Angular Momentum Of Light Beam Research Articles

Ultrafast all-optical second harmonic wavefront shaping

Optical communication can be revolutionized by encoding data into the orbital angular momentum of light beams. However, state-of-the-art approaches for dynamic control of complex optical wavefronts are mainly based on liquid crystal spatial light modulators or miniaturized mirrors, which suffer from intrinsically slow (µs-ms) response times. Here, we experimentally realize a hybrid meta-optical system that enables complex control of the wavefront of light with pulse-duration limited dynamics. Specifically, by combining ultrafast polarization switching in a WSe2 monolayer with a dielectric metasurface, we demonstrate second harmonic beam deflection and structuring of orbital angular momentum on the femtosecond timescale. Our results pave the way to robust encoding of information for free space optical links, while reaching response times compatible with real-world telecom applications.

Control of the total orbital angular momentum of light beams propagating through a turbulent medium.

The robustness of the orbital angular momentum (OAM) of light beams propagating in a turbulent medium, e.g., atmosphere, is critical for many applications such as OAM-based free-space optical communications and remote sensing. However, the total OAM of a beam interacting with the turbulent medium inevitably changes. Here, we demonstrate a practical algorithm to control the total OAM of a beam transmitted through a time-evolving, turbulent medium by dynamically modulating the weights of two coherently superimposed OAM modes, which served as the input beam. A cross-OAM matrix is introduced, and applied for checking whether the desired total OAM in the output plane can be achieved. Furthermore, analytical relations between the weights of two input modes and the output total OAM, as well as its modulation range, are established. As a numerical example, we study the behavior of total OAM of the two-mode beam after passing through a thermal convection occurring in an aqueous medium and suggest a possible application of our strategy.

Transformation of spin and orbital angular momentum in second-harmonic generation process by the near-surface layer of isotropic nonlinear medium.

We investigate the conversion between orbital and spin components of the angular momentum of light beams in the second-harmonic generation process from the near-surface layer of the nonlinear isotropic medium without spatial dispersion at oblique incidence of the elliptically polarized fundamental beam. The conservation of the projection of both spin and orbital angular momenta onto the normal to the surface of the medium during the transformation of the incident wave into a reflected double frequency wave has been demonstrated.

Unitary transformation for Poincar\xe9 beams on different parts of Poincar\xe9 sphere

We construct an experimental setup, consisting of conical refraction transformation in two biaxial cascade crystals and 4f-system, to realize Unitary transformation of light beam and the manipulation of Poincaré beams on the different parts of Poincaré sphere. The spatial structure of the polarization can be controlled by changing the polarization of the incident beam or rotating the angle between these two crystals. The beams with different SoPs covering the full-Poincaré sphere, part-Poincaré sphere and one point on the sphere are generated for the different angles between crystals. The Unitary transformation of light beam is proposed in the experiment with the invariant intensity distribution. Subsequently, the spin angular momentum is derived from the distribution of polarization measured in our experiment. Moreover, the conversion between orbital angular momentum and spin angular momentum of light beam is obtained by changing the angle between crystals. And the conversion progress can also be influenced by the polarization of incident beam. We realized the continuous control of the spatial structure of the angular momentum density, which has potential in the manipulation of optical trapping systems and polarization-multiplexed free-space optical communication.

Nonlinear Beam Shaping in Domain Engineered Ferroelectric Crystals.

Domain engineered ferroelectric crystals are a type of microstructure functional material that is used widely in the area of nonlinear optics. Herein, research processes in the area of nonlinear beam shaping using domain engineered crystals in the past decade are reviewed. The newly developed design methods, such as the nonlinear Huygens-Fresnel principle, nonlinear volume holography, and caustic design, which have analogs in linear optics, are introduced. Using the proposed methods for nonlinear beam shaping, multiple function integration, generation of Airy beams, and arbitrary curved trajectories are realized. As an extra degree of freedom, orbital angular momentum of light beams is generated and manipulated through domain engineering. Discussions and future directions in this field are presented.

Measuring the orbital angular momentum of light beams by using a single intensity distribution

Here, we modify and study a method for measuring the orbital angular momentum (OAM) based on angle-averaged intensity in a single transverse plane. We also simulate propagation of a superposition of two Laguerre-Gaussian modes after passing through a diffuser with a random phase. Despite the distortions caused by the diffuser, the modified method allows OAM measurement with an average error less than 10%. The main advantages of the described technique is that (i) simultaneously with the full OAM the OAM spectrum is obtained and (ii) no any optical elements are needed.

Interconversion between the orbital and spin angular momentum of light beams in three-wave mixing processes in the bulk of an isotropic chiral medium.

A phenomenological description of nondegenerate and degenerate three-wave mixing in the bulk of an isotropic chiral medium is given for the wide class of fundamental light beams with nonuniform polarization distribution and multimode transverse structure. Special attention is paid to the classical and quantum interpretation of interconversion between spin and orbital parts of the angular momentum of the input and output beams.

Geometric phase Doppler effect: when structured light meets rotating structured materials.

We examine the geometric phase Doppler effect that appears when a structured light interacts with a rotating structured material. In our scheme the structured light possesses a vortex phase and the structured material works as an inhomogeneous anisotropic plate. We show that the Doppler effect manifests itself as a frequency shift which can be interpreted in terms of a dynamic evolution of Pancharatnam-Berry phase on the hybrid-order Poincaré sphere. The frequency shift induced by the change rate of Pancharatnam-Berry phase with time is derived from both the Jones matrix calculations and the theory of the hybrid-order Poincaré sphere. Unlike the conventional rotational Doppler effect, the frequency shift is proportional to the variation of total angular momentum of light beam, irrespective of the orbital angular momentum of input beams.

Characterizing coherence vortices through geometry.

We produce coherence vortices experimentally and numerically due to the orbital angular momentum of light beams and study the dependence of their bright ring area and dark region on their different orders. This is a linear dependence with a slope proportional to the bright ring area or dark area. We show that it is possible to estimate any order of coherence vortices, including fractional orders, just by calculating the bright ring area or dark area of the vortices for some specific parameters of the incident beam.

Fractional orbital angular momentum of light beams by topological trajectory

We derive a rigorous quantum formula called topological trajectory to describe orbital angular momentum (OAM) index based on linear momentum density of Laguerre–Gauss (L–G) light beam. By considering the correspondence between optics and quantum theory, we construct a coherent state from two L–G modes light beam. The light beams with fractional OAM are described by the coherent state. By making use of topological trajectory, we present the conditions that OAM index is fractional.

Electron-spin dynamics in elliptically polarized light waves

We investigate the coupling of the spin angular momentum of light beams with elliptical polarization to the spin degree of freedom of free electrons. It is shown that this coupling, which is of similar origin as the well-known spin-orbit coupling, can lead to spin precession. The spin-precession frequency is proportional to the product of the laser-field's intensity and its spin density. The electron-spin dynamics is analyzed by employing exact numerical methods as well as time-dependent perturbation theory based on the fully relativistic Dirac equation and on the nonrelativistic Pauli equation that is amended by a relativistic correction that accounts for the light's spin density.

Optical orbital angular momentum conservation during the transfer process from plasmonic vortex lens to light

We demonstrate the optical orbital angular momentum conservation during the transfer process from subwavelength plasmonic vortex lens (PVLs) to light and the generating process of surface plasmon polaritons (SPPs). Illuminating plasmonic vortex lenses with beams carrying optical orbital angular momentum, the SP vortices with orbital angular momentum were generated and inherit the optical angular momentum of light beams and PVLs. The angular momentum of twisting SP electromagnetic field is tunable by the twisted metal/dielectric interfaces of PVLs and angular momentum of illuminating singular light. This work may open the door for several possible applications of SP vortices in subwavelength region.

Conservation and transfer of orbital angular momentum of light in optically induced photonic lattices

Using analytical and numerical methods, we elucidate the conservation and transfer of orbital angular momentum of light beams propagating in fully periodic and radially periodic photonic lattices, induced in photorefractive crystals. We consider paraxial beams propagating in both fixed and interacting lattices, in both copropagating and counterpropagating geometries, in photorefractive media with saturable nonlinearity. We also discuss the influence of beam intensity and the finite lattice size on the angular momentum transfer in such photonic lattices.

Experimental study of 8-bits information transmission system based on orbital angular momentum of light beams

An experimental setup is built to realize 8-bits information transmission based on orbital angular momentum (OAM) of light beams. In the modulation section, a controllable laser diode array and a stable phase modulator are used to superpose and encode the OAM states. A binary amplitude grating is used to detect the OAM states in the demodulation part. This system transmits 8-bits data successfully.

On the possibility of electrooptic operation by orbital angular momentum of light beams via Pockels effect in crystals

We have shown that some special configurations of electric field created in electrooptic crystals can lead to appearance of orbital angular momentum in the outgoing light beam, provided that the incident circularly polarised beam propagates along the optic axis direction. It has been found that the topological charge of outgoing helical mode is equal to unity. We have also demonstrated on the canonical examples of electrooptic crystals LiNbO3 and LiTaO3 that the efficiency of SAM-to- OAM conversion can be gradually operated by the electric field, using the Pockels effect.

Measurements of piezooptic coefficients pi(14) and pi(25) in Pb5Ge3O11 crystals using torsion induced optical vortex

We show how a singularity of optical fiel d can be used for determination of piezooptic coefficient of crystal s under their torsion, in particular in the situation when the natural optical activity makes direct polar imetric experiments too complicated. Experimental studies of an op tical vortex appearing under torsion of lead germanate crystals have enabled us to determine so-called non-principal coefficients (π14 and π25). A relationship between the spin angular momentum of light beam and the piezooptic coefficients has been derived.

Transverse angular momentum of photons

We develop the quantum theory of transverse angular momentum of light beams. The theory applies to paraxial and quasiparaxial photon beams in vacuum and reproduces the known results for classical beams when applied to coherent states of the field. Both the Poynting vector, alias the linear momentum, and the angular-momentum quantum operators of a light beam are calculated including contributions from first-order transverse derivatives. This permits a correct description of the energy flow in the beam and the natural emergence of both the spin and the angular momentum of the photons. We show that for collimated beams of light, orbital angular-momentum operators do not satisfy the standard commutation rules. Finally, we discuss the application of our theory to some concrete cases.

Spin-induced angular momentum switching

When light is transmitted through optically inhomogeneous and anisotropic media the spatial distribution of light can be modified according to its input polarization state. A complete analysis of this process, based on the paraxial approximation, is presented, and we show how it can be exploited to produce a spin-controlled change in the orbital angular momentum of light beams propagating in patterned space-variant optical axis phase plates. We also unveil a new effect: the development of a strong modulation in the angular momentum change upon variation of the optical path through the phase plates.

Realizing high density optical data storage by using orbital angular momentum of light beam

The orbital angular momentum (OAM) states can be used to construct a high-dimensional Hilbert space, which implies higher data density when applied in data storage. This work proposes a method to realize high density optical data storage by using OAM of light beam, with the encoded area illuminated by a light beam with certain OAM resulting in distribution of power in the OAM states (the spiral spectrum or OAM spectrum), and detection of the power distribution conducing to its decoding. A quadrant staircase structure (QSS) used as an encoded element can realize 57 different states, and it is expected to enhance the data density 6 times as high as the conventional method.

Laguerre-Gaussian Beams and Optical Orbital Angular Momentum

Laguerre-Gaussian beams are solutions of the paraxial wave equation and carry orbital angular momentum, which can be transferred to trapped particles. Orbital angular momentum of light beams can be better understood by considering angular momentum flux. Phase singularity and orbital angular momentum are different concepts and should be distinguished.