Momentum Modulation Research Articles

Electrically optical phase controlling for millimeter wave orbital angular momentum multi-modulation communication

Orbital angular momentum (OAM), an emerging and fascinating degree of freedom, has highlighted an innovation in communication and optical manipulation field. The beams with different OAM state, which manifest as the phase front “twisting” of electromagnetic waves, are mutually orthogonal, which is exactly what a new freedom applied to practical communication eagers for. Herein, we proposed a novel millimeter-wave OAM modulation technique by electrically optical phase controlling. By modulating OAM and phase of optical-millimeter-wave synchronously, the multi-modulation: quadrature orbital angular momentum modulation (QOM) communication system at W band is structured and simulated, allowing a 50Gbit/s signal transmitting with bit-error rates less than 10−4. Our work might suggest that OAM could be compounded to more complex multi-modulation signal, and revealed a new insight into OAM based high capacity wireless and radio-over-fiber communication.

Antideuterons in cosmic rays: sources and discovery potential

Antibaryons are produced in our Galaxy in collisions of high energy cosmic rays with the interstellar medium and in old supernova remnants, and possibly, in exotic sources such as primordial black hole evaporation or dark matter annihilations and decays. The search for signals from exotic sources in antiproton data is hampered by large backgrounds from spallation which, within theoretical errors, can solely account for the current data. Due to the higher energy threshold for antideuteron production, which translates into a suppression of the low energy flux from spallations, antideuteron searches have been proposed as a probe for exotic sources. We perform in this paper a comprehensive analysis of the antideuteron fluxes at the Earth expected from known and hypothetical sources in our Galaxy, and we calculate their maximal values consistent with current antiproton data from AMS-02. We find that supernova remnants generate a negligible flux, whereas primordial black hole evaporation and dark matter annihilations or decays may dominate the total flux at low energies. On the other hand, we find that the {detection of cosmic antideuterons} would require, for the scenarios studied in this paper and assuming optimistic values of the coalescence momentum and solar modulation, an increase of the experimental sensitivity compared to ongoing and planned instruments by at least a factor of 2. Finally, we briefly comment on the prospects for antihelium-3 detection.

Electromagnetic Vortex Imaging Using Uniform Concentric Circular Arrays

Electromagnetic vortex imaging based on orbital angular momentum (OAM) modulation has the ability of azimuthal resolution without relative motion, which needs various OAM modes. However, the main-lobe directions of different OAM beams differ from each other so that the echo energy is limited. In this letter, several concentric circular arrays are employed to adjust the directivity, each array generating a beam carrying special OAM mode. With this scheme, the target can be illuminated by main lobes of all OAM beams simultaneously. Our proposal can benefit the image reconstruction in noisy environment. Moreover, this scheme can successfully reduce the artifacts caused by high sidelobes at the cost of reduction in azimuthal resolution. Simulation results demonstrate the effectiveness of this scheme.

Protocol for observing molecular dipole excitations by attosecond self-streaking

We propose a protocol to probe the ultrafast evolution and dephasing of coherent electronic excitation in molecules in the time domain by the intrinsic streaking field generated by the molecule itself. Coherent electronic motion in the endohedral fullerene \Necsixty~is initiated by a moderately intense femtosecond UV-VIS pulse leading to coherent oscillations of the molecular dipole moment that persist after the end of the laser pulse. The resulting time-dependent molecular near-field is probed through the momentum modulation of photoemission from the central neon atom by a time-delayed attosecond XUV pulse. Our ab-initio time-dependent density functional theory and classical trajectory simulations predict that this self-streaking signal accurately traces the molecular dipole oscillations in real time. We discuss the underlying processes and give an analytical model that captures the essence of our ab-initio simulations.

Dynamic stability during running gait termination: Differences in strategies between children and adults to control forward momentum

Rapid deceleration during running is key for successful participation in most childhood activities and sports; this requires modulation of body momentum and consequent challenges to postural equilibrium. The purpose of this study was to investigate the strategies employed by adults and children to control forward momentum and terminate running gait. Sixteen young adults and 15 pre-pubertal children completed two tasks as fast as possible: an unobstructed run (RUN) and a run and stop (STOP) at a pre-determined location. For STOP, center of mass (COM) approach velocity and momentum prior to deceleration and spatiotemporal characteristics and COM position during deceleration were compared between groups. Position and velocity variables were normalized to height and maximum velocity during RUN, respectively. Children used fewer steps with relatively longer step length to decelerate over a relatively longer distance and longer time than adults. Children approached at higher relative velocity than adults, but adults approached with greater momentum. Adults positioned their COM lower and more posterior than children throughout deceleration. Our results suggest that pre-pubertal children and young adults employ different strategies to modulate body momentum, with adults exhibiting mechanics characteristic of a more stable strategy. Despite less stable mechanics, children and adults achieved similar success.

Orbital-Angular-Momentum-Based Electromagnetic Vortex Imaging

A novel radar imaging technique based on orbital angular momentum (OAM) modulation is presented. First, the generation of electromagnetic (EM) vortex wave, which carries the OAM, using incrementally phased uniform circular array (UCA) is introduced, and factors that affect the phase-front distribution are analyzed. Subsequently, echo signal models of both multiple-in-multiple-out and multiple-in-single-out modes are established. The target images are obtained using the fast Fourier transform (FFT) and back-projection methods. Simulation results demonstrate that orbital angular momentum has the prospect for acquiring the azimuth information of radar target. The signal of both OAM modulation and frequency modulation can be used to obtain two-dimensional radar target image. The work can benefit the development of novel information-rich radar based on orbital angular momentum, as well as radar target recognition.

Potential of vortex beams with orbital angular momentum modulation for deep-space optical communication

In order to achieve multigigabit transmission in deep-space optical communication, our study applies a new modulation mode named orbital angular momentum (OAM) modulation, and uses the encoded OAM states of single photon as data information carriers, thus providing a reliable and high-speed transmission of signals. According to the long link characteristic of deep-space communication, we conduct a reasonable deployment for communication nodes in deep-space environment. First, we present the reliability of deep-space channel and analyze the data rate and spectral efficiency of beams with OAM. Second, we study the characteristics and generations of vortex beams with OAM by simulation. Results show that vortex beams have better spatial multiplexing capability of realizing high capacity data transmission. Finally, we propose an encoding method with OAM states of single photon. The transceiver units are based on spatial light modulators to perform the modulation and demodulation of vortex beams. At the receiver, the charged-coupled device camera is used to detect the signal intensity and decodes the OAM states. Our proposal not only ensures the confidentiality of deep-space optical communication, but also greatly increases the transmission rate.

Einzel lens chopper and behavior of the chopped beam in the KEK digital accelerator

The KEK digital accelerator (KEK-DA), which is a small-scale rapid-cycle induction synchrotron (IS), has commenced operation. A permanent magnet $x$-band electron cyclotron resonance ion source serves as the KEK-DA ion source and delivers various ions. A new Einzel lens beam chopper has been developed to provide the necessary pulse width of a few microseconds. The chopper is implemented by applying a rectangular pulse voltage generated by a solid-state Marx generator to the middle electrode of the Einzel lens. Momentum modulation at the bunch head and tail resulting from chopping in longitudinal direction has been observed. This intrinsic property of the chopped pulse has been clearly observed in a long drift (a free run in the circular ring) in KEK-DA.

Core momentum and particle transport studies in the ASDEX Upgrade tokamak

Core momentum and particle transport in ASDEX Upgrade (AUG) have been examined in a wide variety of plasma discharges and via several different methods. Experiments were performed in which ECRH power was added to NBI heated H-modes causing the electron and impurity ion density profiles to peak and the core toroidal rotation to flatten. Turbulence calculations of these plasmas show a change in the dominant regime from ITG to TEM due to the ECRH induced changes in the electron and ion temperature profiles. The impurity and electron density behavior can be fully explained by the changes in the turbulent particle transport. Momentum transport analyses demonstrate that in the TEM regime there is a core localized, counter-current directed, residual stress momentum flux of the same order of magnitude as the applied NBI torque. The initial results from momentum modulation experiments performed on AUG confirm that the Prandtl number in AUG NBI heated H-modes is close to 1 and that there exists a significant inward momentum pinch. Lastly, an intrinsic toroidal rotation database has been developed at AUG which can be used to test theoretically predicted dependences of residual stress momentum fluxes. Initial results show a linear correlation between the gradient of the toroidal rotation and both the electron density gradient scale length and the frequency of the dominant turbulent mode.

Deep-space and near-Earth optical communications by coded orbital angular momentum (OAM) modulation.

In order to achieve multi-gigabit transmission (projected for 2020) for the use in interplanetary communications, the usage of large number of time slots in pulse-position modulation (PPM), typically used in deep-space applications, is needed, which imposes stringent requirements on system design and implementation. As an alternative satisfying high-bandwidth demands of future interplanetary communications, while keeping the system cost and power consumption reasonably low, in this paper, we describe the use of orbital angular momentum (OAM) as an additional degree of freedom. The OAM is associated with azimuthal phase of the complex electric field. Because OAM eigenstates are orthogonal the can be used as basis functions for N-dimensional signaling. The OAM modulation and multiplexing can, therefore, be used, in combination with other degrees of freedom, to solve the high-bandwidth requirements of future deep-space and near-Earth optical communications. The main challenge for OAM deep-space communication represents the link between a spacecraft probe and the Earth station because in the presence of atmospheric turbulence the orthogonality between OAM states is no longer preserved. We will show that in combination with LDPC codes, the OAM-based modulation schemes can operate even under strong atmospheric turbulence regime. In addition, the spectral efficiency of proposed scheme is N2/log2N times better than that of PPM.

Chromaticity tracking with a phase modulation/demodulation technique in the Tevatron

The Tevatron chromaticity tracker (CT) has been successfully commissioned and is now operational. The basic idea behind the CT is that when the phase of the Tevatron RF is slowly modulated, the beam momentum is also modulated. This momentum modulation is coupled transversely via chromaticity to manifest as a phase modulation on the betatron tune. And so by phase demodulating the betatron tune, the chromaticity can be recovered. However, for the phase demodulation to be successful, it is critical that the betatron tune be a coherent signal that can be easily picked up by a phase detector. This is easily done because the Tevatron has a phase locked loop based tune tracker which coherently excites the beam at the betatron tune.

Renormalization of the electron-phonon interaction in presence of charge fluctuations

We investigate the effect of strong electron correlations on the coupling of electrons to Holstein phonons in the one-band Hubbard model in the presence of charge fluctuations. Specifically, we analyze the momentum modulation of the electron-phonon vertex induced by the electronic correlations. The role of the on-site Coulomb interaction $U$ is investigated in different regions of the Brillouin zone. The results reveal the presence of a strong forward scattering peak at low temperatures and, after an initial decrease with $U$, we find that the electron-phonon vertex starts to increase above an intermediate value of the Coulomb interaction. We show that this behavior is related to an incipient phase separation that takes place at a critical value of $U$ for fixed doping. A comparison with other approaches is also discussed.

On the topological features at the Fermi surface of Bi2212 system revealed by angle scanning photoemission

Here we report use of angle scanning photoemission spectroscopy using polarized synchrotron light to explore the Fermi surface topology of Bi 2Sr 2CaCu 2O 8± δ superconductors. The measurements were made in different experimental geometry to take care of the polarization dependence of photoemission transition probability (matrix element effect) and discriminate the true features along the high symmetry directions. We find an asymmetric suppression of spectral weight around M(π,0) point indicating coupling of the itinerant carriers with an incommensurate charge density wave (ICDW) i.e. stripes, involving modulation of the orbital momentum.

Identification of nonclassical states in neutron spin precession experiments

Highly non-classical states are produced in polarized neutron spin precession experiments due to the coherent superposition of spin-up and spin-down states. These states are described within the Wigner function formalism and the measured momentum modulation in a neutron spin precession experiment is taken as a sign of such non-classical states.

Electron momentum modulation by the stimulated Cerenkov interaction

Momentum modulation of relativistic electrons by laser light using the stimulated Cerenkov interaction is analyzed. This interaction is accomplished by introducing a gas whose index of refraction retards the phase velocity of the light wave. By intersecting the electron and laser beams at the Cerenkov angle, ϑc = cos−1(1/nβ), where n is the index of refraction of the gas, and β is the ratio of the electron velocity over the velocity of light in vacuum; the electrons remain in a field of constant phase resulting in a cumulative energy exchange. The laser-induced change in particle momentum and the electron beam energy spectrum are analyzed as a function of the electron and laser-beam characteristics. The results agree with a computer simulation of the interaction.

The stimulated Cerenkov interaction and its applications

Reviewed are the results of two experiments in which momentum modulation of a relativistic electron beam by laser fields using the stimulated Cerenkov interaction was measured, and coherent Cerenkov radiation from the optically bunched electron beam was observed. In the first experiment, light at 1.06 μm from a 30 MW Nd:YAG laser intersected 102 MeV electrons at an angle of 18 mrad in hydrogen gas, which was used as the phase-matching medium. The change in the electron-beam energy spectrum in the presence of the laser was measured, together with its functional dependence on the index of refraction of the phase-matching medium. In the second experiment, the same laser intersected 55.9 MeV electrons at an angle of 17 mrad, again in hydrogen gas. Coherent Cerenkov radiation at the second harmonic of the laser frequency (0.532 μm) was measured and indicated that the electrons were bunched on the order of the laser wavelength as a result of being velocity modulated by the laser. Applications of these results, such as development of optical klystrons and laser-driven particle accelerators, are considered. The characteristics of these devices and possible design configurations are discussed.

Momentum modulation of an electron beam by laser fields and observation of coherent optical radiation

Momentum modulation of an electron beam by laser fields and observation of coherent optical radiation

Free-electron interactions with light using the inverse Cerenkov effect

An analysis and experimental verification of momentum modulation of relativistic electrons by laser light using the inverse Cerenkov effect is presented. As an alternative to the free-electron laser for achieving energy exchange between particles and photons, the inverse Cerenkov effect uses the index of refraction of a gaseous medium to retard the phase velocity of an electromagnetic wave, enabling the electrons to remain in a field of constant phase. The momentum modulation converts to charge-density modulation by allowing the electrons to drift, thus forming electron bunches separated by optical wavelengths. An analysis is presented for the maximum amount of energy exchange, the energy exchange distribution, and the optimum bunching distance. A computer simulation of the interaction process is also given. These results are compared with the observed momentum modulation of a 102 MeV electron beam by a 30 MW Nd:YAG 1.06 μm laser in both hydrogen and methane gases. Initial observation of coherent optical radiation from a 57 MeV electron beam using the same laser system is also presented. Laser-driven particle accelerators and optical klystrons are possible applications of this interaction.

Free-electron momentum modulation by means of limited interaction length with light

An analysis is performed to determine the energy exchange between light and free electrons when the interaction length is limited. Restricting the length introduces a momentum uncertainty which can be sufficient to allow energy transfer in vacuum if the particles are relativistic and the angle between the direction of particle motion and light wave vector is small.

Momentum modulation of a free electron beam with a laser

A Nd : YAG laser has been used to modulate the energy of a free electron beam. With a single pass of the 2−MW laser light through the 100−MeV electrons we have measured a 37−keV increase in the width of the energy spectrum for the particles. The interaction occurred in helium at standard temperature and pressure over a distance corresponding to 105 optical wavelengths, and a phase synchronism condition was maintained by means of the inverse Cerenkov effect. That is, along the direction of motion of the electrons the phase velocity of the electromagnetic wave equaled the electron velocity. Therefore, the electrons remained in an electric field of constant phase, resulting in significant energy exchange.