We experimentally generate cylindrically polarized wavepackets with transverse orbital angular momentum, demonstrating the coexistence of spatiotemporal optical vortex with spatial polarization singularity. The results in this paper extend the study of spatiotemporal wavepackets to a broader scope, paving the way for its applications in various areas such as light-matter interaction, optical tweezers, spatiotemporal spin-orbit angular momentum coupling, etc.

Experimental demonstration of cylindrical vector spatiotemporal optical vortex

Optical aberrations have been studied for centuries, placing fundamental limits on the achievable resolution in focusing and imaging. In the context of structured light, the spatial pattern is distorted in amplitude and phase, often arising from optical imperfections, element misalignment, or even from dynamic processes due to propagation through perturbing media such as living tissue, free-space, underwater and optical fibre. Here we show that the polarisation inhomogeneity that defines vectorial structured light is immune to all such perturbations, provided they are unitary. By way of example, we study the robustness of vector vortex beams to tilted lenses and atmospheric turbulence, both highly asymmetric aberrations, demonstrating that the inhomogeneous nature of the polarisation remains unaltered from the near-field to far-field, even as the structure itself changes. The unitary nature of the channel allows us to undo this change through a simple lossless operation, tailoring light that appears robust in all its spatial structure regardless of the medium. Our insight highlights the overlooked role of measurement in describing classical vectorial light fields, in doing so resolving prior contradictory reports on the robustness of vector beams in complex media. This paves the way to the versatile application of vectorial structured light, even through non-ideal optical systems, crucial in applications such as imaging deep into tissue and optical communication across noisy channels. 

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Revealing the invariance of vectorial structured light in complex media

Abstract The simultaneous and independent measurement of multiple movement forms is a significant issue to be solved in research. In this paper, we proposed a method that combines the self-interference of conjugated optical vortices and external interference of plane waves, and successfully realize the independent measurement of both rotation and rectilinear motion. Three kinds of interference schemes based on vortex beams are analyzed theoretically and verified experimentally. The results show that the double interference between conjugated optical vortices and Gaussian beam can detect the motion along and perpendicular to the beam propagation direction even under complex motion background, providing a powerful way to detect the multiple movement forms of a target. Our work may pave a new way for the detection of spatial noncooperative targets and stimulate the invention of new detection equipment.

Compound motion detection based on OAM interferometry

The rotational Doppler effect has attracted extensive attention, caused by the angular momentum and energy exchange between rotating objects and waves. However, most previous works used a simple rotation frame, which made use of only a single-round angular momentum and energy exchange. We propose and demonstrate a frame containing a spiral phase plate cascaded with rotating targets to make an amplification of the traditional Doppler shift, and reduce the diffusion of orbital angular momentum modes by half, which means the distance of practical application is doubled theoretically. To this end, an experiment is carried out to verify the frame. It shows a more practical, convenient, and non-destructive method to measure the rotational speed of a remote target.

Remote angular velocity measurement by the cascaded rotational Doppler effect.

Vector optical field (VOF) manipulation greatly extended the boundaries of traditional scalar optics over the past decades. Meanwhile, the newly emerging techniques enabled by structural functional optical materials have driven the research domain into the subwavelength regime, where abundant new physical phenomena and technologies have been discovered and exploited for practical applications. In this Tutorial, we outline the basic principles, methodologies, and applications of VOF via structural functional materials. Among various technical routes, we focus on the metasurface-based approaches, which show obvious advantages regarding the design flexibility, the compactness of systems, and the overall performances. Both forward and inverse design methods based on the rigorous solution of Maxwell's equations are presented, which provide a valuable basis for future researchers. Finally, we discuss the generalized optical laws and conventions based on VOF manipulation. The applications in optical imaging, communications, precision measurement, laser fabrication, etc. are highlighted.

Vector optical field manipulation via structural functional materials: Tutorial

The Doppler effect is a universal wave phenomenon that has spurred a myriad of applications. In early manifestations, it was implemented by interference with a reference wave to infer linear velocities along the direction of motion, and more recently lateral and angular velocities using scalar phase structured light. A consequence of the scalar wave approach is that it is technically challenging to directly deduce the motion direction of moving targets. Here we overcome this challenge using vectorially structured light with spatially variant polarization, allowing the velocity and motion direction of a moving particle to be fully determined. Using what we call a vectorial Doppler effect, we conduct a proof of principle experiment and successfully measure the rotational velocity (magnitude and direction) of a moving isotropic particle. The instantaneous position of the moving particle is also tracked under the conditions of knowing its starting position and continuous tracking. Additionally, we discuss its applicability to anisotropic particle detection, and show its potential to distinguish the rotation and spin of the anisotropic particle and measure its rotational velocity and spin speed (magnitude and direction). Our demonstration opens the path to vectorial Doppler metrology for detection of universal motion vectors with vectorially structured light. The Doppler effect is a wave phenomenon that can find the magnitude of velocity of moving targets with scalar waves. Here, the authors use vectorially structured light with spatially variant polarization to fully determine both the magnitude of velocity and motion direction of a moving particle.

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Vectorial Doppler metrology.

We propose and experimentally verify a flexible and robust rotational Doppler velocimetry for detecting a remotely rotating target in situ, which uses a ring-core fiber that can stably transmit the orbital angular momentum superposed modes.

Flexible and Robust Detection of a Remotely Rotating Target using Fiber-Guided Orbital Angular Momentum Superposed Modes

Optical interferometry has made tremendous development in fundamentals and applications of light. Traditional optical interferometry using simplex plane phase light just allows one dimensional measurement, but incapable of retrieving multidimensional movement. Here, we present new structured light interferometry to achieve simultaneous determination of rotational and translational dimensionalities (magnitudes and directions) of moving objects by fully exploiting multiple degree of freedoms of structured light. Such multidimensional detectability benefits from multiple amplitude and sign distinguishable Doppler shifts of structured light by analyzing relative amplitude and phase information of Doppler signals. We experimentally demonstrate two interferometric schemes to individually retrieve both translational and rotational velocity vectors of two kinds of moving objects (small particles and bulk targets). These interferometric schemes based on structured light show remarkable function enhancement for simultaneous measurement of multidimensional motion vectors, compared with traditional interferometry, and thus may expand optical interferometry to multidimensional metrology in remote sensing, intelligent manufacturing and engineering.

Structured light interferometry

The invention discloses a structured light interference measurement method and a structured light interference measurement device. The structured light interference measurement method achieves the simultaneous measurement of full-vector motion information of translation and rotation speeds and directions of a moving object by means of structured light interference. The structured light interference measurement device comprises a laser, a beam splitter, a reflector, a beam expander, a reflective device, a beam combiner and a detection device, wherein the laser outputs Gaussian light, and the beam splitter splits the Gaussian light into detection light and reference light; the beam expander adjusts the spot size of the detection light, the reflective device is attached to the surface of themoving object to generate structured light, the beam combiner performs interference superposition on the structured light carrying object motion information and reference light to obtain an interference light field, and the detection device detects the interference light field to obtain full-vector motion information of the moving object. The structured light interference measurement method breaksthrough the limitation that the traditional scheme cannot measure the full-vector motion information of the translation and rotation motion speeds of the object at the same time, has a wide application prospect in the aspects of optical measurement, sensing and the like, and fills up the blank in the related technical fields.

Structured light interference measurement method and device

The invention discloses a vector Doppler effect measurement method and device. The method achieves the simultaneous measurement of the movement speed and direction of a transverse moving object through a polarization non-uniform distribution vector light field. The device comprises a polarization non-uniform distribution vector light source and a detection device, wherein the polarization non-uniform distribution vector light source is used for generating a transverse vector light field or a cylindrical vector light field, and the detection device is used for detecting vector Doppler signals obtained by sampling different spatial positions in the vector light field by the transverse moving object, so that the movement speed and direction of the transverse moving object are measured at thesame time. According to the invention, full-vector information of the moving speed and direction of the transverse moving object can be measured simultaneously, the method breaks through the limitation that the traditional scheme can only measure the speed of transverse moving object and cannot distinguish the direction, can also monitor the motion state of the object in real time, has wide application prospects in the aspects of optical measurement, sensing and the like, and fills up the blank in the related technical fields.

Zhenyu Wan

Papers

Vectorial Doppler metrology.

Flexible and Robust Detection of a Remotely Rotating Target using Fiber-Guided Orbital Angular Momentum Superposed Modes

Structured light interferometry

Structured light interference measurement method and device

Vector Doppler effect measurement method and device