Encoding photonic angular momentum information onto surface plasmon polaritons with plasmonic lens

Light orbital angular momentum (OAM) has been recognized as a new promising resource for classical and quantum information applications. In contrast to the spin angular momentum, the OAM is an inherently multidimensional. Thus, the information can be encoded in the higher-dimensional OAM alphabets. Recently, Marrucci et al. have invented a new device named q-plate (QP), made of liquid crystal cell patterned in such a way to introduce a topological charge q at the transverse plane, which is able to generate a well-defined values of photon OAM. My research has been aimed at investigating the physics of the QP, of the optical fields that it generates and of its possible applications for optical communication and quantum information. We studied both theoretically and experimentally a novel set of non-orthogonal but over-complete paraxial modes, named Hypergeometric-Gaussian modes, that is typically associated with OAM-carrying optical fields. We have also found the light propagation kernel inside the QP and we have shown analytically that if small losses due to reflection, absorption, and scattering are neglected, the QP can convert the photon spin into OAM with up to 100 \% efficiency. We implemented a technique to control the QP optical retardation by tuning the QP temperature. At the optimal temperature, the QP can generate a beam with up to 97\% efficiency. Moreover, the OAM state generated by QP can be rotated easily in the 2D OAM Hilbert space by proper manipulation of the input polarization state, a fact which opens a new way of beam shape controlling in MHz scale. We also performed a novel way to encode and read two bits information on the 4D OAM space by using only one QP. Finally, we experimentally demonstrated the transfer of quantum information from spin to OAM and vice versa, including the case of bi-photon states having quantum correlations. Furthermore, by exploiting these quantum information transfer devices, we demonstrated the Hong-Ou-Mandel effect and the optimal quantum cloning with OAM-carrying photons.

Photonic spin‐orbit interactions describe the interactions between spin angular momentum and orbital angular momentum of photons, which play essential roles in subwavelength optics. However, the influence of frequency dispersion on photonic angular‐momentum coupling is rarely studied. Here, by elaborately designing the contribution of the geometric phase and waveguide propagation phase, the dispersion‐enabled symmetry switching of photonic angular‐momentum coupling is experimentally demonstrated. This notion may induce many exotic phenomena and be found in enormous applications, such as the spin‐Hall effect, optical calculation, and wavelength division multiplexing systems. As a proof‐of‐concept demonstration, two metadevices, a multi‐channel vectorial vortex beam generator and a phase‐only hologram, are applied to experimentally display optical double convolution, which may offer additional degrees of freedom to accelerate computing and a miniaturization configuration for optical convolution without collimation operation. These results may provide a new opportunity for complex vector optical field manipulation and calculation, optical information coding, light‐matter interaction manipulation, and optical communication.

Structured light carrying orbital angular momentum (OAM) opens up a new physical dimension for studying light–matter interactions. Despite this, the complex fields created by OAM beams still remain largely unexplored in terms of their effects on surface plasmons. This paper presents a revelation of anomalous plasmon excitations in single particles and plasmon couplings of neighboring nanorods under OAM beams, which are forbidden using non-OAM sources. The plasmon excitation of single nanoparticles is determined both by photon spin angular momentum (SAM) and OAM and influenced by the locations of the nanoparticles. Specifically, when SAM and OAM are equal in magnitude and opposite in direction, a pure plasmon excitation along light propagation direction is achieved. Two plasmon dipoles show end-to-end antibonding coupling and side-by-side bounding coupling, which are the opposite of the typical couplings. Furthermore, we observe Fano resonance with a nanorod dimer: one aligned along light propagation direction acting as the bright mode and the other aligned along the global polarization direction of light acting as the dark mode, which is the opposite of the usual plasmonic Fano resonance. By taking advantage of the unique property of the OAM source, this investigation presents a novel way to control and study surface plasmons, and the research of plasmon behavior with OAM would open new avenues for controlling electromagnetic waves and enriching the spectroscopies with more degrees of freedom.

Orbital angular momentum (OAM) light is nowadays an intriguing resource in classical and quantum optics due to the richness of physical properties it shows in interaction with matter. A key ingredient needed to exploit the full potential of OAM light is the control of quantum interference, a crucial resource in fields like quantum communication and quantum optics. Here, we study the vortex four-wave mixing (FWM) via multi-photon quantum interference in an ultraslow propagation regime. We find that the structured information can be manipulated via two-photon detuning and three photon detuning, which manifests itself as a spatial modulation. The detailed explanations based on the dispersion relation are given, which are in good agreement with our simulations. Furthermore, in order to clearly show the modulated mechanism, we perform the interference between the FWM field and a same-frequency Gaussian beam. It is found that the interference patterns are also manipulated by adjusting the multi-photon detunings. This work may have some potential applications in quantum control based on OAM light.

Orbital angular momentum (OAM) of light has drawn increasing attention due to its intriguingly rich physics and potential for a variety of applications. Having an unbounded set of orthogonal states, OAM has been used to enhance the channel capacity of data transmission. We propose and demonstrate the viability of using OAM to create an automultiscopic 3D display. Multi-view image information is encoded using an OAM beam array, then sorted into different view directions using coordinate transformation elements. A three-view demonstration was achieved to encode and decode 9 × 9 pixel images. These demonstrations suggest that OAM could potentially serve as an additional platform for future 3D display systems.

Detecting the chirality of molecules is of great importance in optics, biomedicine, and materials science. In chiroptical spectroscopy, it's crucial to achieve strong chiroptical signals with a minimal number of chiral molecules. The molecular chiroptical signals, however, are typically weak for chiral molecular sensing in conventional circular dichroism using photonic spin angular momentum, even in the presence of a large number of chiral molecules (micromoles to millimoles). Here, by involving chiral light-matter interaction with photonic orbital angular momentum, we demonstrate strong chiroptical responses that reflect the molecular chirality in a single chiral nanoassembly. We experimentally present the helical dichroism spectra of chiral nanoassemblies synthesized from L/D-cystines, consistent with electromagnetic simulations. The asymmetry factors in the fundamental wavelength and photoluminescence emission reach values of 0.53 and 1.18, respectively, exceeding those observed in the circular dichroism mechanism. To improve the dimensions of helical dichroism spectroscopy, we analyze helical dichroism in wavelength domain, polarization domain, and momentum space. Our findings not only expand the methods for trace chiral molecular sensing but also provide insights into chiral light-matter interactions.

The paper reports the intensity characteristics of fractional vortex beam, and controls the coupling between photon orbital angular momentum and spin angular momentum by coherently manipulating Laguerre–Gaussian vortex beam with linearly polarized plane light and its conjugate beam. By comparing theory with experiment, fractional vortex beams with different topological charges are generated by using pure phase liquid crystal spatial light modulator, and their intensity characteristics and interference characteristics are observed. The results show that the radial notch size of the spot is related to the fractional part of the topological charge value, and the closer the fractional part of the topological charge value is to the half integer, the more obvious the radial notch is. With the increase of integer part of topological charge, the spot radius also increases. In addition, interference with linearly polarized plane light reveals that the number of branches is equal to the number of topological charges. Interfere with its conjugate beam, and judge the topological charge number of vortex beam by the relationship between petal number and topological charge number. The petal number of integer vortex beam is twice the topological charge number, and fractional order vortex beam destroys the ring structure of Laguerre–Gaussian beam.

Angular momentum of light can be divided into spin angular momentum and orbital angular momentum (OAM). Due to the theoretically unlimited orthogonal states, the physical dimension of OAM provides a potential solution to boost the information capacity. The OAM multiplexing and modulation techniques have been implemented to meet the continuous growth of bandwidth requirements, resulting in the concept of OAM optical communication. However, the performances of the traditional optical OAM detection techniques degrade seriously in the practical application of OAM optical communications. Thanks to the powerful data analysis advantages, the cutting-edge machine learning (ML) algorithms have been widely used in the field of image processing, laying the technical foundation for OAM recognition. This paper reviews the recent advances on OAM optical communications that are enhanced by ML methods. More than the traditional OAM detection methods, the OAM demodulation methods based on multiple network architectures, including the support vector machine, self-organizing map, feed-forward neural network, convolutional neural network, and diffractive deep optical neural network (D2NN), have been summarized. We also discuss the development of the spiking neural network and on-chip D2NN, opening a possible way to facilitate the future ultra-low power and ultra-fast OAM demodulation technology.

Separation of spin and orbital angular momentum states from cylindrical vector beams

Summary form only given.The orbital angular momentum of light is distinct from the intrinsic angular momentum associated with the polarization state of the light. Thus linearly polarized laser modes can carry angular momentum. Such orbital angular momentum is associated with light beams with helical wave fronts, one class of which are Laguerre-Gaussian laser modes. The azimuthal phase factor exp(il/spl phi/) in their field distribution, gives rise to an orbital angular momentum equivalent to l/spl planck/ per photon. Our previous investigations of second-harmonic generation using Laguerre-Gaussian modes confirmed that the conservation of orbital angular determines the mode structure of the frequency-doubled beam. It was shown that a Laguerre-Gaussian mode with an azimuthal index l is transformed into a frequency-doubled mode with twice the azimuthal index 2l, thus conserving the angular momentum within the light fields. Degenerate parametric downconversion can be considered as the inverse process to second-harmonic generation. An incoming pump wave generates two new fields, signal and idler, with half the pump frequency. Accordingly, a Laguerre-Gaussian pump beam with an even value of the azimuthal index l produces signal and idler beams that can both have integer azimuthal mode indices of l/2. Away from degeneracy, the signal and idler frequencies do not in general have an integer ratio. Consequently, the orbital angular momentum of the pump cannot be divided to give integer l-values for the signal and idler beams in the same ratio as their energy. Even for degenerate phase matching, an integer division of the orbital angular momentum will not be possible when the pump mode has an odd value of l. We investigate the role that the conservation of orbital angular momentum plays in the downconversion process.

The angular momentum of light can be split into spin and orbital components. Only recently several optical processes involving a conversion of angular momentum from one form to another were conceived and experimentally demonstrated. We will briefly review these processes, and then survey some applications we have demonstrated of these spin-orbit effects in classical and quantum optics. Finally, we will show that analogous spin-orbit effects can be conceived for electron beams, leading to a theoretical proposal for a high-efficiency electron spin filter. If it will prove to be practical, this device could lead to a spin-polarized electron microscopy.

In this communication, a new method for modulating spin angular momentum (SAM) and orbital angular momentum (OAM) based on anisotropic holographic metasurfaces is proposed, which can realize the co-modulation of SAM, high order OAM, and beam’s direction simultaneously. The tensor unit adopts an ellipse patch with three adjustable parameters without slotting, and a design and analysis method suitable for this kind of tensor unit is proposed based on the two-dimensional effective impedance database for geometrical size mapping. Based on the tensor unit and proposed methods, the vortex beams with a single SAM state, a single high order OAM mode, and different directions are generated at 22 GHz. Then, the deflected vortex beams with the same SAM state and mixed OAM modes, or the deflected vortex beams with different SAM states and OAM modes are realized using a single feed and a single metasurface integrated for the first time. Simulation and experimental measurement results show that the OAM mode purity can reach more than 80% of the theoretical best value in the main SAM state. The proposed design methods and the device can achieve arbitrary modulation of SAM and OAM, providing a solution for combining and applying the two to multiplexing communications.

Like the eletron, the photon carries spin and orbital angular momentum caused by the polarization and the spatial phase distribution of light, respectively. Since the first observation of an optical vortex beam with orbital angular momentum (OAM), the use of an optical vortex beam has led to further studies on the light-matter interaction, the quantum nature of light, and a number of applications. In this article, using a metasurface with geometrical phase, we introduce the fundamental origins and some important applications of light with spin-orbit angular momentum as examples, including optical vortex tweezer and quantum entanglement of the spin-orbital angular momentum.

Vortex light carrying orbital angular momentum (OAM) is a beam with a helical phase structure. The OAM light has great potential in the field of communication, due to the fact that it can greatly improve the efficiency and capacity of information transmission. One of the popular information propagation methods is coding and decoding by different single OAM light or combined multiple OAM lights. However, Laguerre–Gaussian (LG) beams (LGB) carrying OAM, which is a classical vortex light, are prone to distortion of intensity under atmospheric turbulence (AT) disturbances. Due to the influence of AT, the effective recognition of the OAM mode in a free space becomes an important challenge for information propagation. For mitigating the influence of AT, a scheme combining AT extraction and OAM modes recognition is proposed, which can efficiently identify both AT intensity and OAM modes. A 99 % identification accuracy of AT can be reached by the proposed scheme. Besides, the obtained results also show that the recognition rate of OAM modes is greatly improved after the introduction of AT extraction module, especially under strong turbulence conditions. Compared to direct-mode-identification method without extracting AT, the recognition accuracy can be improved by 8 % and 3 % when the AT intensity is 1 × 10 − 13 and 5 × 10 − 14 m − 2 / 3 , respectively. Consequently, the proposed scheme can be used to identify the OAM modes with a high accuracy, which is beneficial to OAM coding and decoding in an OAM-based communication system.

Spin angular momentum (SAM) and orbital angular momentum (OAM) are two important fundamental degrees of freedom of light and play crucial roles in various light–matter interactions. SAM usually makes the microparticle rotate around its axis, while OAM causes orbital motion of the microparticles around the beam axis. For an optical field with only SAM, the spin-to-orbit conversion may occur under the tightly focused condition, leading to the orbital motion of probing particles. However, it is invalid for weakly focused conditions. Here, we generated an annular optical field without intrinsic OAM by weakly focusing (i.e., negligible spin-to-orbit conversion) a circularly polarized light with a linearly varying radial phase and then observed a kind of dual orbital motion of asymmetric probing particles (Janus particles) in the focal plane. The two orbital motions have opposite directions on both sides across the strongest ring of the annular optical field. In addition to the SAM, the local angular momentum (AM) density also depends on the radial intensity gradient. The radial intensity gradient has the opposite signs on both sides across the strongest ring of the annular optical field, which results in the opposite orbital motions of trapped particles. The manipulation of the local AM density and the resulting novel dual orbital effect in the absence of intrinsic OAM provide a new scene to understand the physics underlying the light–matter interaction, paving the way to some new applications involving the sorting and delivery of microparticles.