Orbital Angular Momentum Of Light Research Articles

Spatial vortex four-wave mixing in a five-level atomic system

We propose a scheme for modulating the spatial vortex four-wave mixing (FWM) in a five-level atomic system. During nonlinear FWM process, we find the FWM field can be easily manipulated by adjusting the corresponding parameters. Practically, the vortex phase can be spatially modulated. Thus, our results may be helpful to study nonlinear quantum phenomena based on orbital angular momentum light.

Thermal equilibrium spin torque: Near-field radiative angular momentum transfer in magneto-optical media

Spin and orbital angular momentum of light plays a central role in quantum nanophotonics as well as topological electrodynamics. Here, we show that the thermal radiation from finite-sized bodies comprising of nonreciprocal magneto-optical materials can exert a spin torque even in global thermal equilibrium. Moving beyond the paradigm of near-field heat transfer, we calculate near-field radiative angular momentum transfer between finite-sized nonreciprocal objects by combining Rytov's fluctuational electrodynamics with the theory of optical angular momentum. We prove that a single magneto-optical cubic particle in non-equilibrium with its surroundings experiences a torque in the presence of an applied magnetic field (T-symmetry breaking). Furthermore, even in global thermal equilibrium, two particles with misaligned gyrotropic axes experience equal magnitude torques with opposite signs which tend to align their gyrotropic axes parallel to each other. Our results are universally applicable to semiconductors like InSb (magneto-plasmas) as well as Weyl semi-metals which exhibit the anomalous Hall effect (gyrotropy) at infrared frequencies. Our work paves the way towards near-field angular momentum transfer mediated by thermal fluctuations for nanoscale devices.

Spatially dependent hyper-Raman scattering in five-level cold atoms.

We demonstrate a scheme to control the spatially dependent hyper-Raman scattering based on electromagnetically induced transparency in a cold atomic system. By adjusting the different system parameters, one can effectively modulate the phase and intensity of the generated Raman field. Specifically, we show that electromagnetically induced transparency creates quantum interference, which results in greatly enhanced efficiency for the generated Raman field. Such improvement in Raman efficiency makes our scheme suitable for generation of short-wavelength coherent radiation, conversion of frequency, and nonlinear spectroscopy based on orbital angular momentum light.

Switchable X-Ray Orbital Angular Momentum from an Artificial Spin Ice.

Artificial spin ices (ASI) have been widely investigated as magnetic metamaterials with exotic properties governed by their geometries. In parallel, interest in x-ray photon orbital angular momentum (OAM) has been rapidly growing. Here we show that a square ASI with a patterned topological defect, a double edge dislocation, imparts OAM to scattered x rays. Unlike single dislocations, a double dislocation does not introduce magnetic frustration, and the ASI equilibrates to its antiferromagnetic (AFM) ground state. The topological charge of the defect differs with respect to the structural and magnetic order; thus, x-ray diffraction from the ASI produces photons with even and odd OAM quantum numbers at the structural and AFM Bragg conditions, respectively. The magnetic transitions of the ASI allow the AFM OAM beams to be switched on and off by modest variations of temperature and applied magnetic field. These results demonstrate ASIs can serve as metasurfaces for reconfigurable x-ray optics that could enable selective probes of electronic and magnetic properties.

Controllable double-helical microstructures by photonic orbital angular momentum for chiroptical response

Three-dimensional helical microstructures are abundant in nature and can be applied as chiral metamaterials for advanced nanophotonics. Here we report a flexible method to fabricate double-helical microstructures with single exposure by recording the chirality of incident optical vortices. Two coaxial optical vortices can interfere to generate a helical optical field, confirmed by the numerical simulation. The diameters of double-helical microstructures can be tailored by the magnitude of topological charges. This fast manufacturing strategy provides the opportunity to efficiently yield helical microstructures. Finally, the chirality of double-helical microstructures can be reversibly read by optical vortices, demonstrating a strong chiroptical response.

Vector vortex state preservation in Fresnel cylindrical diffraction.

The vector vortex light beam, which exhibits a space-variant polarization state and is coupled with orbital angular momentum of light, has been drawing much attention due to its fundamental interest and potential applications in a wide range. Here we reveal both theoretically and experimentally that a diffractive structure having cylindrical symmetry is shown to be transparent for the vector vortex state of light with arbitrary topology. We demonstrate such an intriguing phenomenon in the Fresnel diffraction condition, where the vector Helmholtz wave equation can be utilized in the paraxial regime. Our demonstration has implications in control and manipulation of vector vortex light beams in diffractive optics, and hence, it may find potential applications.

Toward generation of mid-infrared orbital angular momentum beams by tailoring four-wave mixing in chalcogenide photonic crystal fiber

The nonlinear process of orbital angular momentum (OAM) beams in multimode fiber has attracted renewed interest due to its enhanced intermodal nonlinear interactions. Here, we designed and simulated a novel photonic crystal fiber (PCF) based on A s 2 S e 3 glass that supports stable OAM modes. By optimizing the structure parameters of the fiber, both the effective index separation and the phase-matching condition of OAM modes were satisfied in the fiber. Mid-infrared OAM light can be generated by intermodal four-wave mixing in the chalcogenide fiber. The idler conversion efficiency at the 5 µm waveband and the signal optical gain spectrum near 1.9 µm is calculated when the pump light is around 2.8 µm. Furthermore, the crosstalk of the FWM process from the other vector modes vanished due to the lower overlap factor compared with the interacting OAM modes. Our work demonstrates that the vortex beam at 5 µm could be generated, and its conversion efficiency can reach 21.96% with appropriate optimization.

Study of electric field vector, angular momentum conservation and Poynting vector of nonparaxial beams

The angular momentum (AM) of light, comprising spin and orbital AMs, is conserved and produces a spin-Hall shift in this process for paraxial beams. For nonparaxial beams, the spin and orbital AMs are non-separable and produce many changes in the beams’ spatial profile contrary to paraxial beams. These changes can be manifested as polarization modulation in the transverse plane, and conversion to orbital angular momentum (OAM) structured beams in the transverse and longitudinal planes, which can be estimated by studying the electric field vector in detail. We have calculated theoretically and simulated numerically the electric field vector components in the focal plane, to study the polarization modulation and AM conservation for OAM and Gaussian light beams of circular and linear polarizations and compared the results. Further, we have calculated and simulated the Poynting vector components for the corresponding fields to study the energy flow. We have considered the focusing of light beams using a high Numerical Aperture objective lens to obtain the nonparaxial beam, and presented a detailed theoretical analysis therein. The interpretation studies presented here are new, which may have many applications in nanophotonics and help in understanding the spin–orbit interaction at the fundamental level.

OAM light propagation through tissue

A major challenge in use of the optical spectrum for communication and imaging applications is the scattering of light as it passes through diffuse media. Recent studies indicate that light beams with orbital angular momentum (OAM) can penetrate deeper through diffuse media than simple Gaussian beams. To the best knowledge of the authors, in this paper we describe for the first time an experiment examining transmission of OAM beams through biological tissue with thickness of up to a few centimeters, and for OAM modes reaching up to 20. Our results indicate that OAM beams do indeed show a higher transmittance relative to Gaussian beams, and that the greater the OAM, the higher the transmittance also up to 20, Our results extend measured results to highly multi scattering media and indicate that at 2.6 cm tissue thickness for OAM of order 20, we measure nearly 30% more power in comparison to a Gaussian beam. In addition, we develop a mathematical model describing the improved permeability. This work shows that OAM beams can be a valuable contribution to optical wireless communication (OWC) for medical implants, optical biological imaging, as well as recent innovative applications of medical diagnosis.

Amplitude and phase sorting of orbital angular momentum states at low light levels

We present a method of photon orbital angular momentum selection at very low light levels using spatial interference between a strong local oscillator field and a weak beam. By using Fourier phase recovery techniques familiar in classical interferometry, we can experimentally obtain a quantum-limited Q distribution with a standard deviation consistent with the quantum noise floor. Further, by projecting the complex Fourier peak on a Laguerre–Gauss basis, we can distinguish states of different orbital angular momentum with high fidelity for small numbers of counts per acquisition frame. The noise equivalent photoelectron count for this measurement is 10 − 5 counts per pixel per frame.

Fourth-harmonic generation of orbital angular momentum light with cascaded quasi-phase matching crystals.

Orbital angular momentum (OAM) light, combined with the nonlinear process to expand the frequency range, has drawn increasing research interest in recent years. Here, we implement the first, to the best of our knowledge, experimental fourth-harmonic generation of OAM light with two cascaded quasi-phase-matching crystals. A Laguerre-Gaussian beam was transmitted through a duplet crystals system and frequency-doubled twice by two separate second-harmonic generation processes, which transduced the frequency of the OAM beam from telecom band to visible band and then to ultraviolet (UV) band. The topological charge of the OAM beam was increased substantially in the cascaded frequency conversion processes. In this experiment, we verify the OAM conservation by utilizing a specially designed interferometer, and the results correspond well with the numerical simulation. This work provides an effective method for the generation of UV OAM beams with high topological charges.

The A Satellite-to-Ground Quantum Key Distribution Protocol Based on Orbital Angular Momentum of Light

The orbital angular momentum (OAM) of light has been considered as a promising degree of freedom (DoF) that gives access to a higher-dimensional Hilbert space, which may lead to potential higher capacity quantum communications. Due to the fragility of the OAM state, the traditional view is that turbulence will make OAM-QKD infeasible in satellite-to-ground channels. However, based on the detailed phase screen simulations of the expected atmospheric turbulence, we find that quantum key distribution (QKD) using OAM of the light is feasible in certain system configurations, especially if quantum channel in-formation is utilized in the processing of post-selected states. Therefore, we propose a satellite-to-ground quantum key distribution protocol based on the orbital angular momentum of the light, which uses the principle that OAM-QKD can only be used in high-altitude ground stations with larger receiver apertures with-out using classic optical probes. At the same time, the classically entangled light is used as a probe of the quantum channel and reasonably-sized transmitter-receiver apertures are also employed. Numerical simulation results show that this protocol can lead to positive secret key rates even under circumstances where a sea-level ground station with a reasonable-sized aperture is used. We also found that quantum channel conjugation enables a key rate advantage provided by the higher dimensions of the protocol to be realized.

All-silicon metasurfaces for polarization multiplexed generation of terahertz photonic orbital angular momentum superposition states

The superposition state of photonic orbital angular momentum (OAM) has more degrees of freedom than pure photonic orbital angular momentum, with rich physical implications and engineering application possibilities.

Reconstitution of optical orbital angular momentum through strongly scattering media via feedback-based wavefront shaping method

Orbital angular momentum (OAM) is a fundamental physical characteristic to describe laser fields with a spiral phase structure. Vortex beams carrying OAMs have attracted more and more attention in recent years. However, the wavefront of OAM light would be destroyed when it passes through scattering media. Here, based on the feedback-based wavefront shaping method, we reconstitute OAM wavefronts behind strongly scattering media. The intensity of light with desired OAM states is enhanced to 150 times. This study provides a method to manipulate OAMs of scattered light and is of great significance for OAM optical communication and imaging to overcome complex environment interference.

Gigantic vortical differential scattering as a monochromatic probe for multiscale chiral structures

Spin angular momentum of light is vital to investigate enantiomers characterized by circular dichroism (CD), widely adopted in biology, chemistry, and material science. However, to discriminate chiral materials with multiscale features, CD spectroscopy normally requires wavelength-swept laser sources as well as wavelength-specific optical accessories. Here, we experimentally demonstrate an orbital-angular-momentum-assisted approach to yield chiroptical signals with monochromatic light. The gigantic vortical differential scattering (VDS) of ∼120% is achieved on intrinsically chiral microstructures fabricated by femtosecond laser. The VDS measurements can robustly generate chiroptical properties on microstructures with varying geometric features (e.g., diameters and helical pitches) and detect chiral molecules with high sensitivity. This VDS scheme lays a paradigm-shift pavement toward efficiently chiroptical discrimination of multiscale chiral structures with photonic orbital angular momentum. It simplifies and complements the conventional CD spectroscopy, opening possibilities for measuring weak optical chirality, especially on mesoscale chiral architectures and macromolecules.

Efficient spatiotemporal-vortex four-wave mixing in a semiconductor nanostructure

We propose a scheme to demonstrate spatiotemporal-vortex four-wave mixing (FWM) in an asymmetric semiconductor double quantum-well nanostructure. It is found that the orbital-angular-momentum (OAM) phase is transferred entirely from a unique OAM mode to the FWM field. Interestingly, by adjusting the detuning or the intensity of a control field, one can effectively modulate the phase and intensity of the FWM field. Also, we perform the superposition modes created by the interference between the FWM field and a same-frequency Laguerre-Gaussian mode, which show many interesting properties. Moreover, the conversion efficiency and quality of the output FWM field are studied. It is shown that the generated FWM mode has a maximum fidelity of approximately 100%. Our result may find potential applications in fundamental research and quantum technologies based on OAM light in solids.

Nonlinear beam shaping in periodical χ (2) fork gratings with structural defects

Nonlinear beam shaping provides an efficient way to control the phase and amplitude of optical waves while shifting their frequency. Periodical topological structures with a modulated second-order nonlinear optical coefficient (χ (2)) are commonly used to generate special optical beams at new frequencies, e.g. the second harmonic vortices. In this paper, we theoretically study the influence of structural imperfection on nonlinear beam shaping and demonstrate how to utilize these imperfections for second harmonic generation with controlled orbital angular momentums. The results indicate that the studied nonlinear beam-shaping process has excellent tolerance to structural defects, and introducing designed defects of different scales and shapes constitutes a flexible approach for controlling the orbital angular momentum of light at new frequencies. As a proof of concept example, the nested χ (2) structures are designed for generating optical vortices with predesigned topological charges along different directions. In addition, the performance of the nonlinear generation of vortex beams with a fundamental beam shifted from the center of the fork grating is also discussed. The study opens up new prospects for the optimization of nonlinear χ (2) structures for special beam generation at new frequencies.

Some aspects of scaling the orbital angular momentum of light with conical diffraction

The mechanisms leading to scaling the orbital angular momentum (OAM) from a Gaussian beam with no OAM up to integers 1, 2… have been pointed out in the cascaded conical diffraction process. The study is illustrated with two different biaxial crystals: KGd(WO2)4 and Bi2ZnOB2O6 (non-degenerate cascade). Scaling the OAM is the repetition of the following basic stage: the polarization of the highest OAM state generated by a previous crystal is changed with a quarter-wave plate in order that the OAM increment through the next biaxial crystal is maximized under a projection operator. This latter is constituted from a quarter-wave plate and a linear polarizer. The image of the field at the output of a crystal is repeated through the following one with a two-lenses afocal telescope, preventing its natural spread. The OAM is visualized by interference patterns with a reference beam and by a cylindrical lens. All the experimental patterns (intensity, phase, OAM) are quite well described by a full numerical model. The role of each crystal of the cascade in the OAM increment is simply exhibited in the Fourier space.

Imprinting photon orbital angular momentum during laser-assisted photoemission from quantum wells.

We study theoretically the transfer of the light field orbital angular momentum (OAM) to propagating electrons upon photoemission from quantum well states. Irradiation with a Laguerre-Gaussian mode laser pulse elevates the quantum well state into a laser-dressed Volkov state that can be detected in an angular and energy-resolved manner while varying the characteristics of the driving fields. We derive the photoemission cross section for this process using the S-matrix theory and illustrate how the OAM is embodied in the photoelectron angular pattern with the aid of numerical calculations. The results point to a new type of time-resolved spectroscopy, in which the electronic orbital motion is addressed exclusively, with the potential for a new insight in spin-orbitally or orbitally coupled systems.

Generation of optical orbital angular momentum light by an azimuthally-graded refractive index plate

We propose and numerically demonstrate a method to generate orbital angular momentum (OAM) light by using an azimuthally graded index (AGI) plate of uniform subwavelength thickness. We illustrate, theoretically, through numerical simulations, how a plane polarized fundamental gaussian beam incident onto such an AGI plate acquires a nonzero orbital angular momentum, and exhibits an annular far-field intensity profile with a phase singularity line along its axis. We further confirm the OAM property of the output beam by calculations of the overlap integral between its far-field and the paraxial Laguerre Gaussian modes of nonzero OAM. We also highlight the advantages of the proposed AGI plate over existing methods to generate OAM light.