Optical Angular Momentum Research Articles

Time Evolution of Orbital Angular Momentum Modes for Deep-Routing Multiplexing Channels

Optical orbital angular momentum (OAM) mode multiplexing has emerged as a promising technique for boosting communication capacity. However, most existing studies have concentrated on channel (de)multiplexing, overlooking the critical aspect of channel routing. This challenge involves the reallocation of multiplexed OAM modes across both spatial and temporal domains—a vital step for developing versatile communication networks. To address this gap, we introduce a novel approach based on the time evolution of OAM modes, utilizing the orthogonal conversion and diffractive modulation capabilities of unitary transformations. This approach facilitates high-dimensional orthogonal transformations of OAM mode vectors, altering both the propagation direction and the spatial location. Using Fresnel diffraction matrices as unitary operators, it manipulates the spatial locations of light beams during transmission, breaking the propagation invariance and enabling temporal evolution. As a demonstration, we have experimentally implemented the deep routing of four OAM modes within two distinct time sequences. Achieving an average diffraction efficiency above 78.31%, we have successfully deep-routed 4.69 Tbit·s−1 quadrature phase-shift keying (QPSK) signals carried by four multiplexed OAM channels, with a bit error rate below 10–6. These results underscore the efficacy of our routing strategy and its promising prospects for practical applications.

Optical orbital angular momentum analogy to the Stern-Gerlach experiment.

Symmetry breaking has been shown to reveal interesting phenomena in physical systems. A notable example is the fundamental work of Otto Stern and Walther Gerlach [Stern and Zerlach, Z. Physik9, 349 (1922)10.1007/BF01326983] nearly 100 years ago demonstrating a spin angular momentum (SAM) deflection that differed from classical theory. Here we use non-separable states of SAM and orbital angular momentum (OAM), known as vector vortex modes, to demonstrate how a classical optics analogy can be used to reveal this non-separability, reminiscent of the work carried out by Stern and Gerlach. We show that by implementing a polarization insensitive device to measure the OAM, the SAM states can be deflected to spatially resolved positions.

Azimuthally Modulating Surface Plasmon Polaritons

Abstract We suggest a scheme for the lossless excitation of surface plasmon polaritons (SPPs) at a metal-dielectric interface using a Laguerre Gaussian (LG) beam. We consider a three-layer structure consisting of a top transparent medium, a middle metal thin film, and a dielectric bottom layer. The dielectric medium contains three-level atoms exhibiting electromagnetically induced transparency (EIT). By applying a control field, a LG beam, and a microwave field, we induce azimuthally varying permittivity, which leads to the azimuthal modulation of SPPs. The propagation length of SPPs is increased when excited by a LG beam carrying optical angular momentum (OAM). Moreover, the group velocity of SPPs can be effectively controlled by adjusting the OAM and azimuthal angle of the LG beam, allowing for both slow and fast propagation of SPPs.

Controllable orbital-to-spin angular momentum conversion in tight focusing of spatiotemporal vortex wavepacket

In this paper, we investigate the tight focusing of the radially polarized spatiotemporal vortex (STV) wavepackets. We find that, by changing the initial phase of the incident polarization state, the intensity envelope of the tightly focused first-order radially polarized STV wavepacket can be well controlled, yet the intensity envelope just rotates in whole for the tightly focused high-order radially polarized STV wavepacket. Furthermore, we show that, when the initial phase of incident polarization state takes π/2, the transverse double vortex structure arises in the focal region. More interestingly, when the initial phase takes π/2, the pure longitudinal spin angular momentum and transverse orbital angular momentum can be obtained in the tight focusing of the first-order radially polarized STV wavepacket. These effects are the manifestation of the spin-orbit interaction determined by the transverse orbital angular momentum and the incident polarization state. Our works present a technique to modulate the optical angular momentum in the tight focusing of the radially polarized STOV wavepacket, have potential application in the fields of optical switches, optical capture, quantum communication and nano-manipulation.

Identification of multimodal vortex optical orbital angular momentum in multimode fiber speckle patterns

Orbital angular momentum (OAM) multiplexing technology is a highly promising approach that can significantly increase the data capacity of optical communication systems. However, traditional optical communication systems are prone to atmospheric disturbances such as turbulence, which can degrade transmission quality. To mitigate this issue, multimode fibers (MMFs) have been introduced to reduce the impact of turbulence on signals. Moreover, previous studies have primarily focused on the identification of single vortex beams, facing challenges in accurately recognizing multiplexed modes. To address this challenge, this chapter proposes a multi-label image classification optimization algorithm based on transfer learning. By utilizing the pre-trained MobileNet V2 model as a feature extractor, this network structure can accurately identify 8-bit, 16-bit, and 24-bit multiplexed OAM from speckle patterns in multimode fibers, even with small sample datasets, achieving classification accuracies exceeding 95%. This method overcomes the limitations of traditional optical communication systems that are susceptible to atmospheric disturbances, providing new possibilities for long-distance transmission and increased data capacity in optical communication systems.

A Hybrid Metadetector for Measuring Bell States of Optical Angular Momentum Entanglement.

High-dimensional entanglement of optical angular momentum has shown its enormous potential for increasing robustness and data capacity in quantum communication and information multiplexing, thus offering promising perspectives for quantum information science. To make better use of optical angular momentum entangled states, it is necessary to develop a reliable platform for measuring and analyzing them. Here, we propose a hybrid metadetector of monolayer transition metal dichalcogenide (TMD) integrated with spin Hall nanoantenna arrays for identifying Bell states of optical angular momentum. The corresponding states are converted into path-entangled states of propagative polaritonic modes for detection. Several Bell states in different forms are shown to be identified effectively. TMDs have emerged as an attractive platform for the next generation of on-chip optoelectronic devices. Our work may open up a new horizon for devising integrated quantum circuits based on these two-dimensional van der Waals materials.

Flat lens–based subwavelength focusing and scanning enabled by Fourier translation

Abstract We demonstrate a technique for flexibly controlling subwavelength focusing and scanning, by using the Fourier translation property of a topology-preserved flat lens. The Fourier transform property of the flat lens enables converting an initial phase shift of light into a spatial displacement of its focus. The flat lens used in the technique exhibits a numerical aperture of 0.7, leading to focusing the incident light to a subwavelength scale. Based on the technique, we realize flexible control of the focal positions with arbitrary incident light, including higher-order structured light. Particularly, the presented platform can generate multifocal spots carrying optical angular momentum, with each focal spot independently controlled by the incident phase shift. This technique results in a scanning area of 10 μm × 10 μm, allowing to realize optical scanning imaging with spatial resolution up to 700 nm. This idea is able to achieve even smaller spatial resolution when using higher-numerical-aperture flat lens and can be extended to integrated scenarios with smaller dimension. The presented technique benefits potential applications such as in scanning imaging, optical manipulation, and laser lithography.

Object azimuth measurement based on optical orbital angular momentum phase spectrum under tilted irradiation condition

In optical remote sensing, a beam carrying orbital angular momentum can be used to identify the azimuth of an object. However, most of the studies reported currently require the beam to be illuminated vertically to the surface of the object, and the general case of tilted irradiation for the beam has not been fully discussed. In this paper, an object azimuth measurement method based on orbital angular momentum phase spectrum under tilted irradiation condition is proposed. We analyze the relationship between the measurement error caused by the beam oblique irradiation and the angle of tilt and propose an error compensation method. In the experiment, the orbital angular momentum phase spectrum of the Laguerre-Gaussian beam is obtained by a four-step phase-shifting method, and the error caused by tilt modulation is measured. After implementing the error compensation scheme, the maximum absolute error in azimuth measurement under tilted irradiation conditions is maintained within 2.15°. Across all tilt angles, the average absolute error remains below 0.73°. The method proposed in this paper has a certain reference value for expanding the practical application of orbital angular momentum beams in the field of remote sensing.

Orbital Angular Momentum Multiplexing Architecture for OAM/SDM Passive Optical Networks

Optical Angular Momentum multiplexing (OAM) is a technology of communication systems that enables high-capacity optical communication networks. One of the most important determinants of this technology is the channel capacity, loss of power, and BER that accompany the transmission. This article proposed an OAM/SDM G-PON architecture for a MIMO-type communication system that supports (OAM/SDM G-PON)Technology. The proposed architecture is used to multiplex the downstream OAM channels and the upstream SDM channels, and an OAM multiplexer/demultiplexer (OAM-MUX/DEMUX) is used to multiplex and demultiplex the OAM channels. In the OAM/SDM G-PON system, the signal will propagate through three different mediums, each one having its own nature in influencing the power of the signal that passes through that medium. The experimental study has bidirectional transmissions with the DS/US data rate of 2.4 Gbps and Binary Phase Shift Keying (BPSK) downstream and 1.2 Gbps upstream. The observed results showed that the bit-error rate (BER) is a function of coupling angles and increases with the increasing in the OAM ring size

Transmitter diversity and OAM incorporated 40 Gbps free space optical system

Abstract The present research evaluates optical angular momentum’s (OAM) performance in challenging atmospheric conditions and emphasizes its significance in free space optical (FSO) communication systems. It has been demonstrated that implementing the transmitter diversity (TD) technique effectively suppresses inter-channel interference, improving system performance as a whole. The best option among the studied encodings is found to be the combination of non-return-to-zero (NRZ) and carrier suppressed RZ (CSRZ), which performs better in a variety of weather scenarios and covers a wide FSO range from 72 m to 1450 m. Proposed system offered distance enhancement of 81.25 % under clear sky, 16.66 % under light rain, 10.22 % under moderate rain, 3.4 % under heavy rain, 10 % under light haze, 4 % under moderate haze, 4.44 % under heavy haze, 12.5 % under light fog, 4 % under moderate fog, 7.8 % under heavy fog, 5.8 % under light dust, 7.6 % under medium dust and 12.5 % under heavy dust as compared to existing workIn particular, during bad weather, this research offers significant insights into the design and optimisation of high-speed FSO systems.

Vortex beam induced spatial modulation of quantum-optical effects in a coherent atomic medium

We have studied two-dimensional absorption, gain, and corresponding refractive index profiles in a ladder-type three-level atomic system interacting with three coherent fields. One is a weak probe field considered as a plane wave, while the other two are the control fields taken as two Laguerre–Gaussian doughnut beams. Position dependence of two vortex beams induces the spatially modulated coherence at the condition of resonance, which enables us to obtain space-dependent absorption, transparency, gain without inversion (GWI), and refractive index enhancement in the present scheme. The azimuthal modulation of coherence effects is attributed to the presence of optical angular momentum of the vortex beams. Under the influence of an additional travelling wave field with the presence of two vortex beams, the present model leads us to obtain an ultra-large enhancement of refractive index at the resonant detuning of the fields. This phenomenon makes the atom-field system to play the equivalent role of a high-refractive-index prism. The role of near dipole–dipole (NDD) interaction on the modulation of position-dependent coherence effects has also been investigated. It has been shown that, without any inclusion of the travelling wave field in the system, the phenomenon of resonant enhancement of refractive index may also occur in the presence of NDD effect. The new way of generating spatially controlled GWI and nonlinear refractive index enhancement is specific to the present model. This work seems to be useful for finding its applications in spatially modulated coherence controlled electromagnetically induced transparency-based quantum devices like quantum optical memory, switches, and quantum logic gates, where the refractive index switching phenomenon is a prerequisite.

Generation of arbitrarily structured optical vortex arrays based on the epicycle model.

Optical vortex arrays (OVAs) are complex light fields with versatile structures that have been widely studied in large-capacity optical communications, optical tweezers, and optical measurements. However, generating OVAs with arbitrary structures without explicit analytical expressions remains a challenge. To address this issue, we propose an alternative scheme for customizing OVAs with arbitrary structures using an epicycle model and vortex localization techniques. This method can accurately generate an OVA with an arbitrary structure by pre-designing the positions of each vortex. The influence of the number and coordinates of the locating points on customized OVAs is discussed. Finally, the structures of the OVA and each vortex are individually shaped into specifically formed fractal shapes by combining cross-phase techniques. This unique OVA will open up novel potential applications, such as the complex manipulation of multiparticle systems and optical communication based on optical angular momentum.

Tailoring nonuniform local orbital angular momentum density.

As is well known, a light beam with a helical phase carries an optical orbital angular momentum (OAM), which can cause the orbital motion of trapped microparticles around the beam axis. Usually, the speed of the orbital motion is uniform along the azimuthal direction and depends on the amount of OAM and the light intensity. Here, we present the reverse customized method to tailor the nonuniform local OAM density along the azimuthal direction of the focal field, which has a hybrid polarization distribution and maintains a doughnut-shaped intensity profile. Theoretical analysis and experimental results about the orbital motion of the trapped polystyrene sphere show that the nonuniform local OAM density can be tailored by manipulating the polarization states of the focal field. Our results provide an ingenious way to control the local tangential optical force and the speed of the orbital motion of particles driven by the local OAM density and will promote exciting possibilities for exploring ways to control the mechanical dynamics of microparticles in optical trapping and microfluidics.

Regulation Mechanisms and Recent Progress of Optical Spin Angular Momentum (Invited)

Regulation Mechanisms and Recent Progress of Optical Spin Angular Momentum (Invited)

Optical orbital angular momentum transfer to electronic currents

Light orbital angular momentum (OAM) is transformed into an electronic current loop in a copper coil, by absorption of the optical radiation. The measurement of this current, that is in the 10−17 A range, makes it possible to evaluate and quantify the OAM carried by an electromagnetic wave in the optical domain. In particular, it determines unambiguously the torque generated by the beam and its topological charge. The induced current depends on the optical power and on the wavelength, in good agreement with theoretical predictions. Further possible developments towards an optical torque-meter are then discussed.

Concentric ring optical traps for orbital rotation of particles

Abstract Optical vortices (OVs), as eigenmodes of optical orbital angular momentum, have been widely used in particle micro-manipulation. Recently, perfect optical vortices (POVs), a subclass of OVs, are gaining increasing interest and becoming an indispensable tool in optical trapping due to their unique property of topological charge-independent vortex radius. Here, we expand the concept of POVs by proposing concentric ring optical traps (CROTs) and apply them to trapping and rotating particles. A CROT consists of a series of concentric rings, each being a vortex whose radius and topological charge can be controlled independently with respect to the other rings. Quantitative results show that the generated CROTs have weak sidelobes, good uniformity, and relatively high diffraction efficiency. In experiments, CROTs are observed to trap multiple dielectric particles simultaneously on different rings and rotate these particles with the direction and speed of rotation depending on the topological charge sign and value of each individual ring. In addition, gold particles are observed to be trapped and rotate in the dark region between two bright rings. As a novel tool, CROTs may find potential applications in fields like optical manipulation and microfluidic viscosity measurements.

Conical Emission Induced by the Filamentation of Femtosecond Vortex Beams in Water

Conical emission is a typical nonlinear phenomenon that occurs during the filamentation of femtosecond laser pulses in transparent media. In this work, the conical emission induced by two kinds of typical vortex beams (i.e., Laguerre–Gaussian (LG) and Bessel–Gaussian (BG) beams) in water is experimentally studied. By recording the light spots of different spectra components from the supercontinuum induced by the vortex beams, the characteristics of the conical emission induced by femtosecond vortex beams are studied. It is found that the spots of the supercontinuum induced by the two kinds of vortex beams differ greatly from each other. The spots of the supercontinuum induced by the BG beams are a set of concentric rings like a rainbow with a white center, while the white light spots in the case of the LG beams are circular white disks, which are different from the commonly observed white light spots. By measuring the maximum divergence angle, it is observed that the divergence angle increases with a decrease in the wavelength, while it is merely affected by the topological charge, which is explained by the formation mechanism of conical emission in terms of self-phase modulation. Based on the observed results, we discuss the transfer of optical angular momentum during the supercontinuum induced by the filamentation of femtosecond vortex beams. This work may help to better understand the transfer of optical angular momentum in non-optical parametric processes as well as the interaction of high-intensity pulses with matter.

Radiation of optical angular momentum from a dipole source in a magneto-birefringent disordered environment

Radiation of optical angular momentum from a dipole source in a magneto-birefringent disordered environment

Bessel–Gauss Beams of Arbitrary Integer Order: Propagation Profile, Coherence Properties, and Quality Factor

We present a novel approach to generate Bessel–Gauss modes of arbitrary integer order and well-defined optical angular momentum in a gradient index medium of transverse parabolic profile. The propagation and coherence properties, as well as the quality factor, are studied using algebraic techniques that are widely used in quantum mechanics. It is found that imposing the well-defined optical angular momentum condition, the Lie group SU(1,1) comes to light as a characteristic symmetry of the Bessel–Gauss beams.

Unlocking the out-of-plane dimension for photonic bound states in the continuum to achieve maximum optical chirality

The realization of lossless metasurfaces with true chirality crucially requires the fabrication of three-dimensional structures, constraining experimental feasibility and hampering practical implementations. Even though the three-dimensional assembly of metallic nanostructures has been demonstrated previously, the resulting plasmonic resonances suffer from high intrinsic and radiative losses. The concept of photonic bound states in the continuum (BICs) is instrumental for tailoring radiative losses in diverse geometries, especially when implemented using lossless dielectrics, but applications have so far been limited to planar structures. Here, we introduce a novel nanofabrication approach to unlock the height of individual resonators within all-dielectric metasurfaces as an accessible parameter for the efficient control of resonance features and nanophotonic functionalities. In particular, we realize out-of-plane symmetry breaking in quasi-BIC metasurfaces and leverage this design degree of freedom to demonstrate an optical all-dielectric quasi-BIC metasurface with maximum intrinsic chirality that responds selectively to light of a particular circular polarization depending on the structural handedness. Our experimental results not only open a new paradigm for all-dielectric BICs and chiral nanophotonics, but also promise advances in the realization of efficient generation of optical angular momentum, holographic metasurfaces, and parity-time symmetry-broken optical systems.