Slow light
We present a brief classical discussion of a process to reduce the group velocity of an electromagnetic pulse by many orders of magnitude.
physics.optics↗
arXiv subjects
Kirk T. McDonald
Publications and source records attributed to Kirk T. McDonald.
A mechanical model that exhibits a gravitational critical radius
Circular orbits of a particle sliding on a frictionless surface of revolution about a vertical axis are unstable below a critical radius if the curvature of the surface satisfies a specified condition. This behavior can be realized in a mechanical model, which give a glimpse of the greater complexity of particle orbits in curved rather than flat spacetime.
physics.class-ph↗
Radial Viscous Flow between Two Parallel Annular Plates
An analytic solution to the linearized Navier-Stokes equation is given that describes the radial flow of an incompressible, viscous fluid between two parallel, concentric annular plates.
physics.flu-dyn↗
Bessel Beams
Scalar Bessel beams are derived both via the wave equation and via diffraction theory. While such beams have a group velocity that exceeds the speed of light, this is a manifestation of the "scissors paradox" of special relativty. The signal velocity of a modulated Bessel beam is less than the speed of light. Forms of Bessel beams that satisfy Maxwell's equations are also given.
physics.optics↗
Polarization Dependence of Emissivity
We deduce the emissivity of radiation from a metallic surface as a function of angle and polarization. This effect has found application in the calibration of detectors for cosmic microwave background radiation.
physics.optics↗
Axicon Gaussian Laser Beams
We deduce the simplest form for an axicon Gaussian laser beam, i.e., one with radial polarization of the electric field.
physics.optics↗
Diffraction as a Consequence of Faraday's Law
We consider a linearly polarized electromagnetic wave incident on an opaque screen with square aperture of edge a. An application of Faraday's law to a loop parallel to the screen, on the side away from the source, shows that the wave must have longitudinal components there. The ratio of the longitudinal to transverse field is a measure of the diffraction angle.
physics.optics↗
A bounded source cannot emit a unipolar electromagnetic wave
We show that a bounded source cannot produce a unipolar electromagnetic pulse. As a consequence, there are no three-dimensional electromagnetic solitons in vacuum.
physics.class-ph↗
Temporary Acceleration of Electrons While Inside an Intense Electromagnetic Pulse
A free electron can temporarily gain a very significant amount of energy if it is overrun by an intense electromagnetic wave. In principle, this process would permit large enhancements in the center-of-mass energy of electron-electron, electron-positron and electron-photon interactions if these take place in the presence of an intense laser beam. Practical considerations severely limit the utility of this concept for contemporary lasers incident on relativistic electrons. A more accessible laboratory phenomenon is electron-positron production via an intense laser beam incident on a gas. Intense electromagnetic pulses of astrophysical origin can lead to very energetic photons via bremsstrahlung of temporarily accelerated electrons.
physics.acc-ph↗
Radiation from a Superluminal Source
The sweep speed of an electron beam across the face of an oscilloscope can exceed the velocity of light, although of course the velocity of the electrons does not. Associated with this possibility there should be a kind of Cherenkov radiation, as if the oscilloscope trace were due to a charge moving with superluminal velocity.
physics.acc-ph↗
Time-Reversed Diffraction
We show that a time-reversed formulation of Huygens-Kirchhoff diffraction can be used to deduce the transverse and longitudinal fields of a Gaussian laser beam, starting from a simple assumption of a Gaussian beam profile in the far field. An attempt to apply this technique to the far fields of a Hertzian dipole shows how the laws of diffraction do not permit a wave to be focused to a volume smaller than a cubic wavelength in a charge-free region.
physics.optics↗
Classical "Dressing" of a Free Electron in a Plane Electromagnetic Wave
The energy and momentum densities of the fields of a free electron in a plane electromagnetic wave include interference terms that are the classical version of the ``dressing'' of the electron the arises in a quantum analysis. The transverse mechanical momentum of the oscillating electron is balanced by the field momentum resulting from the interference between the driving wave and the static part of the electron's field. The interference between the wave and the oscillating part of the electron's field leads to a longitudinal field momentum and a negative field energy that compensate for the longitudinal momentum and kinetic energy of the electron. The interference terms are dominated by the near zone, so that as the wave passes the electron by the latter reverts to its energy and momentum prior to the arrival of the wave.
physics.acc-ph↗
The Hawking-Unruh Temperature and Quantum Fluctuations in Particle Accelerators
We wish to draw attention to a novel view of the effect of the quantum fluctuations during the radiation of accelerated particles, particularly those in storage rings. This view is inspired by the remarkable insight of Hawking that the effect of the strong gravitational field of a black hole on the quantum fluctuations of the surrounding space is to cause the black hole to radiate with a temperature T = hbar g / 2 pi c k, where g is the acceleration due to gravity at the surface of the black hole, c is the speed of light, and k is Boltzmann's constant. Shortly thereafter Unruh argued that an accelerated observer should become excited by quantum fluctuations to a temperature T = hbar a* / 2 pi c k, where a* is the acceleration of the observer in its instantaneous rest frame. In a series of papers Bell and co-workers have noted that electron storage rings provide a demonstration of the utility of the Hawking-Unruh temperature, with emphasis on the question of the incomplete polarization of the electrons due to quantum fluctuations of synchrotron radiation. Here we expand slightly on the results of Bell et al., and encourage the reader to consult the literature for more detailed understanding.
physics.acc-ph↗
The Hawking-Unruh Temperature and Damping in a Linear Focusing Channel
The Hawking-Unruh effective temperature, hbar a* / 2 pi c k, due to quantum fluctuations in the radiation of an accelerated charged-particle beam can be used to show that transverse oscillations of the beam in a practical linear focusing channel damp to the quantum-mechanical limit. A comparison is made between this behavior and that of beams in a wiggler.
physics.acc-ph↗
Limits on the Applicability of Classical Electromagnetic Fields as Inferred from the Radiation Reaction
Can the wavelength of a classical electromagnetic field be arbitrarily small, or its electric field strength be arbitrarily large? If we require that the radiation-reaction force on a charged particle in response to an applied field be smaller than the Lorentz force we find limits on the classical electromagnetic field that herald the need for a better theory, i.e., one in better accord with experiment. The classical limitations find ready interpretation in quantum electrodynamics. The examples of Compton scattering and the QED critical field strength are discussed. It is still open to conjecture whether the present theory of QED is valid at field strengths beyond the critical field revealed by a semiclassical argument.
physics.class-ph↗
Energy Balance in an Electrostatic Accelerator
The principle of an electrostatic accelerator is that when a charge e escapes from a conducting plane that supports a uniform electric field of strength E_0, then the charge gains energy e E_0 d as it moves distance d from the plane. Where does this energy come from? We that the mechanical energy gain of the electron is balanced by the decrease in the electrostatic field energy of the system.
physics.acc-ph↗
The Grating Accelerator
In optics, a reflective grating is a conducting surface with a ripple. The typical use of such a grating involves an incident electromagnetic wave with wave vector k in the x-z plane, and interference effects lead to a discrete set of reflected waves also with wave vectors in the x-z plane. Consider, instead, an incident plane electromagnetic wave with wave vector in the y-z plane and polarization in the x direction: We show that for small ripples, this leads to a reflected wave plus two surface waves that are attenuated exponentially with z. For certain values of the grating wavelength and the optical wavelength, the component of the phase velocity of the surface waves is the speed of light c. In this case, a charged particle moving with v_x ~ c could extract energy from the wave, which is the principle of the "grating accelerator".
physics.acc-ph↗
The Laser-Driven Vacuum Photodiode
A vacuum photodiode driven by a pulsed laser can sustain an average current very similar to that of a continous diode, where the latter is described by Child's law.
physics.acc-ph↗