8,450 research outputs found
The physics of angular momentum radio
Wireless communications, radio astronomy and other radio science applications
are predominantly implemented with techniques built on top of the
electromagnetic linear momentum (Poynting vector) physical layer. As a
supplement and/or alternative to this conventional approach, techniques rooted
in the electromagnetic angular momentum physical layer have been advocated, and
promising results from proof-of-concept radio communication experiments using
angular momentum were recently published. This sparingly exploited physical
observable describes the rotational (spinning and orbiting) physical properties
of the electromagnetic fields and the rotational dynamics of the pertinent
charge and current densities. In order to facilitate the exploitation of
angular momentum techniques in real-world implementations, we present a
systematic, comprehensive theoretical review of the fundamental physical
properties of electromagnetic angular momentum observable. Starting from an
overview that puts it into its physical context among the other Poincar\'e
invariants of the electromagnetic field, we describe the multi-mode quantized
character and other physical properties that sets electromagnetic angular
momentum apart from the electromagnetic linear momentum. These properties
allow, among other things, a more flexible and efficient utilization of the
radio frequency spectrum. Implementation aspects are discussed and illustrated
by examples based on analytic and numerical solutions.Comment: Fixed LaTeX rendering errors due to inconsistencies between arXiv's
LaTeX machine and texlive in OpenSuSE 13.
Multiplication and division of the orbital angular momentum of light with diffractive transformation optics
We present a method to efficiently multiply or divide the orbital angular
momentum (OAM) of light beams using a sequence of two optical elements. The
key-element is represented by an optical transformation mapping the azimuthal
phase gradient of the input OAM beam onto a circular sector. By combining
multiple circular-sector transformations into a single optical element, it is
possible to perform the multiplication of the value of the input OAM state by
splitting and mapping the phase onto complementary circular sectors.
Conversely, by combining multiple inverse transformations, the division of the
initial OAM value is achievable, by mapping distinct complementary circular
sectors of the input beam into an equal number of circular phase gradients. The
optical elements have been fabricated in the form of phase-only diffractive
optics with high-resolution electron-beam lithography. Optical tests confirm
the capability of the multiplier optics to perform integer multiplication of
the input OAM, while the designed dividers are demonstrated to correctly split
up the input beam into a complementary set of OAM beams. These elements can
find applications for the multiplicative generation of higher-order OAM modes,
optical information processing based on OAM-beams transmission, and optical
routing/switching in telecom.Comment: 28 pages, 10 figure
Preparation of circular Rydberg states in helium using the crossed fields method
Helium atoms have been prepared in the circular
Rydberg state using the crossed electric
and magnetic fields method. The atoms, initially travelling in pulsed
supersonic beams, were photoexcited from the metastable 1s2s\,^3S_1 level to
the outermost, Rydberg-Stark state with in the presence of
a strong electric field and weak perpendicular magnetic field. Following
excitation, the electric field was adiabatically switched off causing the atoms
to evolve into the circular state with defined with respect to
the magnetic field quantization axis. The circular states were detected by
ramped electric field ionization along the magnetic field axis. The dependence
of the circular state production efficiency on the strength of the excitation
electric field, and the electric-field switch-off time was studied, and
microwave spectroscopy of the circular-to-circular
transition at ~GHz
was performed.Comment: 10 pages, 8 figure
Orbital Angular Momentum (OAM) of Rotating Modes Driven by Electrons in Electron Cyclotron Masers
The well-defined orbital angular momentum (OAM) of rotating cavity modes operating near the cutoff frequency excited by gyrating electrons in a high-power electron cyclotron maser (ECM)-a gyrotron-has been derived by photonic and electromagnetic wave approaches. A mode generator was built with a high-precision 3D printing technique to mimic the rotating gyrotron modes for precise low-power measurements and shows clear natural production of higher-order OAM modes. Cold-test measurements of higher-order OAM mode generation promise the realization towards wireless long-range communications using high-power ECMs
Parallel axis theorem for free-space electron wavefunctions
We consider the orbital angular momentum of a free electron vortex moving in
a uniform magnetic field. We identify three contributions to this angular
momentum: the canonical orbital angular momentum associated with the vortex,
the angular momentum of the cyclotron orbit of the wavefunction, and a
diamagnetic angular momentum. The cyclotron and diamagnetic angular momenta are
found to be separable according to the parallel axis theorem. This means that
rotations can occur with respect to two or more axes simultaneously, which can
be observed with superpositions of vortex states
On Small Beams with Large Topological Charge II: Photons, Electrons and Gravitational Waves
Beams of light with a large topological charge significantly change their
spatial structure when they are focused strongly. Physically, it can be
explained by an emerging electromagnetic field component in the direction of
propagation, which is neglected in the simplified scalar wave picture in
optics. Here we ask: Is this a specific photonic behavior, or can similar
phenomena also be predicted for other species of particles? We show that the
same modification of the spatial structure exists for relativistic electrons as
well as for focused gravitational waves. However, this is for different
physical reasons: For electrons, which are described by the Dirac equation, the
spatial structure changes due to a Spin-Orbit coupling in the relativistic
regime. In gravitational waves described with linearized general relativity,
the curvature of space-time between the transverse and propagation direction
leads to the modification of the spatial structure. Thus, this universal
phenomenon exists for both massive and massless elementary particles with Spin
1/2, 1 and 2. It would be very interesting whether other types of particles
such as composite systems (neutrons or C) or neutrinos show a similar
behaviour and how this phenomenon can be explained in a unified physical way.Comment: 8 pages, 3 figure
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