Twisted Beams Research Articles

Generation and manipulation of multi-twisted beams via azimuthal shift factors

We develop the effective experimental approach to generate multi-twisted beams (MTBs) with twisted intensity lobes by superimposing helical phases consisting of multiple independent sub-phases with different azimuthal shift factors. The MTBs' energy flows and propagation properties are also investigated, indicating that such beams exhibit twisted properties. The azimuthal shift factor determines the twisted intensity distributions, and the number of twisted lobes depends on the sub-phase number. The bright lobes of a MTB possess the shapes of thin spiral lines, and the intensity pattern depends on the topological charge. Diverse MTBs can be generated by flexibly manipulating the azimuthal shift factors and the sub-phase number. Also, various mirror-symmetrical twisted beams are constructed using the matrix flip scheme, further enriching the light structures of MTBs. Numerical simulation and experimental results are consistent. Furthermore, the capture and guide of microspheres via the MTBs are experimentally executed and demonstrate the feasibility and practicability of our generated MTBs. The various MTBs will likely give rise to potential applications in fabricating chiral nanostructures and manipulating microparticles.

Noise self-canceling picoscale twisted interferometer.

We show a noise self-canceling real-time picometer scale interferometer by exploiting the unique spiral phase structure of twisted light. We use a single cylindrical interference-lens to implement the twisted interferometer and perform simultaneous measurement on N phase-orthogonal single-pixel intensity pairs chosen on the petal of the daisy-flower-like interference pattern. A cancellation of various noises by three orders of magnitude was achieved in our setup compared with a conventional single-pixel detection, enabling a sub-100 picometer resolution in measuring a non-repetitive intracavity dynamic event in real-time. Furthermore, the noise cancellation capability of the twisted interferometer scales up statistically for higher radial and azimuthal quantum numbers of the twisted light. The proposed scheme could find applications in precision metrology and in developing analogous ideas for twisted acoustic beam, electron beams, and matter waves.

Strengthening Reinforced Beams Subjected to Pure Torsion by Near Surface Mounted Rebars

This paper investigates the possibility of strengthening Reinforced Concrete (RC) beams under pure torsion loadings. The torsional behaviour of strengthened RC beams with near-surface mounted steel and CFRP bars was investigated. The verification with the experimental work was performed to ensure the validity and accuracy which revealed a good agreement through the torque-rotation relationship, ultimate torque, and rotation, and crack pattern. This numerical study included testing of thirteen specimens (one of them was control beams while the remaining 12 were strengthened beams) with several parameters such as mounting spacing and configuration. The analytical results revealed that the addition of NSM rebar redistributed the internal stresses and enhanced the ultimate torsional strength, torque-rotation capacity, ductility, and energy absorption of the concrete beams. Most of the strengthened beams revealed the appearance of the cracks at a phase less than the reference beam by an average of (9%). Concerning the NSM strengthening, the CFRP bars provided a higher enhancement ratio when compared with the beams that strengthened with NSM steel rebar especially for the strengthening space equal to 130 mm and more. The ultimate torsional strength increased by (3.5%) and rotation decreased by (4%) approximately when the steel rebar was replaced by the carbon bar. The ductility and energy absorption of the analysed beams showed that the strengthening enhanced the ductility of the twisted beams. The ductility values varied according to the method of strengthening used, as it showed the highest values of the beam that was strengthened small spacing.

Effect of Twisting Phases on the Polarization Dynamics of a Vector Optical Field

The effect of twisting phases on the polarization dynamics of a vector vortex beam with a spatially variant state of polarization (SoP) is investigated theoretically and experimentally. The twisted vector vortex optical field with the cross-phase modulation is experimentally generated and observed. The presence of the twisting phases on the vector optical field results in novel propagation dynamics and the evolution of polarization states. The optical field performs the process of compressing and stretching the orthogonal polarization components and the linear-circular polarization conversions during propagation. Different from the scalar twisted beams, the orthogonal polarization components of twisted vector beams can be compressed along the same azimuthal angle or the orthogonal azimuthal angles, depending on the signs of the twist intensity coefficients. For an anisotropic twisted optical field, the SoP and the optical field rotate, and the rotation behaviors are sensitively dependent on the twisting coefficients. Furthermore, the two vortex topological charges embedded within the orthogonal polarization component of a twisted vector vortex beam can be simultaneously identified by looking at the interference fringes of the twisting phases. This work reveals that a vector vortex beam and its SoP can be dynamically manipulated by twisting phases.

Particle acceleration by twisted laser beams

We consider particle acceleration in plasmas, using twisted laser beams, or beams with orbital angular momentum. We discuss different acceleration processes using two LG laser modes, which include donut wakefield, beat-wave and self-torque acceleration, and compare the respective properties. We show that a self-torque configuration is able to produce azimuthal acceleration and can therefore be considered as an alternative method to produce helical electron beams.

Direct Monitoring of Conical Intersection Passage via Electronic Coherences in Twisted X-Ray Diffraction.

Quantum coherences in electronic motions play a critical role in determining the pathways and outcomes of virtually all photophysical and photochemical molecular processes. However, the direct observation of electronic coherences in the vicinity of conical intersections remains a formidable challenge. We propose a novel time-resolved twisted x-ray diffraction technique that can directly monitor the electronic coherences created as the molecule passes through a conical intersection. We show that the contribution of electronic populations to this signal is canceled out when using twisted x-ray beams that carry a light orbital angular momentum, providing a direct measurement of transient electronic coherences in gas-phase molecules.

Polarization and coherence properties in self-healing propagation of a partially coherent radially polarized twisted beam.

With the help of generalized Huygens-Fresnel integral, an analytical expression for the self-healing of a partially coherent radially polarized twisted (PCRPT) beam is derived. The coherence and polarization properties of the PCRPT beam in self-healing propagation are studied in detail. It shows that the existence of the twist phase is a double-edged sword for the self-healing properties of the beam. With the increase of the twist factor, the self-healing ability of beam intensity distribution decreases. However, the anti-disturbance performance of beam polarization improves at the same time. Besides, the polarization and coherence distribution of the beam are proved that own a slight self-healing ability when the obstacle is small. Our results will be helpful to the fields of optical tweezers, microscopy, optical communication, and so on.

Geometric metasurface for polarization synthesis and multidimensional multiplexing of terahertz converged vortices

The investigation of converged twisted beams with a helical phase structure has a remarkable impact on both fundamental physics and practical applications. Geometric metasurfaces consisting of individually orientated metal/dielectric meta-atoms provide an ultracompact platform for generating converged vortices. However, it is still challenging to simultaneously focus left-handed and right-handed circularly polarized incident beams with pure geometric phase modulation, which hinders the independent operation on topological charges between these two helical components. Here we propose and experimentally demonstrate an approach to design terahertz geometric metasurfaces that can generate helicity-independent converged vortices with homogeneous polarization states by the superposition of two orthogonal helical vortices with identical topological charges. Furthermore, the multiplexing of polarization-rotatable multiple vortices in multiple dimensions, i.e., in both longitudinal and transverse directions, and a vortex with an extended focal depth is confirmed by embedding polarization modulation into the geometric metasurfaces. The demonstrated approach provides a new way to simultaneously manipulate orthogonal helical components and expand the design dimension, enabling new applications of geometric metasurface devices in polarization optics, twisted-beam related image and edge detection, high capacity optical communication, and quantum information processing, to name a few.

Two-dimensional excitons from twisted light and the fate of the photon's orbital angular momentum

As the bound state of two oppositely charged particles, excitons emerge from optically excited semiconductors as the electronic analogue of a hydrogen atom. In the two-dimensional (2D) case, realized either in quantum well systems or truly 2D materials such as transition metal dichalcogenides, the relative motion of an exciton is described by two quantum numbers: the principal quantum number $n$, and a quantum number $j$ for the angular momentum along the perpendicular axis. Conservation of angular momentum demands that only the $j=0$ states of the excitons are optically active in a system illuminated by plane waves. Here we consider the case for spatially structured light sources, specifically for twisted light beams with non-zero orbital angular momentum per photon. Under the so-called dipole approximation where the spatial variations of the light source occur on length scales much larger than the size of the semiconductor's unit cell, we show that the photon (linear and/or angular) momentum is coupled to the center-of-mass (linear and/or angular) momentum of the exciton. Our study establishes that the selection rule for the internal states of the exciton, and thus the exciton spectrum, is independent from the spatial structure of the light source.

Abruptly Autofocusing Twisted Optical Bottle Beams

Versatile twisted optical bottles (TOBs) generated from a series of abruptly autofocusing beams are derived numerically and experimentally. This kind of TOB beam utilizes dual-autofocus, optical-vortex, and astigmatic phase properties to structure one or more three-dimensional closed dark regions, the bottle space. Closed off-axis multiple optical bottles are generated and observed experimentally. The twist angles can individually affect the rotation of the TOB. The experimental results agree well with our numerical simulations. The relationships between the astigmatic phase, the length of the TOB, the topological order of the vortex, and the width of the TOB are determined. Additionally, stable mesocarbon particles trapped by TOB beams are observed in side view successfully. These TOBs offer more possibilities to trap, sort, and clean microparticles or nanoparticles and improve optical manipulation systems in the future.

Twisted elliptical multi-Gaussian Schell-model beams and their propagation properties: reply.

The correction pointed out by Nemes [J. Opt. Soc. Am. A39, 667 (2022)JOAOD60740-323210.1364/JOSAA.435756] to our paper [J. Opt. Soc. Am. A37, 89 (2020)JOAOD60740-323210.1364/JOSAA.37.000089] is welcome. The author claims that the beam propagation factors Mx2, My2 are inadequate to characterize twisted elliptical multi-Gaussian Schell-model (TEMGSM) beams, which are introduced in our paper. Further, the author points out that there are two pairs of independent, normalized invariants to classify the TEMGSM beams. In this reply, we present the numerical example of the normalized intrinsic propagation invariants of the TEMGSM beams.

Twisted elliptical multi-Gaussian Schell-model beams and their propagation properties: comment.

The inadequacy of using beam propagation factors Mx2, My2 for general astigmatic beams as those analyzed in the paper J. Opt. Soc. Am. A37, 89 (2020)JOAOD60740-323210.1364/JOSAA.37.000089 is discussed, and the appropriate beam invariants to be considered are presented.

Propagation characteristics of partially coherent twisted Laguerre-Gaussian beam in atmospheric turbulence with anisotropy

Analytical formulas for average intensity of partially coherent twisted Laguerre-Gaussian beam (PCTLGB) propagating through anisotropic atmospheric turbulence are derived based on the extended Huygens–Fresnel principle. Our results indicate that PCTLGB gradually evolves from dark hollow distribution to flat-topped distribution and finally degenerates to Gaussian distribution both in free space and atmospheric turbulence. We also find that the effect of atmospheric turbulence on propagation characteristics for PCTLGB can be effectively reduced by adjusting twisted factor, topological charge and beam order. An important result is that PCTLGB has stronger anti-turbulence ability when vortex phase and twisted phase have the same chirality, and it can resist atmospheric turbulence more effectively than the beam without twisted phase under the same conditions. Our results will be useful in free space optical communication, lidar detection and imaging.

Radially Polarized Twisted Rectangular Multi-Gaussian Schell-Model Beam and Its Propagation Properties

Radially Polarized Twisted Rectangular Multi-Gaussian Schell-Model Beam and Its Propagation Properties

Twisted electromagnetic elliptical multi-Gaussian Schell-model beams and their transmission in random media.

We extend the scalar elliptical multi-Gaussian Schell-model (EMGSM) beams with twist phase to the electromagnetic domain and obtain the analytical expression for the propagation of the electromagnetic twisted EMGSM beams through random media. The twist phase-induced changes of the spectral density and degree of polarization of such beams on propagation are studied numerically. Results show that by adjusting the twist factor and the correlated parameters of the source, both the spectral density and degree of polarization not only rotate around the propagation axis but also exhibit diverse shapes. The flattopped ellipse-like and diamond-like shape maintain over a relatively long propagation distance and finally involve into Gaussian-like shape due to stronger atmospheric turbulence. The results will be useful in optical trapping and optical communication.

Generating a twisted Gaussian Schell-model beam with a coherent-mode superposition

Twist phase is a nontrivial second-order phase that only exists in a partially coherent field, providing a new degree of freedom for manipulating statistical properties of random light. However, in the existing experimental methods either it is difficult to control the twist phase or requires a large number of pseudo/random modes to approximately synthesize the twisted beams. In this work, we demonstrate a simple and efficient approach, based on the superposition of mutually orthogonal Laguerre-Gaussian modes with appropriate mode weights, to generate the twisted beams with a controllable twist phase. We show that a smaller number of modes are required to synthesize the twisted beams, compared to the pseudo-mode superposition, because the orthogonal coherent modes are used in the present method. We experimentally generate the twisted Gaussian Schell-model beams with controllable strength of twist phase and measure their degree of coherence and average intensity behavior during propagation. The experimental results agree well with the theoretical predictions. Our approach will promote the application of the twisted partially coherent beams in optical imaging and free-space optical communications.

Torus-knot angular momentum in twisted attosecond pulses from high-order harmonic generation

Bicircular twisted Laguerre-Gaussian beams possess a definite torus knot angular momentum (TKAM) $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{{J}_{\ensuremath{\gamma}}}=\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{L}+\ensuremath{\gamma}\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{S}$ as an alternative a form of angular momentum. TKAM is conserved in nonlinear atomic processes such as high harmonic generation and can be classified by a time delay parameter $\ensuremath{\tau}$ and a coordination parameter $\ensuremath{\gamma}$. These parameters are defined by the respective projected orbital angular momentum ${\ensuremath{\ell}}_{i}$ and the energy $\ensuremath{\hbar}{p}_{i}{\ensuremath{\omega}}_{0}$ of the two superimposed Laguerre-Gaussian beams. We derive a consistent geometric method to determine $\ensuremath{\tau}$ and $\ensuremath{\gamma}$ from the driving beam as well as from the high harmonic radiation. The method relates both invariance parameters ($\ensuremath{\tau}$ and $\ensuremath{\gamma}$) to a torus knot which can be constructed from the emitted high harmonic radiation. These knots are constructed from the spatiotemporal evolution of the electric field of the respective high harmonic radiation or the driving beam. We demonstrate the classification of the invariance parameters for a planar atomic gas target irradiated by bicircular Laguerre-Gaussian beams explicitly. In addition, we demonstrate that the respective torus knots determined by $\ensuremath{\tau}$ and $\ensuremath{\gamma}$ can be mapped onto each other within minor modifications. The numerical calculations are done within the strong-field approximation and the associated quantum orbit approach. Therefore, we also briefly review high harmonic generation by bicircular twisted light beams. This introduced geometric method is a different approach to interpret the invariance parameters $\ensuremath{\tau}$ and $\ensuremath{\gamma}$, as well as their underlying relations, compared to a purely formal derivation. The investigations presented in this work are in good agreement with previous findings and provide insight into the dynamical symmetry of TKAM in the context of high harmonic generation induced by bicircular twisted Laguerre-Gaussian beams.

Generating non-uniformly correlated twisted sources.

The inverse method of proving the twistability of cross-spectral density (CSD) inevitably falls into spontaneous difficulties. Based on a nonnegative self-consistent design guideline for generating genuine CSDs introduced by Gori and Santarsiero, we demonstrate a feasible way for twisting partially coherent sources by sticking a Schell-model function to CSDs, which also determines the upper bound of the twisting strength. Analysis shows that the degree of coherence of a new class of twisted pseudo-Gaussian Schell-model beam is neither shift invariant nor shift-circular symmetric. In the presence of a vortex phase, the two different types of chiral phases affect each other and together control the propagation behavior. We further carry out an experiment to generate this non-uniformly correlated twisted beam using weighted superposition of mutually uncorrelated pseudo modes. The result is beneficial for devising nontrivial twisted beams and offers new opportunities.

Three-dimensional spiral motion of microparticles by a binary-phase logarithmic-spiral zone plate.

Acoustic vortex beams, which have both linear and angular momentum, can be used to make precise acoustic tweezers. Limited by the symmetry of a normal vortex beam, these tweezers are usually used for trapping or rotating particles in two dimensions. Here, the three-dimensional spiral motion of two soft particles of different sizes was realized using a vortex beam with a twisted focus, which was synthesized by a silicone binary-phase logarithmic-spiral zone plate. Numerical simulations and experimental measurements demonstrated that the beam had anisotropic focuses of crescent transverse intensity profiles and a screw phase dislocation with a singularity at the center. Experiments showed that a small particle (k0r ≈ 1.3) can follow the twisted intensity of the beam, but a large particle (k0r ≈ 4.7) spirals up away from the twisted field pattern. This is attributed to the dominant gradient force for the small particle, whereas the scattering effect induced a scattering force combined with a gradient force for the large particle. This focused twisted beam, which was generated with a structured silicone plate, and the three-dimensional spiral motion of microparticles, advance the development of simple, compact, and disposable acoustic devices for the precise and diverse manipulation of microparticles.

Quadrupole absorption rate for atoms in circularly-polarized optical vortices

Twisted light beams, or optical vortices, have been used to drive the circular motion of microscopic particles in optical tweezers and have been shown to generate vortices in quantum gases. Recent studies have established that electric quadrupole interactions can mediate an orbital angular momentum exchange between twisted light and the electronic degrees of freedom of atoms. Here we consider a quadrupole atomic transition mediated by a circularly-polarized optical vortex. We evaluate the transfer rate of the optical angular momentum to a Ca+ ion involving the 42S1∕2→32D5∕2 quadrupole transition and explain how the polarization state and the topological charge of the vortex beam determine the selection rules.