Optical vortices 30 years on: OAM manipulation from topological charge to multiple singularities

Intracavity spatially modulated metasurfaces for a wavelength-tunable figure-9 vortex fiber laser

Intracavity optical metasurfaces with compact and flexible light manipulation capabilities, effectively enrich the implementation of miniaturized and user-friendly orbital angular momentum (OAM) laser sources. Here we demonstrate a wavelength-tunable figure-9 Yb-doped vortex fiber laser solely with standard non-polarization-maintaining single-mode fibers, which utilizes a gap-surface plasmon (GSP) metasurface as the intracavity mode regulation component to generate OAM beams, extending the avenues and related applications for cost-effective OAM laser sources. Gained by the broadband operation range of the metasurface, the figure-9 fiber laser could emit OAM light with center wavelength tunable from 1020 nm to 1060 nm and of high mode purity (about 90%). OAM beams with different topological charges such as l = ±1 have been obtained by changing the metasurface design. The proposed fiber laser with the intracavity GSP metasurface provides a reliable and customized output of OAM beams at the laser source, holding great promise for a wide range of applications in optical communications, sensing, and super-resolution imaging.

Fourier reciprocity between generalized elliptical Gaussian and elegant elliptical Hermite-Gaussian beams carrying orbital angular momenta

We explore two distinct families of orbital angular momentum carrying light beams, which we refer to as generalized elliptical Gaussian and elegant elliptical Hermite-Gaussian vortex beams, respectively. We show that the fields of the two vortex families are related via a Fourier transform. Hence, one family can be viewed as a source of the far-field intensity distribution of the other and vice versa. We also examine the orbital angular momentum evolution of both beam families on their free space propagation and establish a relationship between the orbital angular momentum, TC, and beam ellipticity factors. Our results may find applications to optical communications and imaging with structured light.

Reuleaux triangle core fiber with triple rotational symmetry

A Reuleaux triangle core fiber (RTF) with triple rotational symmetry is proposed and fabricated. Then the RTF is twisted to form the chiral fiber grating, which converts the core mode into a vortex mode containing 3rd-order orbital angular momentum (OAM). Based on the Fourier expansion of the core boundary, the straight-sided and arc-sided triangular core profiles were analyzed, revealing the mechanism of high-efficiency OAM3 generation. The experimental results show a 3rd-order vortex mode with a high conversion efficiency and purity, and the polarization-independent characteristics endowed by the core shape are also confirmed. The proposed RTF provides a new, to the best of our knowledge, way for higher-order vortex beam generation, which can be used in optical fiber communication systems with OAM multiplexing.

Tight focusing through scattering media via a Bessel-basis transmission matrix

The transmission matrix (TM) is a powerful tool for focusing light through scattering media. Here, we demonstrate a Bessel-basis TM that enables tight focusing through the scattering media and reduces the full width at half maximum of the focus by 23% on average, as compared to the normally used Hadamard-basis TM. To measure the Bessel-basis TM, we establish a common-path inter-mode interferometer (IMI), which can fully utilize the pixels of the spatial light modulator, leading to an enhancement in the peak-to-background intensity ratio (PBR) of the focus. Experimental results suggest that the Bessel-basis TM can achieve a tighter focus behind the scattering media, and the PBR of the focus obtained by the IMI is around 14.3% higher than that achieved using the normal peripheral reference interferometry.

Topological charge identification of superimposed orbital angular momentum beams under turbulence using an attention mechanism

Due to the unique features, orbital angular momentum (OAM) beams have been widely explored for different applications. Accurate determination of the topological charge (TC) of these beams is crucial for their optimal utilization. In this paper, we propose a method that combines adaptive image processing techniques with a simple, parameter-free attention module (SimAM) based convolutional neural network to accurately identify the TC of high-order superimposed OAM beams. Experimental results demonstrate that under the combined influence of non-extreme light intensity and turbulence, it can achieve >95% identification accuracy of TCs ranging from ±1 to ±40. Moreover, even under partial-pattern-missing conditions, our method maintains an accuracy rate of over 80%. Compared with traditional attention mechanisms, SimAM does not require additional network design, significantly reducing the computational costs. Our approach showcases remarkable efficiency, robustness, and cost-effectiveness, making it adaptable to challenging factors such as non-uniform lighting and partially occluded light paths. This research provides a new direction for recognizing OAM modes with valuable implications for the future of communication systems.

Dynamical Control of Topology in Polar Skyrmions via Twisted Light

Twisted light carries a nonzero orbital angular momentum, that can be transferred from light to electrons and particles ranging from nanometers to micrometers. Up to now, the interplay between twisted light with dipolar systems has scarcely been explored, though the latter bear abundant forms of topologies such as skyrmions and embrace strong light-matter coupling. Here, using first-principles-based simulations, we show that twisted light can excite and drive dynamical polar skyrmions and transfer its nonzero winding number to ferroelectric ultrathin films. The skyrmion is successively created and annihilated alternately at the two interfaces, and experiences a periodic transition from a markedly "Bloch" to "Néel" character, accompanied with the emergence of a "Bloch point" topological defect with vanishing polarization. The dynamical evolution of skyrmions is connected to a constant jump of topological number between "0" and "1" over time. These intriguing phenomena are found to have an electrostatic origin. Our study thus demonstrates that, and explains why this unique light-matter interaction can be very powerful in creating and manipulating topological solitons in functional materials.

Propagation of integral and fractional perfect vortex beams in a gradient-index medium

The analytical expressions for the complex amplitude of integral and fractional perfect vortex (PV) beams propagating in a gradient-index (GRIN) medium are derived. The intensity and phase distributions, propagation trajectories, Poynting vectors, and the effects of topological charge and refractive index at the medium axis on the intensity of both beams in the medium are numerically investigated. It is shown that both beams propagate periodically in the GRIN medium with alternating spot focusing and reconstruction. Unlike the integral PV beam, the fractional PV beam has a dark line in intensity profiles and a line edge dislocation in phase distributions along the positive x-axis. These properties persist during the beam propagation in the GRIN medium. Moreover, the topological charge and the refractive index at the medium axis have little effect on the intensity of the PV beam propagating in the GRIN medium. The results presented in this paper may be useful for the application of integral and fractional PV beams in optical guiding and optical communications.

Adaptive methods of generating complex light arrays

Structured light arrays of various shapes have been a cornerstone in optical science, driven by the complexities of precise and adaptable generation. This study introduces an approach using a spatial light modulator (SLM) as a generator for these arrays. By projecting a holographic mask onto the SLM, it functions simultaneously as an optical convolution device, focusing mechanism, and structured light beam mask. Our approach offers unmatched versatility, allowing for the experimental fabrication of traditional beam arrays like azimuthal Laguerre-Gaussian (LG), Bessel-Gaussian (BG), and Hermite-Gauss (HG) in the far-field. Notably, it has enabled a method of generating Ince-Gauss (IG) and LG radial mode beam arrays using a convolution solution. Our system provides exceptional control over array periodicity and intensity distribution, bypassing the Talbot self-imaging phenomenon seen in traditional setups. We provide an in-depth theoretical discussion, supported by empirical evidence, of our far-field results. This method has vast potential for applications in optical communication, data processing, and multi-particle manipulation. It paves the way for rapid generation of structured light with high spatial frequencies and complex shapes, promising transformative advances in these domains.

Remote focusing optical tweezers for 3D imaging

We present a remote focusing optical tweezer utilizing a 4f symmetrical optical system to compensate the high-order aberration during annular light refocusing. The position of the optical trap can be adjusted beyond the range of one hundred micrometers in the axial direction by means of tuning the position of the mirror placed in the focal region of the illumination objective lens. This optical tweezer can be combined with a sectioning microscope to realize three-dimensional (3D) imaging, e.g., a confocal microscope using a single water immersion objective lens. All optical elements are placed in one side of the sample, which is very useful for application in fields such as radiation biology, where radiation or magnetism disturbance must be introduced on the other side of the sample. In the experiment, a 10 µm diameter silicon dioxide microsphere and pollen cells immersed in the water are translated along the axis using the optical tweezer and, meanwhile, the sectioning images are obtained using the confocal microscope.

Polarization-insensitive vortex beam generator by the holographic grating on an integrated multi-layer waveguide

An integrated polarization-insensitive vortex beam generator is proposed in this study. It is composed of a holographic grating on a multi-layer waveguide, which enables conversion of Transverse Electric (TE) and Transverse Magnetic (TM) waveguide modes to y-polarized and x-polarized optical vortex beams, respectively. The conversion efficiency and the phase fidelity are numerically analyzed, and the working bandwidth is about 100 nm from 1500 nm to 1600 nm with a phase fidelity above 0.7. Moreover, the vortex beam with the superposition of the y-polarization and x-polarization states can be obtained with the incident of the superposition of TE and TM waveguide modes.

High-efficiency integer multiplier for the orbital angular momentum of light

The spiral transformation has attracted an increasing interest in switching orbital angular momentum (OAM) modes. However, the efficiency is deteriorated by the inevitable gap between the turns of the spiral strips. In order to overcome the problem, a multiple-ring conformal mapping scheme is proposed for efficient multiplication of the OAM of light. The OAM mode at the input plane is divided into concentric rings, which are mapped to multiple sectors and connected into a ring at the output plane. This point-to-point mapping mechanism can avoid the generation of high-order diffraction, leading to high conversion efficiency. The scheme may underpin the development of optical communication and quantum key distribution in OAM-based systems.

Metasurface Holography with Multiplexing and Reconfigurability

Metasurface holography offers significant advantages, including a broad field of view, minimal noise, and high imaging quality, making it valuable across various optical domains such as 3D displays, VR, and color displays. However, most passive pure-structured metasurface holographic devices face a limitation: once fabricated, as their functionality remains fixed. In recent developments, the introduction of multiplexed and reconfigurable metasurfaces breaks this limitation. Here, the comprehensive progress in holography from single metasurfaces to multiplexed and reconfigurable metasurfaces is reviewed. First, single metasurface holography is briefly introduced. Second, the latest progress in angular momentum multiplexed metasurface holography, including basic characteristics, design strategies, and diverse applications, is discussed. Next, a detailed overview of wavelength-sensitive, angle-sensitive, and polarization-controlled holograms is considered. The recent progress in reconfigurable metasurface holography based on lumped elements is highlighted. Its instant on-site programmability combined with machine learning provides the possibility of realizing movie-like dynamic holographic displays. Finally, we briefly summarize this rapidly growing area of research, proposing future directions and potential applications.

Micro-Ring Resonator-Based Tunable Vortex Beam Emitter

Light beams bearing orbital angular momentum (OAM) are used in various scientific and engineering applications, such as microscopy, laser material processing, and optical tweezers. Precise topological charge control is crucial for efficiently using vortex beams in different fields, such as information encoding in optical communications and sensor systems. This work presents a novel method for optimizing an emitting micro-ring resonator (MRR) for emitting vortex beams with variable orders of OAM. The MRR consists of a ring waveguide with periodic structures side-coupled to a bus waveguide. The resonator is tunable due to the phase change material Sb2Se3 deposited on the ring. This material can change from amorphous to crystalline while changing its refractive index. In the amorphous phase, it is 3.285 + 0i, while in the transition to the crystalline phase, it reaches 4.050 + 0i at emission wavelength 1550 nm. We used this property to control the vortex beam topological charge. In our study, we optimized the distance between the bus waveguide and the ring waveguide, the bending angle, and the width of the bus waveguide. The optimality criterion was chosen to maximize the flux density of the radiated energy emitted by the resonator. The numerical simulation results proved our method. The proposed approach can be used to optimize optical beam emitters carrying OAM for various applications.

On-demand flat-top wideband OAM mode converter based on a cladding-etched helical fiber grating

A new method enabling to provide an on-demand flat-top wideband orbital angular momentum (OAM) mode converter is proposed and experimentally demonstrated, which is based on utilization of a cladding-etched helical long-period fiber grating (CEHLPG). By appropriately selecting the grating period and precisely controlling the diameter of the CEHLPG in-situ, both the radial order and central wavelength of the flat-top band for the generated OAM mode can be flexibly tailored according to specific requirements. As typical examples, the first azimuthal order OAM modes with a flat-top bandwidth of 95 nm at -20 dB, a central operating wavelength of ∼1500 nm, and the radial-orders of 9, 8, 5, and 2, respectively, have been demonstrated consecutively. The proposed method provides an excellent flexibility and robustness in controlling both the radial order and the central wavelength of the resulting flat-top wideband OAM mode conversion, which may support a variety of practical optical vortex applications.

All-fiber function devices for twisted lights

Lights carrying orbital angular momentum (OAM), also called twisted lights, have been applied in fields of optical manipulation, imaging, quantum communication, and mode-division-multiplexing (MDM) optical communication systems. Traditional approaches for manipulating twisted lights carrying OAM in free space paths such as Q-plates, spiral phase plates (SPPs), and spatial light modulators (SLMs) that are usually affected by diffraction effect and imperfect alignment between different optical components, limiting the practical applications of twisted lights. Here we design, fabricated, and package all-fiber function devices for twisted light carrying OAM such as all-fiber broadband OAM generator, all-fiber OAM (de)multiplexer, all-fiber OAM & WDM coupler, and all-fiber OAM 1 × 2 coupler. Base on coupled mode theory and phase-matching condition, twisted light can be generated and detected by pre-tapered single mode fiber (SMF) fusing with multi-mode fiber (MMF). The results show that the proposed all-fiber function devices for twist light have large working broadband (at least C band), high purity (above 95%), and low insert loss (less than 3 dB). The proposed devices will open a reliable way for twisted light applied in optical fiber communications and optical interconnections.

Control of the annular spatial profile of high harmonics using a Bessel-Gaussian beam carrying the nonzero orbital angular momentum

We propose to generate vortex high harmonics in the extreme ultraviolet (XUV) with a controllable spatial profile by using a Bessel-Gaussian (BG) beam carrying a nonzero orbital angular momentum (OAM). Such BG beam has quite a different intensity profile at the focus compared to the generally used BG beam without carrying the OAM. We show that the BG beam is capable of generating single-ring structured high harmonics, which is quite different from an Laguerre-Gaussian (LG) beam with a similar intensity distribution at the laser focus. We reveal that favorable phase-matching conditions can be achieved off-axis and away from the laser focus because a single-atom intrinsic phase due to the short electron trajectory can be well compensated by a geometric phase of the BG beam. We thus give a general rule that vortex high harmonics with a single annular profile can be efficiently generated when a gas medium is located at 1.5zred to 2.0zred before or after the laser focus of the BG beam, here zred is a reduced length. We also show the validity of this rule when the BG beam carries a higher OAM. This work is expected to be useful for synthesizing attosecond vortex pulses.

GaN vortex metasurface for interference and broadband characteristics

We experimentally demonstrate a highly efficient metasurface-based optical vortex beam (OVB) composed of high-aspect-ratio gallium nitride (GaN) meta-structures with an exceptional simulated absolute polarization conversion efficiency (APCE) of up to 98%. A flower-like interference pattern emerges at the converging distance of the device with the helicity switching in spiral and dislocation interference patterns beyond this point, as confirmed through meticulous Mach-Zehnder interferometer analysis. The device also performs broadband capabilities across visible wavelengths. Experimentally demonstrated, the annular shape adeptly expands its diameter with increasing incident wavelengths. This phenomenon is rooted in the fascinating anomalous refractive and reflective characteristics inherent to subwavelength-period metasurfaces.

Switchable rotation of metal nanostructures in an intensity chirality-invariant focus field

Light-induced rotation is a fundamental motion form that is of great significance for flexible and multifunctional manipulation modes. However, current optical rotation by a single optical field is mostly unidirectional, where switchable rotation manipulation is still challenging. To address this issue, we demonstrate a switchable rotation of non-spherical nanostructures within a single optical focus field. Interestingly, the intensity of the focus field is chiral invariant. The rotation switch is a result of the energy flux reversal in front and behind the focal plane. We quantitatively analyze the optical force exerted on a metal nanorod at different planes, as well as the surrounding energy flux. Our experimental results indicate that the direct switchover of rotational motion is achievable by adjusting the relative position of the nanostructure to the focal plane. This result enriches the basic motion mode of micro-manipulation and is expected to create potential opportunities in many application fields, such as biological cytology and optical micromachining.

Generation of femtosecond optical vortices with multiple separate phase singularities from a Kerr-lens mode-locked Yb:KGW oscillator

Femtosecond optical vortices with a phase singular point have diverse applications such as microscopic particles manipulation, special-structure micro-processing and quantum information. Raising the number of singularity points can provide additional dimensions of control. Here we report for what we believe is the first time the generation of femtosecond optical vortices with multiple (two and five) singularities directly from a laser oscillator. The average powers and pulse durations of the resulting vortex pulses are several hundred milliwatts and less than 300 fs, respectively. This work represents an innovate way for obtaining femtosecond multi-vortices, opening the way to the further studies of optical vortex crystals and their applications.

Broadband optical vortex beam generation using flat-surface nanostructured gradient index vortex phase masks

We developed a new kind of compact flat-surface nanostructured gradient index vortex phase mask, for the effective generation of optical vortex beams in broadband infrared wavelength range. A low-cost nanotechnological material method was employed for this work. The binary structure component consists of 17,557 nano-sized rods made of two lead-bismuth-gallium silicate glasses which were developed in-house. Those small rods are spatially arranged in such a way that, according to effective medium theory, the refractive index of this internal structure is constant in the radial direction and linearly changes following azimuthal angle. Numerical results demonstrated that a nanostructured vortex phase mask with a thickness of 19 μm can convert Gaussian beams into fundamental optical vortices over 290 nm wavelength bandwidth from 1275 to 1565 nm. This has been confirmed in experiments using three diode laser sources operating at 1310, 1550, and 1565 nm. The generation of vortex beams is verified through their uniform doughnut-like intensity distributions, clear astigmatic transformation patterns, and spiral as well as fork-like interferograms. This new flat-surface component can be directly mounted to an optical fiber tip for simplifying vortex generator systems as well as easier manipulation of the generated OVB in three-dimensional space.

Manipulation of Giant Multipole Resonances via Vortex γ Photons

Traditional photonuclear reactions primarily excite giant dipole resonances, making the measurement of isovector giant resonances with higher multipolarities a great challenge. In this Letter, the manipulation of collective excitations of different multipole transitions in even-even nuclei via vortex γ photons is investigated. We develop the calculation method for photonuclear cross sections induced by the vortex γ photon beam using the fully self-consistent random-phase approximation plus particle-vibration coupling (RPA+PVC) model based on Skyrme density functional. We find that the electromagnetic transitions with multipolarity J<|m_{γ}| are forbidden for vortex γ photons due to the angular momentum conservation, with m_{γ} being the projection of total angular momentum of γ photon on its propagation direction. For instance, this allows for probing the isovector giant quadrupole resonance without interference from dipole transitions using vortex γ photons with m_{γ}=2. Furthermore, the electromagnetic transition with J=|m_{γ}|+1 vanishes at a specific polar angle. Therefore, the giant resonances with specific multipolarity can be extracted via vortex γ photons. Moreover, the vortex properties of γ photons can be meticulously diagnosed by measuring the nuclear photon-absorption cross section. Our method opens new avenues for photonuclear excitations, generation of coherent γ photon laser and precise detection of vortex particles, and consequently, has significant impact on nuclear physics, nuclear astrophysics and strong laser physics.

Maximum helical dichroism enabled by an exceptional point in non-Hermitian gradient metasurfaces

Helical dichroism (HD) utilizing unbounded orbital angular momentum degree of freedom, has provided an important means of exploring chiral effects in diverse wave systems, surpassing the two-state constraint in circular dichroism that relies on intrinsic spin. However, the naturally feeble chiral signals that arise during wave-matter interactions pose significant challenges to the effective enlargement of HD. Here, we introduce a new paradigm for realizing maximum HD through non-Hermitian gradient metasurfaces by engineering a chiral exceptional point (EP) in intrinsic topological charge. The non-Hermitian gradient metasurfaces are empowered by the asymmetric coupling feature at the EP, enabling flexible construction to realize versatile chirality control in extreme circumstances where one chiral vortex is totally reflected and the opposite counterpart is completely absorbed or transmitted into the customized vortex modes. As the manifestation of the maximum HD, we present the first experimental demonstration of perfect chirality-selected vortex transmission in acoustics. Our findings open new venues to achieve maximum chirality and explore chiral physics of wave-matter interactions, which can boost many vortical applications in asymmetric chirality manipulation, one-way propagation, and information multiplexing.

Reconfigurable perfect vortex beam generator based on a liquid crystal spiral phase plate

A transmissive adaptable optical setup to generate a range of perfect vortex beams (PVBs) carrying different topological charges (TC) without using moving parts is presented. The setup is composed of an ad hoc transparent reconfigurable liquid crystal (LC) spiral phase plate (SPP), a refractive axicon and a convergent refractive lens. The LC SPP electrodes are manufactured ablating indium-tin oxide (ITO) glass substrates using direct laser writing (DLW) resulting in a very high fill factor device. In-house tailored electronics drive the 72 LC SPP electrodes giving rise to 72 different configurations with orbital angular momentum. In this work, the generation of PVBs with 36 positive or 36 negative TCs using this optical setup is accomplished.

Switch of orbital angular momentum flux density of partially coherent vortex beams

We investigate the orbital angular momentum (OAM) flux density of beams which are the incoherent superposition of partially coherent vortex (PCV) beams with different topological charges and beam widths. Simulation results show that such beams can exhibit counter-rotating radial regions of the OAM flux density, and that we can "switch" the order of these regions by adjusting the topological charges and beam widths in the source plane. Furthermore, these counter-rotating regions can switch on propagation in free space without any change to the beam parameters. We discuss how these unusual OAM dynamics may find use in OAM-based applications.

On-demand multiplexed vortex beams for terahertz polarization detection based on metasurfaces

The manipulation of polarization states is crucial for tailoring light-matter interactions and has great applications in fundamental science. Nevertheless, conventional polarization measurement approaches are extremely challenging to determine the polarization state of incident terahertz (THz) beams. The combination of metasurfaces and inhomogeneous vector vortex beams (VVBs) provides a new solution for integrated polarization-related functional devices. Herein, a general design strategy for spin-multiplexing all-silicon metasurfaces is presented and demonstrated in THz polarization detection. The employment of basic building blocks with a high aspect ratio (AR) imparts a greater degree of freedom for generating vector beams, and those basic blocks are subsequently utilized to explore the visualized polarization state. With the assistance of a THz near-field scanning system, we evaluate the capability of reconstructing the incident polarization state from the longitudinal polarization component multiplexed by vortex beams with tight focusing characteristics. Not only that, we also utilize the polarization with dynamically varying behavior as the illumination method to elucidate the evolution trend of the polarization state under a single snapshot and establish a visualized parametric model. This work paves the way to realize ultra-compact THz polarization detection-related devices for future applications in remote sensing, high-resolution imaging, and communications.

Multi-channel data transmission through a multimode fiber based on OAM phase encoding

Data transmission based on the transmission matrix method has realized the multiplexing of a large number of orbital angular momentum (OAM) modes under scattering, which encodes the data by modulating the amplitude of the OAM modes. However, this amplitude modulation (amplitude encoding) method has obvious cross talk when the number of output modes is small, resulting in a non-negligible bit error rate. Here, a multi-channel data transmission method based on OAM phase modulation (phase encoding) under scattering is proposed. This method can resist the multiple-scattering effect of multimode fibers and realize accurate data transmission with very few rows of camera pixels for output mode measurement, which is suitable for high-speed data transmission under scattering. Experimentally, we have achieved a bit error rate of less than 0.005% in the data transmission of a color image through a 60 m multimode fiber with only 2 rows of camera pixels for output mode measurement. Experiments also showed that the proposed method has a higher stability than amplitude encoding when the proportion of "1" or "0" in the code changes.

Geometric Phase in Twisted Topological Complementary Pair

Geometric phase enabled by spin-orbit coupling has attracted enormous interest in optics over the past few decades. However, it is only applicable to circularly-polarized light and encounters substantial challenges when applied to wave fields lacking the intrinsic spin degree of freedom. Here, a new paradigm is presented for achieving geometric phase by elucidating the concept of topological complementary pair (TCP), which arises from the combination of two compact phase elements possessing opposite intrinsic topological charge. Twisting the TCP leads to the generation of a linearly-varying geometric phase of arbitrary order, which is quantified by the intrinsic topological charge. Notably distinct from the conventional spin-orbit coupling-based theories, the proposed geometric phase is the direct result of the cyclic evolution of orbital-angular-momentum transformation in mode space, thereby exhibiting universality across classical wave systems. As a proof of concept, the existence of this geometric phase is experimentally demonstrated using scalar acoustic waves, showcasing the remarkable ability in the precise manipulation of acoustic waves at subwavelength scales. These findings engender a fresh understanding of wave-matter interaction in compact structures and establish a promising platform for exploring geometric phase, offering significant opportunities for diverse applications in wave systems.

Broadband Achromatic Metalens for Tunable Focused Vortex Beam Generation in the Near-Infrared Range

Vortex beams accompanied with orbital angular momentum have attracted significant attention in research fields due to their formidable capabilities in various crucial applications. However, conventional devices for generating vortex beams still suffer from bulky sizes, high cost, and confined performances. Metalens, as an advanced platform to arbitrarily control the optical waves, has promising prospects to address the predicament for conventional devices. Although great progress has been demonstrated in the applications of vortex beams, they are still confronted with fixed functionality after fabrication that severely hinders their application range. In this work, the phase-change material of Ge2Sb2Te5 (GST) is employed to design the meta-atoms to realize tunable optical responses. Moreover, the focused vortex beam can be accomplished by superimposing a helical phase and hyperbolic phase, and the chromatic aberrations in near-infrared (NIR) range can be corrected by introducing an additional phase compensation. And the design strategy is validated by two different metalenses (BAMTF-1 and BAMTF-2). The numerical results indicate that the chromatic aberrations for two metalens can be corrected in 1.33-1.60 μm covering the telecom range. Moreover, the average focusing efficiency of BAMTF-1 is 51.4%, and that of BAMTF-2 is 39.9%, indicating the favorable performances of designed BAMTF. More importantly, their average focal lengths have a relative tuning range of 38.82% and 33.17% by altering the crystallization ratio of GST, respectively. This work may provide a significant scheme for on-chip and tunable devices for NIR imaging and communication systems.

Cascaded metasurfaces for high-purity vortex generation

We introduce a new paradigm for generating high-purity vortex beams with metasurfaces. By applying optical neural networks to a system of cascaded phase-only metasurfaces, we demonstrate the efficient generation of high-quality Laguerre-Gaussian (LG) vortex modes. Our approach is based on two metasurfaces where one metasurface redistributes the intensity profile of light in accord with Rayleigh-Sommerfeld diffraction rules, and then the second metasurface matches the required phases for the vortex beams. Consequently, we generate high-purity LGp,l optical modes with record-high Laguerre polynomial orders p = 10 and l = 200, and with the purity in p, l and relative conversion efficiency as 96.71%, 85.47%, and 70.48%, respectively. Our engineered cascaded metasurfaces suppress greatly the backward reflection with a ratio exceeding -17 dB. Such higher-order optical vortices with multiple orthogonal states can revolutionize next-generation optical information processing.

Optical Manipulation of Soft Matter

Optical manipulation has emerged as a pivotal tool in soft matter research, offering superior applicability, spatiotemporal precision, and manipulation capabilities compared to conventional methods. Here, an overview of the optical mechanisms governing the interaction between light and soft matter materials during manipulation is provided. The distinct characteristics exhibited by various soft matter materials, including liquid crystals, polymers, colloids, amphiphiles, thin liquid films, and biological soft materials are highlighted, and elucidate their fundamental response characteristics to optical manipulation techniques. This knowledge serves as a foundation for designing effective strategies for soft matter manipulation. Moreover, the diverse range of applications and future prospects that arise from the synergistic collaboration between optical manipulation and soft matter materials in emerging fields are explored.

Nanoprinted Diffractive Layer Integrated Vertical-Cavity Surface-Emitting Vortex Lasers with Scalable Topological Charge

Vertical-cavity surface-emitting lasers (VCSELs) represent an attractive light source to integrate with OAM structures to realize chip-scale vortex lasers. Although pioneering endeavors of VCSEL-based vortex lasers have been reported, they cannot achieve large topological charges (less than l = 5) due to the insufficient space-bandwidth product (SBP) caused by the inherent limited device size. Here, by integrating a nanoprinted OAM phase structure on the VCSELs, we demonstrate a vortex microlaser with a low threshold and simple structure. A monolithic microlaser array with addressable control of vortex beams with different topological charges (l = 1 to l = 5) was achieved. Nanoprinting offers high degrees of freedom for the manipulation of spatial structures. To address the challenge of insufficient SBP, two-layer cascaded spiral phase plates were designed. Thereby, a vortex beam with l = 15 and mode purity of 83.7% was obtained. Our work paves the way for future chip-scale OAM-based information multiplexing with more channels.

Multi-focus composite spiral zone plate to generate focused vortices with the comparable intensity based on genetic algorithm

In this study, we introduce an optical element, named Multi-focus Composite Spiral Zone Plate (MFCSZP), to generate multi focused vortices with approximately equal intensity along the optical axis. The genetic algorithm (GA) is used to optimize the parameters of the MFCSZP, which avoids manual parameter adjustment and improves computational efficiency. We analyze the focusing properties of the constructed MFCSZP theoretically and experimentally. The results provide evidence for its capability to generate multiple focused vortices with comparable peak intensities verified through experiment. This work shows the powerful ability of intelligent algorithms in the optimization of complex optical elements. The proposed optical element showcases potential applications within research areas of optical trapping and laser machining.

Spiral-phase-objective for a compact spiral-phase-contrast microscopy

Spiral-phase-contrast imaging, which utilizes a spiral phase optical element, has proven to be effective in enhancing various aspects of imaging, such as edge contrast and shadow imaging. Typically, the implementation of spiral-phase-contrast imaging requires the formation of a Fourier plane through a 4f optical configuration in addition to an existing optical microscope. In this study, we present what we believe to be a novel single spiral-phase-objective, integrating a spiral phase plate, which can be easily and simply applied to a standard microscope, such as a conventional objective. Using a new hybrid design approach that combines ray-tracing and field-tracing simulations, we theoretically realized a well-defined and high-quality vortex beam through the spiral-phase-objective. The spiral-phase-objective was designed to have conditions that are practically manufacturable while providing predictable performance. To evaluate its capabilities, we utilized the designed spiral-phase-objective to investigate isotropic spiral phase contrast and anisotropic shadow imaging through field-tracing simulations, and explored the variation of edge contrast caused by changes in the thickness of the imaging object.

Topologically crafted spatiotemporal vortices in acoustics

Vortices in fluids and gases have piqued the human interest for centuries. Development of classical-wave physics and quantum mechanics highlighted wave vortices characterized by phase singularities and topological charges. In particular, vortex beams have found numerous applications in modern optics and other areas. Recently, optical spatiotemporal vortex states exhibiting the phase singularity both in space and time have been described. Here, we report the topologically robust generation of acoustic spatiotemporal vortex pulses. We utilize an acoustic meta-grating with broken mirror symmetry which exhibits a topological phase transition with a pair of phase singularities with opposite topological charges emerging in the momentum-frequency domain. We show that these vortices are topologically robust against structural perturbations of the meta-grating and can be employed for the generation of spatiotemporal vortex pulses. Our work paves the way for studies and applications of spatiotemporal structured waves in acoustics and other wave systems.

Revisiting the Design Strategies for Metasurfaces: Fundamental Physics, Optimization, and Beyond

Over the last two decades, the capabilities of metasurfaces in light modulation with subwavelength thickness have been proven, and metasurfaces are expected to miniaturize conventional optical components and add various functionalities. Herein, various metasurface design strategies are reviewed thoroughly. First, the scalar diffraction theory is revisited to provide the basic principle of light propagation. Then, widely used design methods based on the unit-cell approach are discussed. The methods include a set of simplified steps, including the phase-map retrieval and meta-atom unit-cell design. Then, recently emerging metasurfaces that may not be accurately designed using unit-cell approach are introduced. Unconventional metasurfaces are examined where the conventional design methods fail and finally potential design methods for such metasurfaces are discussed.

Extended-depth-of-focus wavefront design from pseudo-umbilical space curves

Designing extended-depth-of-focus wavefronts is required in multiple optical applications. Caustic location and structure analysis offer a powerful tool for designing such wavefronts. An intrinsic limitation of designing extended-depth-of-focus wavefronts is that any smooth surface, with a non-constant mean curvature, unavoidably introduces a separation between caustic sheets, which is proportional to the ratio of change of the mean curvature along a curve embedded in the wavefront. We present a method to obtain extended-depth-of-focus wavefronts where the mean curvature variation ratio is reduced thanks to using a long circle-involute space curve effectively filling the wavefront surface. Additionally, we present a variant of the method in which the wavefront is modified within a small tubular neighborhood of the circle involute in order to partially meet the umbilical condition along that tubular region. Finally, we provide some numerical results showing the potential of our method in an application example.

Generating the 1.5 kW mode-tunable fractional vortex beam by a coherent beam combining system

As an essential component of the vortex beam, the fractional vortex beam has significantly advanced various applications, such as optical imaging, optical communication, and particle manipulation. However, practical applications face a significant challenge as generating high average power fractional vortex beams remains difficult. Here, we proposed and experimentally demonstrated a high average power mode-tunable fractional vortex beam generator based on an internally sensed coherent beam combining (CBC) system. We presented the first, to the best of our knowledge, successful generation of a 1.5 kW continuous wave fractional vortex beam. Moreover, real-time tuning of the topological charge (TC) from -2/3 to +2/3 was easily achieved using the programmable liquid crystals (LCs). More importantly, the fractional vortex beam copier was presented as well, and the generated fractional vortex beam could be easily transformed into a fractional vortex beam array by changing the fill factor of the laser array. This work can pave the path for the practical implementation of high average power structured light beams.

Wide-Angle Optical Metasurface for Vortex Beam Generation

In this work, we have achieved an advancement by integrating wide-angle capacity into vortex beams with an impressive topological charge (TC) of 12. This accomplishment was realized through the meticulous engineering of a propagation-phase-designed metasurface. Comprising gallium nitride (GaN), meta-structures characterized by their high-aspect ratio, this metasurface exhibits an average co-polarization transmission efficiency, reaching a remarkable simulated value of up to 97%. The intricate spiral patterns, along with their respective quantification, have been meticulously investigated through tilt-view scanning electron microscopy (SEM) and were further analyzed through the Mach-Zehnder interferometer. A captivating revelation emerged, a distinctive petal-like interference pattern manifests prior to the metasurface's designed focal distance. The occurrence of this petal-like pattern at a specific z-axis position prompts a deliberate manipulation of the helicity of the spiral branches. This strategic helicity alteration is intrinsically tied to the achievement of a minimized donut diameter at the designed focal length. In regard to the angular capability of the device, the captured images continuously showcase prominent attributes within incident angles spanning up to 30 degrees. However, as incident angles surpass the 30-degree threshold, the measured values diverge from their corresponding theoretical projections, resulting in a progressive reduction in the completeness of the donut-shaped structure.

Optical vector fields with kaleidoscopic quasicrystal structures by multiple beam interference

An easily accessible approach is proposed to create structured beams with various quasicrystal structures and polarization distributions based on multi-beam interference. By controlling the azimuthally-dependent polarization for Q evenly and circularly distributed beams to be interfered, the intensity and polarization structures for the generated quasicrystal field with Q-fold rotational symmetry are flexibly adjusted. Using the diffraction theory for interfering Q vector Gaussian beams, an analytical wave function is derived to reconstruct the polarization-resolved intensities and the distributions of Stokes parameters measured in the experiment. With good agreement between the numerical and experimental results, the derived wave function is further employed to characterize the propagation-variant states of polarization, providing fundamentally important information for the vector quasicrystal beams.

Transversal optical singularity induced precision measurement of step-nanostructures

Optical singularities indicate zero-intensity points in space where parameters, such as phase, polarization, are undetermined. Vortex beams such as the Laguerre-Gaussian modes are characterized by a phase factor eilθ, and contain a phase singularity in the middle of its beam. In the case of a transversal optical singularity (TOS), it occurs perpendicular to the propagation, and its phase integral is 2π in nature. Since it emerges within a nano-size range, one expects that TOSs could be sensitive in the light-matter interaction process and could provide a great possibility for accurate determination of certain parameters of nanostructure. Here, we propose to use TOSs generated by a three-wave interference to illuminate a step nanostructure. After interaction with the nanostructure, the TOS is scattered into the far field. The scattering direction can have a relation with the physical parameters of the nanostructure. We show that by monitoring the spatial coordinates of the scattered TOS, its propagation direction can be determined, and as consequence, certain physical parameters of the step nanostructure can be retrieved with high precision.

Amplification of light pulses with orbital angular momentum (OAM) in nitrogen ions lasing

Nitrogen ions pumped by intense femtosecond laser pulses give rise to optical amplification in the ultraviolet range. Here, we demonstrated that a seed light pulse carrying orbital angular momentum (OAM) can be significantly amplified in nitrogen plasma excited by a Gaussian femtosecond laser pulse. With the topological charge of ℓ = ±1, we observed an energy amplification of the seed light pulse by two orders of magnitude, while the amplified pulse carries the same OAM as the incident seed pulse. Moreover, we show that a spatial misalignment of the plasma amplifier with the OAM seed beam leads to an amplified emission of Gaussian mode without OAM, due to the special spatial profile of the OAM seed pulse that presents a donut-shaped intensity distribution. Utilizing this misalignment, we can implement an optical switch that toggles the output signal between Gaussian mode and OAM mode. This work not only certifies the phase transfer from the seed light to the amplified signal, but also highlights the important role of spatial overlap of the donut-shaped seed beam with the gain region of the nitrogen plasma for the achievement of OAM beam amplification.

Vortex rings in paraxial laser beams

Interference of a fundamental vortex-free Gaussian beam with a co-propagating plane wave leads to nucleation of a series of vortex rings in the planes transverse to the optical axis; the number of rings grows with vanishing amplitude of the plane wave. In contrast, such interference with a beam carrying on-axis vortex with winding number l results in the formation of |l| rings elongated and gently twisted in propagation direction. The twist handedness of the vortex lines is determined by the interplay between dynamic and geometric phases of the Gaussian beam and the twist angle grows with vanishing amplitude of the plane wave. In the counter-propagating geometry the vortex rings nucleate and twist with half-wavelength period dominated by the interference grating in propagation direction.

Silicon metaoptics for the compact generation of perfect vector beams in the telecom infrared

Perfect vortices have attracted considerable attention as orbital angular momentum (OAM) beams with customizable ring-like intensity distribution. More recently, the non-separable combination of perfect vortices with opposite OAMs and spins, yielding so-called perfect vector beams, has further expanded their applications in the fields of optical manipulation and imaging, high-resolution lithography, and telecommunications. Exploiting the combined manipulation of dynamic and geometric phases using silicon anisotropic metaunits, here we present the design, fabrication, and characterization of novel, to the best of our knowledge, dielectric metaoptics for the compact generation of perfect vector beams in the telecom infrared using a single metasurface. These devices pave the way to integrated optical architectures with applications in information and communication technologies in both the classical and quantum regimes.

Stable higher-charge vortex solitons in the cubic-quintic medium with a ring potential

We put forward a model for trapping stable optical vortex solitons (VSs) with high topological charges m. The cubic-quintic nonlinear medium with an imprinted ring-shaped modulation of the refractive index is shown to support two branches of VSs, which are controlled by the radius, width, and depth of the modulation profile. While the lower-branch VSs are unstable in their nearly whole existence domain, the upper branch is completely stable. Vortex solitons with m ≤  12 obey the anti-Vakhitov-Kolokolov stability criterion. The results suggest possibilities for the creation of stable narrow optical VSs with a low power, carrying higher vorticities.

Flexible generation of broadly wavelength- and OAM-tunable Laguerre-Gaussian (LG) modes from a random fiber laser

Broadband wavelength tunable Laguerre-Gaussian (LG) mode with flexibly manipulated topological charge is greatly desired for large-capacity optical communication. However, the operating wavelengths achieved for the current LG modes are significantly restricted either by the emission spectrum of the intracavity gain medium or by the operation wavelengths of mode-conversion or modulation components. Here, broadband wavelength-tunable LG modes with a controllable topological charge are generated based on a random fiber laser (RFL) and a digital micromirror device (DMD). The RFL can produce broadly wavelength-tunable laser emissions spanning from 1044 to 1403 nm with a high spectral purity and an excellent beam quality, benefiting from the cascaded random Raman gain starting from a ytterbium fiber based active gain. A commercially available broadband DMD is then utilized to excite the LG modes with a flexibly tunable topological charge of up to 100 order through the super-pixel wavefront shaping technique. The combination of the RFL and the DMD greatly broadens the operating wavelength region of the LG modes to be achieved, which facilitates the capacity scaling-up in the orbital angular momentum multiplexed optical communication application.

Spatial multiplexing for robust optical vortex transmission with optical nonlinearity

Optical vortex beams, with phase singularity characterized by a topological charge (TC), introduces a new dimension for optical communication, quantum information, and optical light manipulation. However, the evaluation of TCs after beam propagation remains a substantial challenge, impeding practical applications. Here, we introduce vortices in lateral arrays (VOILA), a novel spatial multiplexing approach that enables simultaneous transmission of a lateral array of multiple vortices. Leveraging advanced learning techniques, VOILA effectively decodes TCs, even in the presence of strong optical nonlinearities simulated experimentally. Notably, our approach achieves substantial improvements in single-shot bandwidth, surpassing single-vortex scheme by several orders of magnitude. Furthermore, our system exhibits precise fractional TC recognition in both linear and nonlinear regimes, providing possibilities for high-bandwidth communication. The capabilities of VOILA promise transformative contributions to optical information processing and structured light research, with significant potential for advancements in diverse fields.

Recognition of the orbital-angular-momentum spectrum for hybrid modes existing in a few-mode fiber via a deep learning method

In this study, we theoretically and experimentally demonstrate that the convolutional neural network (CNN) in combination with the residual blocks and the regression methods can be used to precisely and quickly reconstruct the OAM spectrum of a hybrid OAM mode no matter how the consistent OAM modes have the same or different order indices in both the azimuthal and the radial direction. For cases of the simulation testing, the mean errors of all recognized parameters for hybrid OAM modes in a four-mode fiber (4MF) and a six-mode fiber (6MF) are smaller than 0.003 and 0.008, respectively. To the best of our knowledge, this is the first time that all the OAM modes, probably existing in the core of 4MFs or 6MFs, can be precisely and quickly recognized from intensity distribution of the hybrid OAM mode itself via the deep learning method.

High-order femtosecond vortices up to the 30th order generated from a powerful mode-locked Hermite-Gaussian laser

Femtosecond vortex beams are of great scientific and practical interest because of their unique phase properties in both the longitudinal and transverse modes, enabling multi-dimensional quantum control of light fields. Until now, generating femtosecond vortex beams for applications that simultaneously require ultrashort pulse duration, high power, high vortex order, and a low cost and compact laser source has been very challenging due to the limitations of available generation methods. Here, we present a compact apparatus that generates powerful high-order femtosecond vortex pulses via astigmatic mode conversion from a mode-locked Hermite-Gaussian Yb:KGW laser oscillator in a hybrid scheme using both the translation-based off-axis pumping and the angle-based non-collinear pumping techniques. This hybrid scheme enables the generation of femtosecond vortices with a continuously tunable vortex order from the 1st up to the 30th order, which is the highest order obtained from any femtosecond vortex laser source based on a mode-locked oscillator. The average powers and pulse durations of all resulting vortex pulses are several hundred milliwatts and <650 fs, respectively. In particular, 424-fs 11th-order vortex pulses have been achieved with an average power of 1.6 W, several times more powerful than state-of-the-art oscillator-based femtosecond vortex sources.

Full C-band covered and DWDM channelized high channel-count all-fiber orbital-angular-momentum mode generator based on the fiber gratings

To generate the orbital-angular-momentum (OAM) modes at multiple wavelengths, which exactly fit with the dense-wavelength-division-multiplex (DWDM) channel grids, is important to the DWDM-based OAM mode-division-multiplex (MDM) fiber communication system. In this study, a full C-band covered and DWDM channelized OAM mode generator is firstly proposed and experimentally demonstrated, which is realized especially by using a broadband helical long-period fiber grating (HLPG) combined with a phase-only sampled multichannel fiber Bragg grating (MFBG). As a proof-of-concept example, the DWDM channelized two complementary 51-channel OAM mode generators have been successfully demonstrated, each of which has a channel spacing of 100 GHz (∼0.8 nm), an effective bandwidth of ∼40 nm, a high azimuthal-mode conversion efficiency of 90%, and high uniformities in both inter- and intra-channel spectra as well. To the best of our knowledge, this is the first time for proposal and experimental demonstration of such a high channel-count and DWDM channelized first-order OAM mode (l = 1) generator, which can also be used for multichannel higher-order OAM mode generation as long as the utilized HLPG is capable of generating a broadband higher-order OAM mode. The proposed device has potential applications to DWDM-based OAM fiber communications, OAM comb lasers, OAM holography, and OAM sensors as well.

Spin-orbit Rabi oscillations in optically synthesized magnetic fields

Rabi oscillation has been proven to be one of the cornerstones of quantum mechanics, triggering substantial investigations in different disciplines and various important applications both in the classical and quantum regimes. So far, two independent classes of wave states in the Rabi oscillations have been revealed as spin waves and orbital waves, while a Rabi wave state simultaneously merging the spin and orbital angular momentum has remained elusive. Here we report on the experimental and theoretical observation and control of spin-orbit-coupled Rabi oscillations in the higher-order regime of light. We constitute a pseudo spin-1/2 formalism and optically synthesize a magnetization vector through light-crystal interaction. We observe simultaneous oscillations of these ingredients in weak and strong coupling regimes, which are effectively controlled by a beam-dependent synthetic magnetic field. We introduce an electrically tunable platform, allowing fine control of transition between different oscillatory modes, resulting in an emission of orbital-angular-momentum beams with tunable topological structures. Our results constitute a general framework to explore spin-orbit couplings in the higher-order regime, offering routes to manipulating the spin and orbital angular momentum in three and four dimensions. The close analogy with the Pauli equation in quantum mechanics, nonlinear optics, etc., implies that the demonstrated concept can be readily generalized to different disciplines.