Radially dependent angular acceleration of twisted light

Generation of superposed orbital angular momentum beams using a free-electron laser oscillator

With wavelength tunability, free-electron lasers (FELs) are well-suited for generating orbital angular momentum (OAM) beams in a wide photon energy range. We report here the first experimental demonstration of OAM beam generation using an oscillator FEL with the tens of picosecond pulse duration. Lasing around 458 nm, we have produced the four lowest orders of superposed Laguerre-Gaussian beams using a very long FEL resonator of 53.73 m. The produced beams have good beam quality, excellent stability, and substantial average power. We have also developed a pulsed operation mode for these beams with a highly reproducible temporal structure for a range of repetition rate of 1-30 Hz. This development can be extended to short wavelengths, for example to x-rays using a future x-ray FEL oscillator. The OAM operation of such a storage-ring FEL also paves the way for the generation of OAM gamma-ray beams via inverse Compton scattering.

Optical vortices shape optical tornados

We demonstrate that by seeding an accelerating ring-Airy beam with a finite number of off-axis optical vortices, it transforms into a tornado wave (ToW) upon propagation. Using numerical simulations, we show that both the spiraling high-intensity lobes and the optical vortices exhibit angular acceleration and follow interwinding braid-like trajectories. Likewise, we study the effect of the number, position, and topological charge of the vortices on the propagation dynamics and reveal the connection between optical vortices and optical tornados.

Switchable optical ring lattice in free space

Optical lattices with spatially regular structures have recently attracted considerable attention across physics and optics communities. In particular, due to the increasing emergence of new structured light fields, diverse lattices with rich topology are being generated via multi-beam interference. Here, we report a specific ring lattice with radial lobe structures generated via superposition of two ring Airy vortex beams (RAVBs). We show that the lattice morphology evolves upon propagation in free space, switching from a bright-ring lattice to dark-ring lattice and even to fascinating multilayer texture. This underlying physical mechanism is related to the variation of the unique intermodal phase between the RAVBs as well as topological energy flow with symmetry breaking. Our finds provide an approach for engineering customized ring lattices to inspire a wide variety of new applications.

Experimental observation and manipulation of optical tornado waves

We report experimental realization and manipulation of optical tornado waves (ToWs). By controlling the self-focusing length, total angular momentum, and foci deviation of ToWs, the propagation properties of optical ToWs, especially their angular velocity, can be manipulated. Controlling the accumulated rotation angle of the main intensity lobes of ToWs from 0° through 1100° is experimentally demonstrated, and their angular velocity is predicted to be the highest around the foci overlap situation. Our experimental results are in good agreement with numerical results.

Generation and control of tornado waves by means of ring swallowtail vortex beams

Tornado waves (ToWs), which refer to a light that accelerates and twists over both the radial and the angular directions, have gained a great deal of interest since the concept was introduced by Brimis et al [Opt. Lett.45, 280 (2020)10.1364/OL.45.000280]. In this paper, we superimpose two pairs of ring swallowtail vortex beams (RSVBs) to generate ToWs and we call them tornado swallowtail waves (ToSWs). Each pair consists of RSVBs while carrying orbital angular momentum of opposite helicity and slightly different with the radius of the main ring of RSVBs. The waves spiral forward and reveal intensity maxima, exhibiting a tornado-like intensity profile during propagation. Meanwhile, the angular acceleration of the ToSWs is illustrated via tracing the angular position of the high-intensity main lobes. It is found that ToSWs present very high values of angular acceleration. Compared with typical tornado waves, ToSWs are more diverse and tunable, giving a new degree of freedom to tailor the propagation dynamics due to the flexibility of the swallowtail diffraction catastrophe. In addition, we confirm such waves experimentally and the results match well with the numerical ones. Also, we demonstrate the ability of optical manipulation of ToSWs for the first time in that they allow for particles not only to be trapped but also to be rotated. Finally, we analyze the poynting vectors and power exchange of ToSWs to demonstrate convincingly the physical mechanism.

Experimental generation of the polycyclic tornado circular swallowtail beam with self-healing and auto-focusing

In this paper, the polycyclic tornado circular swallowtail beam (PTCSB) with autofocusing and self-healing properties is generated numerically and experimentally and their properties are investigated. Compared with the circular swallowtail beam (CSB), the optical distribution of the PTCSB presents a tornado pattern during the propagation. The number of spiral stripes, as well as the orientation of the rotation, can be adjusted by the number and the sign of the topological charge. The Poynting vectors and the orbital angular momentum are employed to investigate the physical mechanism of beam-rotating. In addition, we also introduce a sector-shaped opaque obstacle to investigate the self-healing property of the PTCSB, passing through it with different center angles and discuss the influence of the scaling factor along the propagation direction. Our results may expand the potential applications in the optical spanner and material processing.

Helical plasma filaments from the self-channeling of intense femtosecond laser pulses in optical fibers

We experimentally and numerically study the ignition of helical-shaped plasma filaments in standard optical fibers. Femtosecond pulses with megawatt peak power with proper off-axis and tilted coupling in the fiber core produce plasma skew rays. These last for distances as long as 1000 wavelengths thanks to a combination of linear waveguiding and the self-channeling effect. Peculiar is the case of graded-index multimode fibers; here the spatial self-imaging places constraints on the helix pitch. These results may find applications for fabricating fibers with helical-shaped core micro-structuration as well as for designing laser components and three-dimensional optical memories.

Tiny velocity measurement using rotating petal-like mode of orbital angular momentum

A novel, to the best of our knowledge, tiny velocity measurement system is proposed and demonstrated. This proposed system employs an interference structure in which the reference and measurement paths are filled by two light beams carrying opposite-sign orbital angular momentum (OAM), respectively. The tiny velocity to be measured in the measurement path causes the change of the light path and results in a time-varying phase shift between the reference and measurement paths. This time-varying phase shift leads to the rotation of the petal-like light spot obtained by the interference between two paths. The rotating angular velocity of the petal-like light spot is proportional to the time-varying phase shift caused by the tiny velocity, and it is measured by a chopper and a single-point detector instead of array detectors. This proposed system has a simple structure and achieves a high-accuracy tiny velocity measurement with a measurement error rate that is less than 10 nm/s.

High-accuracy longitudinal position measurement using self-accelerating light

Radially self-accelerating light exhibits an intensity pattern that describes a spiraling trajectory around the optical axis as the beam propagates. In this article, we show in simulation and experiment how such beams can be used to perform a high-accuracy distance measurement with respect to a reference using simple off-axis intensity detection. We demonstrate that generating beams whose intensity pattern simultaneously spirals with fast and slow rotation components enables a distance measurement with high accuracy over a broad range, using the high and low rotation frequency, respectively. In our experiment, we achieve an accuracy of around 2 µm over a longitudinal range of more than 2 mm using a single beam and only two quadrant detectors. Because our method relies on single-beam interference and only requires a static generation and simple intensity measurements, it is intrinsically stable and could find applications in high-speed measurements of longitudinal position.

Accelerating polarization structures in vectorial fields

We generate optical fields whose polarization structures not only rotate about their propagation axis but also can be controlled to accelerate independently from their spatial profile. We show that by combining accelerated intensity transport with orthogonal polarization states, we can produce a vector beam that displays optical activity with periodical acceleration and deceleration of the Stokes vector during propagation. We achieve this with orthogonal, scalar fields, represented by weighted superpositions of oppositely charged Bessel beams. In addition to their creation, we show that the Stokes vector can be made to accelerate or decelerate at specific locations along the Poincaré sphere by tailoring the generating basis. We also witness an optical current, or intensity transport, between local positions in the field that corresponds with the occurrence of the state-of-polarization accelerating or decelerating.

Trajectories and rotations controlled off-axis winding beams in nonlocal nonlinear media

Off-axis winding beams are discussed in nonlocal nonlinear media, whose trajectories and rotations can be controlled by the optical power and the orbital angular momentum (OAM). By changing optical power or the OAM, optical beams can propagate along different trajectories including ellipses, circles, rectangles, rhombus and so on. Due to the OAM stemming from a combination of the cross phase and the vortex phase, different parts of the optical beam exhibit distinct rotating characteristics. The elliptic envelope revolves around the beam center resulting from the cross phase, while optical peaks spin about their own axes because of the vortex phase. The two rotating velocities can be adjusted separately by the optical power and the OAM, respectively. Optical power plays a global role on the revolving velocity and the spinning velocity and can enhance the two rotations synchronously. In contrast, the OAM has a local effect and can change the relative magnitudes of these two velocities. Our results may provide a tailoring tool of evolution properties for optical beams. The property of controllable trajectories may find applications in all-optical controlling. While, the controllable rotating property can be used as in optical spanners to operate micro- or nanoparticles.

Controllable conversion between Hermite Gaussian and Laguerre Gaussian modes due to cross phase

The inter-conversion between the Hermite-Gaussian (HG) modes and the Laguerre-Gaussian (LG) modes is discussed. The HG beams carrying a cross phase can evolve into the LG modes, and vice versa, a LG mode with the cross phase can also transform to the HG mode. This conversion process is accompanied by the intensity rotations of optical beams, and their angular velocities and acceleration are both radially dependent. Initially, the outer intensity peak and the inner intensity hollow rotate in the opposite directions. After that they tend to rotate in the same direction with different velocities. Different patterns can be generated in a controllable way by adjusting the cross phase coefficients. The theoretical results provide a controllable approach for modes generation by engineering the phase structure.

Generation of structured light by multilevel orbital angular momentum holograms

Structured light has been created by a myriad of near-and far-field techniques and has found both classical and quantum applications. In the case of orbital angular momentum (OAM), continuous spiral phase patterns in dynamic or geometric phase are often employed with the phase patterns existing across the entire transverse plane. Here, we exploit the uncertain relationship between OAM and angle in order to create structured OAM fields by using multilevel OAM holograms. We show theoretically and experimentally that only a multilevel angular phase contour in the near-field is needed to create structured OAM light in the far-field, exploiting the reciprocal nature of angular momentum and angle. We use this approach to demonstrate exotic 3D structured light control to show the Poynting vector's evolution in such fields and to highlight the physics underlying this phenomenon.

Control on helical filaments by twisted beams in a nonlinear CS2 medium

Curved filament with large bent angle and controllable propagation behavior has always been great expectation and challenge due to its novelty and complexity. The unique properties of curved filaments make it possible to achieve many applications in micro-fabrication, spectroscopy and meteorology. Here we realize experimentally and theoretically control on helical filaments induced by twisted beams in CS₂. The results show that helical filaments exhibit a robust pattern and high rotation rate. Specific intensity pattern of the twisted beam confines the filaments in fixed relative position and the azimuthal energy flux drives the rotating of the filamentation pattern. In addition, we demonstrated that the global orbital angular momentum (OAM) of twisted beams is still conservative to be zero, but local OAMs exhibit distinct variation during nonlinear propagation. Our idea has its significance which realizes the construction of helical filaments with flexibility and controllability and then facilitates to push the development of related researches.

Hyperbolic accelerating beams and their relation with Hermite-Gaussian beams

We derive the initial distributions of phase and complex amplitude of accelerating beams with arbitrary predesigned hyperbolic trajectories using the caustic-design method and explore the relation between these beams and Hermite-Gaussian beams. The results show the hyperbolic accelerating beams are a larger class of beams than Hermite-Gaussian beams. When the bending parameter is an integer, the hyperbolic accelerating beams have a similar initial complex amplitude distribution and almost the same propagating characteristics as Hermite-Gaussian beams. Through the analysis of the ray-based method, we also derive an approximate expression for the initial complex amplitude of Hermite-Gaussian beams after introducing an amplitude distribution function. Although the proposed approximate expressions of complex amplitude are more complex than the usually used Hermite-Gaussian function, they explicitly indicate the information on local amplitude, wave vector, and internal ray structure (including caustics) of these beams and thus provide us clearer geometrical insights into these beams.

Real and virtual propagation dynamics of angular accelerating white light beams

Accelerating waves have received significant attention of late, first in the optical domain and later in the form of electron matter waves, and have found numerous applications in non-linear optics, material processing, microscopy, particle manipulation and laser plasma interactions. Here we create angular accelerating light beams with a potentially unlimited acceleration rate. By employing wavelength independent digital holograms for the creation and propagation of white light beams, we are able to study the resulting propagation in real and virtual space. We find that dephasing occurs for real propagation and that this can be compensated for in a virtual propagation scheme when single plane dynamics are important. Our work offers new insights into the propagation dynamics of such beams and provides a versatile tool for further investigations into propagating structured light fields.