Conservation Of Orbital Angular Momentum Research Articles

Turbulence effects mitigation for twin-photons generated by high order transverse modes

Turbulence presents challenges for free-space light-based protocols. In this work, we investigated the effect of turbulence on the correlations of twin photons prepared in Spontaneous Parametric Down Conversion (SPDC) by a pump beam with different transverse modes. We demonstrate that the strategy of coordinate inversion for mitigating turbulence-induced distortions on transverse coincidence profiles is effective for twin photons generated by HG/LG modes. The conservation of orbital angular momentum in SPDC enables us to encode information in the transverse degrees of freedom of the down-converted photon pairs. This information can have its correlation protected by the coordinate inversion technique against weak turbulence effects in free-space links.

Generation of subnatural-linewidth orbital angular momentum entangled biphotons using a single driving laser in hot atoms.

Orbital angular momentum (OAM) entangled photon pairs with narrow bandwidths play a crucial role in the interaction of light and quantum states of matter. In this article, we demonstrate an approach for generating OAM entangled photon pairs with a narrow bandwidth by using a single driving beam in a 85Rb atomic vapor cell. This single driving beam is able to simultaneously couple two atomic transitions and directly generate OAM entangled biphotons by leveraging the OAM conservation law through the spontaneous four-wave mixing (SFWM) process. The photon pairs exhibit a maximum cross-correlation function value of 27.7 and a linewidth of 4 MHz. The OAM entanglement is confirmed through quantum state tomography, revealing a fidelity of 95.7% and a concurrence of 0.926 when compared to the maximally entangled state. Our scheme is notably simpler than previously proposed schemes and represents the first demonstration of generating subnatural-linewidth entangled photon pairs in hot atomic systems.

Second harmonic of higher-order Poincaré sphere beam with two orthogonal 5%MgO:PPLN crystals

In this work, the second harmonic (SH) of higher-order Poincaré sphere (HOPS) beam was introduced and demonstrated with two orthogonal 5%MgO:PPLN crystals. Based on the quasi-phase-matching technique, the vectorial coupled wave equations were derived to simulate the SH of HOPS beams through the two crystals, including the cylindrical vector beams (CVBs), elliptically polarized CVBs (EPCVBs), and circularly polarized vortex beams. Then, the experimental setup was established to reveal that the SH of CVBs and EPCVBs present the four-lobed structure and still exhibit vector characteristics. Meanwhile, the circularly polarized vortex beams become the linearly polarized vortex beams with double phase topology, confirming the conservation of orbital angular momentum. Moreover, the maximum SH conversion efficiency of CVBs, EPCVBs, and circularly polarized vortex beams can reach 25.3%, 23.4%, and 29.4%, respectively, which may be instructive for promoting the SH generation of vector vortex beams with high efficiency.

Combinations of orbital angular momentum in two degenerate four-wave mixing processes in Rb vapor

We present experimental measurements showing the combination of the orbital angular momentum (OAM) content of incident light beams in two distinct processes of degenerate four-wave mixing induced in hot rubidium vapor. These two processes, driven by the same fields, must satisfy distinct topological charge (TC) selection rules, which impose OAM conservation. These selection rules are readily obtained from the so-called overlap integral of the incident beams and allow us to control the relations between the two nonlinear signals, in particular, to characterize the conditions to obtain the two signals with symmetric or anti-symmetric TCs. The tilted lens method was employed to measure the OAM content of the output fields. We also discuss the transition from the near- to the far-field distributions of the generated signals.

Asymmetric wavefront shaping with nonreciprocal 3D nonlinear detour phase hologram.

Asymmetric control of light with nonlinear material is of great significance in the design of novel micro-photonic components, such as asymmetric imaging devices and nonreciprocal directional optical filters. However, the use of nonlinear photonic crystals for asymmetric optical transmission, to the best of our knowledge, is still an untouched area of research. Herein we propose the 3D nonlinear detour phase holography for realizing asymmetric SH wavefront shaping by taking advantage of the dependence of the SH phase on the propagation direction of the excitation beam. With the proposed method, the designed nonreciprocal 3D nonlinear detour phase hologram yields SH phases with opposite signs for the forward and backward transmission situations. Moreover, the quasi-phase-matching scheme and orbital angular momentum conservation in the asymmetric SH wavefront shaping process are also discussed. This study conceptually extends the 2D nonlinear detour phase holography into 3D space to build the nonreciprocal 3D nonlinear detour phase hologram for achieving SH twin-image elimination and asymmetric SH wavefront shaping, offering new possibilities for the design of nonreciprocal optical devices.

Observation of Anomalous Orbital Angular Momentum Transfer in Parametric Nonlinearity.

Orbital angular momentum (OAM) conservation plays an important role in shaping and controlling structured light with nonlinear optics. The OAM of a beam originating from three-wave mixing should be the sum or difference of the other two inputs because no light-matter OAM exchange occurs in parametric nonlinear interactions. Here, we report anomalous OAM transfer in parametric upconversion, in which a Hermite-Gauss mode signal interacts with a specially engineered pump capable of astigmatic transformation, resulting in Laguerre-Gaussian mode sum-frequency generation (SFG). The anomaly here refers to the fact that the pump and signal both carry no net OAM, while their SFG does. We reveal experimentally that there is also an OAM inflow to the residual pump, having the same amount of that to the SFG but with the opposite sign, and thus holds system OAM conservation. This unexpected OAM selection rule improves our understanding of OAM transfer among interacting waves and may inspire new ideas for controlling OAM states via nonlinear optics.

Theory of Excitons in Gated Bilayer Graphene Quantum Dots

We present a theory of excitons in gated bilayer graphene (BLG) quantum dots (QDs). Electrical gating of BLG opens an energy gap, turning this material into an electrically tunable semiconductor. Unlike in laterally gated semiconductor QDs, where electrons are attracted and holes repelled, we show here that lateral structuring of metallic gates results in a gated lateral QD confining both electrons and holes. Using an accurate atomistic approach and exact diagonalization tools, we describe strongly interacting electrons and holes forming an electrically tunable exciton. We find these excitons to be different from those found in semiconductor QDs and nanocrystals, with exciton energy tunable by voltage from the terahertz to far infrared (FIR) range. The conservation of spin, valley, and orbital angular momentum results in an exciton fine structure with a band of dark low-energy states, making this system a promising candidate for storage, detection and emission of photons in the terahertz range.

Conservation of orbital angular momentum throughout amplification of high order harmonics in Ni-like krypton and silver plasmas.

In this article we present modelling results of the amplification of High Order Harmonics (HOH) carrying orbital angular momentum (OAM) in plasma amplifiers created from krypton gas and silver solid targets. The resulting amplified beam is characterized in terms of intensity, phase and decomposition in helical and Laguerre-Gauss modes. Results show that the amplification process conserves OAM, although some degradation is apparent. Several structures appear in the intensity and phase profiles. These structures have been characterized with our model and related to refraction and interference with the plasma self-emission. Thus, these results not only demonstrate the capability of plasma amplifiers to deliver HOH amplified beams carrying OAM but also pave the way towards using HOH carrying OAM as a probe beam to diagnose the dynamics of hot, dense plasmas.

Nonlinear Conformal Transformation for In Situ IR‐Visible Detection of Orbital Angular Momentum

AbstractLinear conformal transformation provides an effective way to detect orbital angular momentum (OAM) of photons, and particularly, their coherent superposition states that are significant for OAM‐based technologies. However, these methods have limited applicability—they are applied to single or few separated wavelengths and cannot achieve nondestructive detection—although those features are attractive for practical applications. Here, the second‐harmonic spiral transformation is theoretically described and experimentally demonstrated through IR‐visible detection of OAM states from 900 to 1400 nm, with a low energy loss of ≈10−6. Remarkably, a record high optical finesse of ≈5.52 is predicted and observed, indicating that a nonlinear enhancement factor resulting from OAM conservation significantly improves separation efficiency. Additionally, this scheme allows flexibility to achieve lower energy losses or higher sensitivity by adjusting phase‐matching conditions. These results can be applicable to classical and quantum areas and promote conformal transformation into nonlinear regions.

Theoretical Analysis of Continuous-Wave Mid-Infrared Optical Vortex Source Generated by Singly Resonant Optical Parametric Oscillator

Due to the important application in the study of vibrational circular dichroism and helical dichroism of chiral molecules, the tunable vortex beam at mid-infrared region has attracted increasing attention. Based on orbital angular momentum (OAM) conservation in nonlinear interactions, the vortex pumped singly resonant optical parametric oscillator (SRO) is recognized as a versatile source of coherent vortex radiation providing high power and broad wavelength coverage from a single device. However, the low parametric gain and high oscillation threshold under continuous wave (cw) pumping has so far been the most challenging factor in generating cw tunable vortex beams. To predict the output characteristic of vortex pumped SRO, a theoretical model describing the vortex pumped SRO is needed. In this study, the theoretical model describing the vortex pumped SRO is set up under collimated Gaussian beam approximation. Output characteristics of different SROs are simulated numerically. By proper selection of pump scheme (such as double-pass pumping scheme), the vortex pumped mid-infrared SRO can oscillate at a relatively low pump power. By controlling the gain (mode overlap ratio between the pump and resonant wave in the nonlinear crystal) and loss (employing a spot-defect mirror with different defect size as the output coupler) of the resonant signal mode in the SRO, the OAM of the pump beam can be directionally transferred to a specific down converted beam. The transfer mechanism of the OAM among the pump light and the down-converted beams and factors affecting the transfer are studied. Our study provides the guidelines for the design and optimization of vortex pumped SRO under cw operation.

Quantized Constrained Molecular Chains: Vibrations, Internal Rotations and Polymerization

The present work has a twofold purpose. a) It proposes a quantum-mechanical approach to constrained molecular chains and their small vibrations and rotations, by employing in a compact way vector variables and operators associated to the constituent units of the chain. The methods here differ from standard approaches based upon cartesian coordinates and normal modes and generalize previous quantum Hamiltonians describing only rotational degrees of freedom. Several models in D = 2, 3 spatial dimensions, with new Hermitean Hamiltonians, are formulated and analyzed. The chains studied successively display an increasing number of constraints: freely-jointed, freely-rotating and with constrained torsions. Conservation of total orbital angular momentum is analyzed. As a partial test, by using the present approach, the vibrational frequencies of certain triatomic molecules (water vapour, hydrogen sulfide, heavy water and sulfur dioxide) are computed and shown to be consistent with experimental data. b) A new (quantum-mechanical) analysis of polymerization, namely, the growth of a freely-jointed molecular chain (of the kind considered above) by binding an additional unit 1 to the chain, is presented. They move in a very dilute solution in a fluid at rest in thermal equilibrium about room temperature. The analysis is based upon a mixed (quantum-classical) distribution function in phase-space: a quantum Wigner-like one for unit 1 and a classical Liouville one for the chain. That leads to an approximate Schmolukowski equation for unit 1 alone and, through it, to compute the mean first passage time (MFPT) for unit 1 to become bound by the chain. The resulting MFPT displays a temperature dependence consistent with the Arrhenius formula for rate constants in chemical reactions.

Conservation of orbital angular momentum and polarization through biological waveguides

A major roadblock to the development of photonic sensors is the scattering associated with many biological systems. We show the conservation of photonic states through optically self-arranged biological waveguides, for the first time, which can be implemented to transmit light through scattering media. The conservation of optical properties of light through biological waveguides allows for the transmission of high bandwidth information with low loss through scattering media. Here, we experimentally demonstrate the conservation of polarization state and orbital angular momentum of light through a self-arranged biological waveguide, several centimeters long, in a sheep red blood cell suspension. We utilize nonlinear optical effects to self-trap cells, which form waveguides at 532 nm and 780 nm wavelengths. Moreover, we use the formed waveguide channels to couple and guide probe beams without altering the information. The formed biological waveguides are in a sub-diffusive scattering regime, so the photons’ information degrades insignificantly over several centimeters of propagation through the scattering media. Our results show the potential of biological waveguides as a methodology for the development of novel photonic biosensors, biomedical devices that require optical wireless communication, and the development of new approaches to noninvasive biomedical imaging.

Up-conversion detection of mid-infrared light carrying orbital angular momentum

Frequency up-conversion is an effective method of mid-infrared (MIR) detection by converting long-wavelength photons to the visible domain, where efficient detectors are readily available. Here, we generate MIR light carrying orbital angular momentum (OAM) from a difference frequency generation process and perform up-conversion on it via sum frequency conversion in a bulk quasi-phase-matching crystal. The maximum quantum conversion efficiencies from MIR to visible are 34.0%, 10.4%, and 3.5% for light with topological charges of 0, 1, and 2, respectively, achieved by utilizing an optimized strong pump light. We also verify the OAM conservation with a specially designed interferometer, and the results agree well with the numerical simulations. Our study opens up the possibilities for generating, manipulating, and detecting MIR light that carries OAM, and will have great potential for optical communications and remote sensing in the MIR regime.

Theoretical Investigation of Backward Optical Parametric Oscillator Pumped by Vortex Beams

Nonlinear generation and manipulation of vortex beams have emerged as a research hot topic in recent years. During nonlinear frequency conversions, orbital angular momentum will transfer from the fundamental wave to harmonic waves. In this work, we study theoretically the backward optical parametric oscillator pumped by vortex beams. The orbital angular momentum conservation law has been disclosed for the counter propagation nonlinear process. In addition, the oscillation threshold and the conversion efficiency have been investigated in detail. Our results will be helpful for the experimental demonstration of backward optical parametric oscillator pumped by vortex beams.

Necklace-structured high-harmonic generation for low-divergence, soft x-ray harmonic combs with tunable line spacing.

The extreme nonlinear optical process of high-harmonic generation (HHG) makes it possible to map the properties of a laser beam onto a radiating electron wave function and, in turn, onto the emitted x-ray light. Bright HHG beams typically emerge from a longitudinal phased distribution of atomic-scale quantum antennae. Here, we form a transverse necklace-shaped phased array of linearly polarized HHG emitters, where orbital angular momentum conservation allows us to tune the line spacing and divergence properties of extreme ultraviolet and soft x-ray high-harmonic combs. The on-axis HHG emission has extremely low divergence, well below that obtained when using Gaussian driving beams, which further decreases with harmonic order. This work provides a new degree of freedom for the design of harmonic combs—particularly in the soft x-ray regime, where very limited options are available. Such harmonic beams can enable more sensitive probes of the fastest correlated charge and spin dynamics in molecules, nanoparticles, and materials.

Mode Conversion and Transfer of Orbital Angular Momentum Between Hermite-Gaussian and Laguerre-Gaussian Beams

The cross phase (CP) opens up a new horizon for the generation and measurement of vortex beams, and the conversion between Hermite-Gaussian (HG) and Laguerre-Gaussian (LG) modes can be done with a CP. However, the conversion plane is in the far plane, and the width cannot be adjusted. In this article, we present a more convenient and practical mode converter between HG and LG modes based on CP combined with a lens. The feasibility of the converter for the conversion between two modes is theoretically analyzed. The conversion can be accomplished in the determined plane and the width of converted beam can be controlled by adjusting the parameter of converter. Proof-of-concept experiment is carried out to verify theoretical analysis and the experimental results are agree well with theoretical simulations. The transfer of orbital angular momentum (OAM) between two modes after passing through the converter is also analyzed. From the perspective of OAM conservation, the required parameter conditions for the converter are discussed. In principle, such a converter as proposed here might even be applied to microwave, millimeter wave, and terahertz OAM fields.

Arithmetic operation of orbital angular momentum of light via slow-light four-wave mixing

Orbital angular momentum (OAM) of light has attracted much attention because of its applications in many fields. Electromagnetically induced transparency (EIT) is well known as an effective method to manipulate the light fields. Now there are some increasing interests in using the EIT technology to manipulate OAM. In this work, we experimentally report on arithmetic operation of OAM of light in a solid driven by EIT. In an EIT four-level double lambda configuration, we use the optical vortex beams carrying OAM to perform slow-light four-wave mixing (FWM) without experiencing EIT storage. The conservation of OAM is satisfied in this FWM process, and the topological charge of the generated FWM signal is determined by those of the applied fields. The slow-light FWM signal accumulates the total OAM of the applied laser fields. The arithmetic operation of addition, subtraction and cancellation of OAM is effectively performed in experiment. Such OAM manipulation could have practical applications in information processing and quantum information.

Conservation of extremal ellipticity for coherent single mode Gaussian beams propagating in rotationally invariant media

We demonstrate that the conservation of extremal ellipticity for an astigmatic Gaussian beam propagating in a rotationally invariant medium, for example, an arbitrary sequence of free space propagation and stigmatic lenses, initially identified by Lo et al. (2017), is a direct consequence of the conservation of orbital angular momentum (OAM) of the beam. We derive explicit analytical expressions for the first few non-zero moments of the beam OAM and show that, apart from fundamental constants, they depend only on the beam’s extremal ellipticity. This unexpected connection uncovers the actual origin of a new propagation invariant, namely extremal ellipticity, for a broad class of coherent, single-mode Gaussian beams. We expect this principle to find use in various applications where spot circularity along an extended region of the beam is essential; such as laser micromachining, imaging, optical trapping, and inertial confinement fusion.

Properties of the generation and propagation of spatiotemporal optical vortices.

Spatiotemporal optical vortex (STOV) light is a new type of vortex light with transverse orbital angular momentum (OAM) which is different from conventional spatial vortex light. Understanding the properties of STOV are meaningful before STOV are applied. We present a theoretical study on the generation and propagation of spatiotemporal vortices step by step based on diffraction theory. The properties of the output pulses with different topological charges generated using 4 f pulse shaper in both the near-field and the far-field are analyzed. Using spiral phase mask, the intensity profiles of the output pulses immediately after the 4 f pulse shaper are of multi-lobe structures. With energies circulating around the phase singularity in the space-time plane, energy coupling occurs between the spatial and temporal domains in the wave packets during propagation, then the intensity profiles evolve into multi-hole shapes, and the holes tend to be merged for higher order STOV. The conservation of OAM in the space-time domain is shown clearly. The profiles of the output pulses in the near-field form donut rectangle shapes using π-step mask, and in the far-field, they split into a multi-lobe structure. The rules of the generation and evolution of STOV are revealed. The results demonstrate the physical properties of the STOV and the generation and propagation processes directly and clearly. It provides a guidance on the application of STOV.

The planar two-body problem for spheroids and disks

We outline a new method suggested by Conway (CMDA 125:161–194, 2016) for solving the two-body problem for solid bodies of spheroidal or ellipsoidal shape. The method is based on integrating the gravitational potential of one body over the surface of the other body. When the gravitational potential can be analytically expressed (as for spheroids or ellipsoids), the gravitational force and mutual gravitational potential can be formulated as a surface integral instead of a volume integral and solved numerically. If the two bodies are infinitely thin disks, the surface integral has an analytical solution. The method is exact as the force and mutual potential appear in closed-form expressions, and does not involve series expansions with subsequent truncation errors. In order to test the method, we solve the equations of motion in an inertial frame and run simulations with two spheroids and two infinitely thin disks, restricted to torque-free planar motion. The resulting trajectories display precession patterns typical for non-Keplerian potentials. We follow the conservation of energy and orbital angular momentum and also investigate how the spheroid model approaches the two cases where the surface integral can be solved analytically, i.e., for point masses and infinitely thin disks.