent vortex wave field are analyzed. Expressions are derived for a pair of points on the axis of symmetry and for a pair of points in the focal plane. It is found that the longitudinal and transverse coherence lengths in the focal region change with the variation of the topological charge and the normalized coherence length of the vortex field. In addition, the degree of coherence is shown to possess phase singularities.