Orbital Angular Momentum States Research Articles

A five-level cold atomic system for the orbital angular momentum exchange based on a four-wave mixing setup

In this paper, we suggest a theoretical model for the exchange of the orbital angular momentum (OAM) state of laser fields in a real cold atomic system for realization in an experimental setup. By using four-wave mixing (FWM) processes, we study the spatial dependence of the new weak generated signal light under electromagnetically induced transparency conditions when one of the laser lights becomes an optical vortex. We discuss the spatial dependence of FWM processes via experimental parameters for different conditions of the OAM of vortex light. We have found that the intensity and phase distributions of the new generated light depends strongly on the OAM number of the optical vortex light. Moreover, we investigate the absorption spectrum of the new generated signal light for different OAM of the optical vortex light. Our obtained results may have potential applications in quantum information science.

Stacked ring resonators for optical vortices

In this paper, we study transmission of higher-order modes, optical vortices (OVs) included, through a bus fiber coupled to a vertical stack of ring resonators—stack multi-ring resonator (SMR). We show that transmission curves for OVs feature a characteristic gap-like structure, whose presence we explain as a manifestation of the band structure of an infinite stack of coupled ring resonators. We show a principal possibility to use SMRs as elements of delay lines for fiber optics communications on orbital angular momentum (OAM) states. We emphasize the advantages of such systems as delay lines for communications that use as signal carriers odd and even Laguerre–Gaussian (LG) modes. We also show that SMRs are able to exert super-efficient parametric control over the OAM of the transmitted optical field, as well as to generate OAM beams from odd and even LG modes.

On-chip sorting of orbital angular momentum beams using Bloch surface wave structures

Owing to their unique optical properties and new degrees of freedom, orbital angular momentum (OAM) beams have been applied in various fields. Detection of the topological charges (TCs) of OAM beams is the key step for their applications. However, on-chip sorting of OAM beams with large TCs still remains a challenge. In this paper, Bloch surface wave (BSW) structures with five semi-ring shaped nanoslits are modeled. A spatial separation of 135 nm on the chip is obtained between two neighboring OAM states. OAM beams with TCs up to 35 can be successfully sorted by the BSW structures, which is much larger than that using metallic structures (only seven). BSW structures exhibit better OAM sorting performances than metallic structures. We systematically show how the lower attenuation of BSW structures leads to far superior separation ability compared to surface plasmons propagating on metallic structures. In addition, sorting of two OAM beams with different TCs simultaneously can be achieved in this way. Our results reveal that BSW structures should be an excellent solution for OAM sorting with large TCs, which is beneficial for applications in integrated on-chip devices and optical communications.

Nested SU(2) symmetry of photonic orbital angular momentum

The polarization state is described by a quantum mechanical two-level system, which is known as special unitary group of degree 2 [SU(2)]. Polarization is attributed to an internal spin degree of freedom inherent to photons, while photons also possess an orbital degree of freedom. A fundamental understanding of the nature of spin and orbital angular momentum of photons is significant to utilize the degrees of freedom for various applications in optical communications, computations, sensing, and laser-patterning. Here, we show that the orbital angular momentum of coherent photons emitted from a laser diode can be incremented using a vortex lens, and the magnitude of orbital angular momentum increases with an increase in the topological charge inside the mode. The superposition state of the left and right vortices is described by the SU(2) state, similar to polarization; however, the radius of the corresponding Poincaré sphere depends on the topological charge. Consequently, we expect a nested SU(2) structure to describe various states with different magnitudes in orbital angular momentum. We have experimentally developed a simple system to realize an arbitrary SU(2) state of orbital angular momentum by controlling both amplitudes and phases of the left and right vortices using a spin degree of freedom, whose interplays were confirmed by expected far-field images of dipoles and quadruples.

Realization of circular polarized multiple band multi-mode OAM antenna using a ring patch for IoT applications

A multiband and multi-mode antenna with circular polarized conical patterns is suitable for achieving desired spectral efficiency, increased capacity, and spatial diversity for IoT applications. However, simultaneous excitation of such circular polarized multiple Orbital Angular Momentum (OAM) modes through a single patch antenna is challenging due to the complexity of simultaneously fulfilling distinct requirements of each mode. In this paper, a ring patch antenna is designed to excite different OAM states at different frequencies simultaneously. First, characteristic mode analysis is used to analyze the possibility of simultaneous excitation of multiple OAM modes at corresponding frequencies through a simple ring patch antenna. Then, a dual port ring patch antenna is designed and fabricated to verify the capability of generating multiple OAM states at corresponding frequencies. Furthermore, it also presents the guidance to suppress unwanted OAM modes.

Metasurface‐Based Orbital Angular Momentum Multi‐Dimensional Demultiplexer and Decoder

AbstractLight beams possess various dimensions, such as wavelength, spin angular momentum (SAM), and orbital angular momentum (OAM), which are indispensable in numerous optical applications. However, traditional techniques entail multiple attempts to disclose desired optical information, utilizing unwieldy multi‐pass systems or mechanically moving parts that are cumbersome to integrate into compact and integrated optical systems. Here, a single‐layer dielectric metasurface platform is proposed that enables multi‐dimensional demultiplexing and decoding, circumventing the need for conventional bulk optical elements. The platform can demultiplex 3D light parameters (wavelength, SAM, and OAM) and focus them at distinct spatial positions on a designated focal plane, providing a superior alternative to far‐field beams for integration with optical fiber and communication chips. The study verifies 132‐bit independent channels for a multi‐dimensional demultiplexer and demonstrates the metasurface‐based component's ability to function as an elliptic polarization decoder through a 4‐bit elliptic polarization coding transmission experiment. This work provides a compact and efficient platform for demultiplexing multi‐dimensional SAM and OAM states over a broad wavelength range, with potential implications for optical communications, optical data storage, optical information encryption, and quantum information sciences.

Alignment‐Free Angular Momentum Detection via Spin‐Independent Astigmatic Transformation

AbstractLight beam carrying spin angular momentum (SAM) and orbital angular momentum (OAM) have attracted great interest due to their great potential for high‐capacity optical communication, optical tweezers, and quantum information. However, the current detection methods suffer from the lack of robustness to misalignments in the optical system, which limits the detection accuracy and the application for off‐axis vortex array detection. Here, a high‐efficiency, broadband, and single‐layer metasurface design is proposed for alignment‐free angular momentum detection via spin‐independent astigmatic transformation (SIAT). The SAM and OAM states can be recognized from the diffraction direction and patterns, respectively. Proof‐of‐concept demonstrations are experimentally carried out on different incident beams including circular‐polarization, linear‐polarization, and vector vortex beams, as well as vortex beam arrays, showing a high diffraction efficiency of ≈81% at 1064 nm while operating within the broadband region of 900–1300 nm. The alignment‐free merit for the incident position, incident angle, and detector distance grants the device great potential for integrated quantum systems and multiparticle micromanipulation with optical tweezers.

High-quality tunable optical vortex arrays with multiple states of orbital angular momentum

We demonstrate the generation of high-quality optical vortex arrays (OVAs) with multiple states of orbital angular momentum (OAM) by uploading computer-generated holograms that are specially designed onto a liquid crystal spatial light modulator. The method of encoding both the amplitude and phase of an optical field into a phase-only holograms enables precise control of spatial transverse vortex modes. By combining the spiral, checkerboard and blazed-grating phase, the zero-order diffracted light can be efficiently suppressed while energy is redistributed onto higher orders to improve the quality of vortex array. Furthermore, the intensity distribution and spatial position can be flexibly modulated as well as OAM state of each vortex laser beam. It significantly increases the versatility of OAM, broadening its application in high-capacity optical communication and manipulation of multiple particles.

Experimental demonstration of acoustically induced polarization-dependent fiber optical vortex inversion.

A recently proposed theoretical model of acousto-optic interaction in optical fibers with a traveling flexural acoustic wave of the fundamental order [M.A. Yavorsky, et al., Opt. Lett.44, 598 (2019)10.1364/OL.44.000598] is experimentally examined. We show the effect of inversion of topological charge of optical vortices, which is governed by the direction of incident linear polarization. This vector effect of a coupling of polarization and orbital degrees of freedom proves the inconsistency of the conventional microbending-based model and confirms the recently suggested approach of the description of acousto-optic interaction that is based on the actual displacement vector. In addition, the obtained results demonstrate the realization of a controlled-NOT gate for orbital angular momentum (OAM) states.

Detection of topological charge for composite perfect vortex beams in atmospheric turbulence

Aiming at the problem of detecting the orbital angular momentum (OAM) states of the composite perfect vortex beams (CPVBs) under atmospheric turbulence (AT) channels, we propose and investigate a joint scheme of combing an optical detection method and the Gerchberg-Saxton (GS) algorithm. We firstly used the GS algorithm to pre-compensate the distorted CPVBs, and then the optical intensity distribution, together with the far-field diffraction patterns of the CPVBs after they passing through an elliptical aperture which shields part of the azimuth angle (EA-shielded), can tell us the topological charges (TCs) carried by each sub beam. The results show that the tailing direction of the bright stripes in the CPVBs intensity distribution denotes the sign of the higerh-order TC, counterclockwise is positive, clockwise is negative. The difference between the TCs of the two beams is indicated by the number of stripes, that is, the number of stripes is equal to the absolute values of differences between the two beams. After passing through the EA-shielded, the far-field diffraction intensity patterns is related to the magnitude and sign of the lower-order TC. If the bright stripes are distributed above, the TC is positive, otherwise, the TC is negative, and the number of dark stripes is equal to the lower-order TC-1. The results provide a new method for detecting the TC information of the CPVBs after phase distortion.

Phase vortex lattices in neutron interferometry

Neutron Orbital Angular Momentum (OAM) is an additional quantum mechanical degree of freedom, useful in quantum information, and may provide more complete information on the neutron scattering amplitude of nuclei. Various methods for producing OAM in neutrons have been discussed. In this work we generalize magnetic methods which employ coherent averaging and apply this to neutron interferometry. Two aluminium prisms are inserted into a nested loop interferometer to generate a phase vortex lattice with significant extrinsic OAM, 〈Lz〉 ≈ 0.35, on a length scale of ≈ 220 μm, transverse to the propagation direction. Our generalized method exploits the strong nuclear interaction, enabling a tighter lattice. Combined with recent advances in neutron compound optics and split crystal interferometry our method may be applied to generate intrinsic neutron OAM states. Finally, we assert that, in its current state, our setup is directly applicable to anisotropic ultra small angle neutron scattering.

Spatiotemporal Acoustic Vortex Beams with Transverse Orbital Angular Momentum.

Recently, the discovery of optical spatiotemporal (ST) vortex beams with transverse orbital angular momentum (OAM) has attracted increasing attention and is expected to extend the research scope and open new opportunities for practical applications of OAM states. The ST vortex beams are also applicable to other physical fields that involve wave phenomena, and here we develop the ST vortex concept in the field of acoustics and report the generation of Bessel-type ST acoustic vortex beams. The ST vortex beams are fully characterized using the scalar approach for the pressure field and the vector approach for the velocity field. We further investigate the transverse spreading effect and construct ST vortex beams with an ellipse-shaped spectrum to reduce the spreading effect. We also experimentally demonstrated the orthogonality relations between ST vortex beams with different charges. Our study successfully demonstrates the versatility of the acoustic system for exploring and discovering spatiotemporally structured waves, inspiring further investigation of exotic wave physics.

Transition from optical bistability to multistability in a tunneling quantum dot via structure hybrid light

In this paper, we proposed a new model based on the electron tunneling effect in a four-level quantum dot molecule (QDM) for studying the optical bistability (OB) and optical multistability (OM). The QDM interacts with a probe and two coupling and Laguerre–Gaussian (LG) fields. We found that by adjusting the electron tunneling effect and the parametric controlling of LG light, the transition from OB to OM or vice versa is possible. Moreover, due to the simultaneous interaction between coupling and LG lights with the same optical transition adjusting the threshold of OB and OM by orbital angular momentum (OAM) of the LG light becomes achievable. Our results show that by adjusting the simultaneous effect of electron tunneling and OAM state of the vortex light, the favorable OB and OM patterns with adjustable intensity thresholds are achievable. Our proposed model may have potential application in quantum information science based on quantum dot (QD) devices.

Twisting and entangling gravitons in high-dimensional orbital angular momentum states via photon-graviton conversion

Twisting and entangling gravitons in high-dimensional orbital angular momentum states via photon-graviton conversion

Orbital angular momentum holographic multicasting for switchable and secure wireless optical communication links.

The physical dimension of orbital angular momentum (OAM) states of light has been successfully implemented as information carrier in wireless optical communication (WOC) links. However, the current OAM data coding strategies in WOC are mainly limited to the temporal domain, rarely involving the degree of freedom of spatial domain to transmit an image directly. Here, we apply OAM holographic multiplexing technology for spatial information encoding in WOC links. Further, we demonstrate the new concept of OAM holographic multicasting, wherein a beam-steering grating has been utilized for information decoding. To distribute the OAM multiplexing information appropriately in the receiving terminal, the beam-steering grating with controllable topological charges and amplitude weighting coefficients of each diffraction order in the spatial frequency domain has been designed. An iterative algorithm has been introduced to obtain the intensity uniformity >98% at target diffraction orders. As such, this scheme experimentally allows four separate users to receive independent images, which can be switched by modulating the topological charges of the beam-steering gratings at each diffraction order. In addition, this leads to a beam-steering grating-encrypted WOC links, wherein the information can only be decoded by the grating phase with 7 pre-set spatial frequency components. Our results mark a new parallel decoding paradigm of OAM multiplexing holography, which opens up the door for future high-capacity and high-security all-optical holographic communications.

Continuous heralding control of vortex beams using quantum metasurface

Metasurfaces utilize engineered nanostructures to achieve control on all possible dimensions of light, encouraging versatile applications, including beam steering, multifunctional lensing, and multiplexed holograms. Towards the quantum optical regime for metasurfaces, although significant efforts have been put into generating and analyzing specific quantum states, control schemes to further manipulate these quantum states or information are still limited. Here, based on a metasurface, we propose and experimentally demonstrate a continuous heralding scheme to remotely control a vortex beam with high robustness to noise using polarization-entangled photon pairs. Our metasurface entangles polarization and orbital angular momentum (OAM) and the polarization selection on heralding photon erases the which-OAM information on signal photon. It induces an interference of two different OAM states remotely, manifesting a continuous orbital rotation. Our results show that metasurfaces have potential applications in quantum communication and information processing in entangling information with increasing complexity in the content.

Single-shot Kramers–Kronig complex orbital angular momentum spectrum retrieval

Orbital angular momentum (OAM) spectrum diagnosis is a fundamental building block for diverse OAM-based systems. Among others, the simple on-axis interferometric measurement can retrieve the amplitude and phase information of complex OAM spectra in a few shots. Yet, its single-shot retrieval remains elusive, due to the signal–signal beat interference inherent in the measurement. Here, we introduce the concept of Kramers–Kronig (KK) receiver in coherent communications to the OAM domain, enabling rigorous, single-shot OAM spectrum measurement. We explain in detail the working principle and the requirement of the KK method and then apply the technique to precisely measure various characteristic OAM states. In addition, we discuss the effects of the carrier-to-signal power ratio and the number of sampling points essential for rigorous retrieval and evaluate the performance on a large set of random OAM spectra and high-dimensional spaces. Single-shot KK interferometry shows enormous potential for characterizing complex OAM states in real time.

Atmospheric Turbulence Effects on the Performance of Orbital Angular Momentum Multiplexed Free-Space Optical Links Using Coherent Beam Combining

Atmospheric effects including absorption and scattering, and turbulence could introduce signal power loss and severe mode crosstalk for the orbital angular momentum (OAM)-based free-space optical communication (FSOC). Therefore, it is of great significance to simultaneously increase signal power and mitigate mode crosstalk. In this paper, for the OAM beam from a coherent laser array with a discrete vortex (CLA-DV) based on coherent beam combining, we investigate its propagation characteristics by employing theoretical derivation and the random phase screens simulation in atmospheric propagation, respectively. The probability density and OAM spectrum are given and compared for CLA-DV and Gaussian vortex beam. The results demonstrate that the Gaussian vortex beam exhibits smaller mode crosstalk under weak atmospheric turbulence conditions, while CLA-DV shows a good performance on crosstalk mitigation for strong atmospheric turbulence conditions in long-distance links. Furthermore, with a specially designed radial phase-locked Gaussian laser array composed of two orthogonal polarized coherent laser arrays carrying different OAM states, a scheme of optical communication system possessing simultaneously polarization-division multiplexing and OAM multiplexing is proposed. The normalized energy weight matrices of all 16 non-zeroth-order OAM modes are numerically calculated. To verify the feasibility of the proposed scheme, the performance of an eight-bit grayscale Lena image facing various atmosphere turbulences is evaluated. The quality of transmitted images becomes worse with the turbulence strength and transmission distance increase, which is confirmed by the trend of average optical signal error rates. This work will provide theoretical insight for improving the performance of OAM-based FSOC under scattering conditions.

Photon-counting-based underwater wireless optical communication employing orbital angular momentum multiplexing.

Underwater wireless optical communication (UWOC) is a critical technology for underwater communication, providing high speed, low latency, and security advantages. However, the strong attenuation in the water channel still limits the UWOC systems and their performances require further improvement. In this study, an orbital angular momentum (OAM) multiplexing UWOC system that uses photon-counting detection is experimentally demonstrated. By employing a single-photon counting module to receive photon signals, we analyze the bit error rate (BER) and photon-counting statistics by building a theoretical model that fits the real system, and demodulate the OAM states in single photon level and implement signal processing using field programmable gate array (FPGA) programming. Based on these modules, a 2-OAM multiplexed UWOC link is established over a water channel of 9 m. By using on-off keying modulation and 2-pulse position modulation, we achieve a BER of 1.26×10-3 with data rate of 20Mbps and 3.17×10-4 with data rate of 10Mbps respectively, which below the forward error correction (FEC) threshold of 3.8×10-3. The total transmission loss is 37 dB under an emission power of 0.5 mW, which is equivalent to the attenuation of 283 m Jerlov I type seawater from the perspective of energy loss. Our verified communication scheme will benefit the development of long-range and high-capacity UWOC.

Coherent transfer of optical vortices via backward four-wave mixing in a double- Λ atomic system

We propose and analyze an efficient scheme for the coherent transfer of optical vortices in a cold atomic ensemble with four-level double-$\mathrm{\ensuremath{\Lambda}}$ configuration. The orbital angular-momentum (OAM) information can be transferred from the incident vortex fields to the generated backward signal field via resonant backward four-wave mixing (FWM). Considering the single vortex probe field initially acting on one transition of the atomic ensemble and using experimentally achievable parameters, we identify the conditions under which the resonant backward FWM allows us to greatly improve the conversion efficiency of optical vortices beyond what is achievable in the resonant forward OAM transfers. It is found that the complete energy conversion of optical vortices originates from the perfect competition between linear absorption and parametric gain. We also demonstrate that the phase mismatching would be detrimental to the high-efficiency transfer of optical vortices. Furthermore, we show that the generated backward signal field develops a new pure or mixing OAM state when the control field is also a vortex beam. Finally, we investigate the composite vortex beam generated by collinear superposition of the incident vortex probe and signal fields, which can controlled via adjusting the intensity of the control field. Our scheme may have potential applications in OAM-based optical communication and optical information processing.