Faculty Publications, Department of Physics and Astronomy Copyright (c) 2013 University of Nebraska - Lincoln All rights reserved. http://digitalcommons.unl.edu/physicsfacpub Recent documents in Faculty Publications, Department of Physics and Astronomy en-us Thu, 24 Jan 2013 20:27:25 PST 3600 Phase and Absorption Gratings for Electrons http://digitalcommons.unl.edu/physicsfacpub/126 http://digitalcommons.unl.edu/physicsfacpub/126 Thu, 17 Jan 2013 10:06:39 PST We report the experimental realization of phase and absorption gratings for electrons. Phase gratings made with standing waves of light with a periodicity of 266 nm are used to diffract 380 eV electrons [1]. Material gratings of 100 and 200 nm periodicity are used to diffract 500 eV electrons. We are exploring the possibility to use these gratings for low energy electron interferometry.

]]> Hong Gao et al. Observation of the Kapitza-Dirac Effect http://digitalcommons.unl.edu/physicsfacpub/125 http://digitalcommons.unl.edu/physicsfacpub/125 Wed, 16 Jan 2013 13:37:00 PST In their famous 1927 experiment, Davisson and Germer observed the diffraction of electrons by a periodic material structure, so showing that electrons can behave like waves. Shortly afterwards, Kapitza and Dirac predicted that electrons should also be diffracted by a standing light wave. This Kapitza-Dirac effect is analogous to the diffraction of light by a grating, but with the roles of the wave and matter reversed. The electron and the light grating interact extremely weakly, via the ‘ponderomotive potential,’ so attempts to measure the Kapitza-Dirac effect had to wait for the development of the laser. The idea that the underlying interaction with light is resonantly enhanced for electrons in an atom led to the observation that atoms could be diffracted by a standing wave of light. Deflection of electrons by high-intensity laser light, which is also a consequence of the Kapitza-Dirac effect, has also been demonstrated. But the coherent interference that characterizes wave diffraction has not hitherto been observed. Here we report the diffraction of free electrons from a standing light wave—a realization of the Kapitza-Dirac effect as originally proposed.

]]> Daniel L. Freimund et al. Feynman’s Relativistic Electrodynamics Paradox and the Aharonov-Bohm Effect http://digitalcommons.unl.edu/physicsfacpub/124 http://digitalcommons.unl.edu/physicsfacpub/124 Wed, 16 Jan 2013 10:47:06 PST An analysis is done of a relativistic paradox posed in the Feynman Lectures of Physics involving two interacting charges. The physical system presented is compared with similar systems that also lead to relativistic paradoxes. The momentum conservation problem for these systems is presented. The relation between the presented analysis and the ongoing debates on momentum conservation in the Aharonov-Bohm problem is discussed.

]]> Adam Caprez et al. Simplest Atomic System for Sub-Doppler Laser Cooling http://digitalcommons.unl.edu/physicsfacpub/123 http://digitalcommons.unl.edu/physicsfacpub/123 Mon, 05 Nov 2012 10:16:05 PST Sub-Doppler laser cooling requires optical pumping among differently light-shifted ground-state sublevels. We describe a study of the simplest possible angular-momentum configuration that permits all sub-Doppler cooling phenomena. The Jg = 1 ⟹ Je = 0 angular-momentum configuration shows recoil-limited cooling in the two most well-known types of polarization gradient, magnetically induced laser cooling, velocity-selective resonances, transient cooling, and velocity-selective population trapping.

]]> R. Gupta et al. Slowing of <sup>85</sup>Rb Atoms with Isotropic Light http://digitalcommons.unl.edu/physicsfacpub/122 http://digitalcommons.unl.edu/physicsfacpub/122 Mon, 05 Nov 2012 10:06:12 PST We have demonstrated the slowing of a rubidium atomic beam by isotropic monochromatic light. The results agree with a model calculation, thus allowing its use for designing a practical isotropic light slower. The large hyperfine splittings of rubidium lead to natural multifrequency slowing, which is also included in our model.

]]> Herman Batelaan et al. Electrons, Stern–Gerlach Magnets, and Quantum Mechanical Propagation http://digitalcommons.unl.edu/physicsfacpub/121 http://digitalcommons.unl.edu/physicsfacpub/121 Mon, 05 Nov 2012 10:01:33 PST Quantum corrections to Newton’s equations are obtained and used to illustrate that classical dynamics is embedded explicitly in quantum dynamics. Originally, the resultant set of dynamical equations has been used to shed light on quantum chaos. We show that the method can provide insight into the dynamics of free particles and the harmonic oscillator. We then use it to determine whether Stern–Gerlach magnets can be constructed for free electrons.

]]> Herman Batelaan Light Storage with Light of Arbitrary Polarization http://digitalcommons.unl.edu/physicsfacpub/120 http://digitalcommons.unl.edu/physicsfacpub/120 Mon, 05 Nov 2012 09:51:13 PST We have demonstrated the phase coherence of stored light in Rb vapor with a completely optical technique. Combining this technique with polarization measurements provides strong evidence that arbitrary polarizations can be stored. The fidelity obtained exceeds 95% for all polarizations. We view the capability to store polarizations as a first step towards building a quantum memory in such a system.

]]> Hong Gao et al. Kapitza-Dirac Diffraction without Standing Waves: Diffraction without a Grating? http://digitalcommons.unl.edu/physicsfacpub/119 http://digitalcommons.unl.edu/physicsfacpub/119 Mon, 05 Nov 2012 09:46:15 PST We discuss electron diffraction from two counterpropagating light waves with two different frequencies.We show that, even though these waves do not form a standing wave, electron diffraction similar to the conventional Kapitza-Dirac effect, i.e., scattering on a standing wave, is still possible. The nonlinear response of the electron to the laser fields creates a stationary diffraction grating from which the same electron scatters.

]]> Olga Smirnova et al. Low-Pressure Source of Slow Metastable Rare Gas Atoms http://digitalcommons.unl.edu/physicsfacpub/118 http://digitalcommons.unl.edu/physicsfacpub/118 Mon, 05 Nov 2012 09:41:04 PST We investigate the properties of a commercial inverted magnetron pressure gauge for use as a source of slow metastable rare gas atoms. We find that the velocity distribution of the atoms as well as the pressure dependence of the output flux agree with a simple model. This shows that the low-velocity output of the source is enhanced over the Maxwell–Boltzmann form due to a velocity-dependent excitation probability. For argon, the center-line intensity per unit area of the source is measured to be greater than 4.2x1015 Ar 1s5 atoms/(s srm2) at a pressure of 23 mPa. When observing the entire source area, the center-line intensity is at least 2.6x1011 Ar 1s5 atoms/(s sr).

]]> M. H.L. van der Velden et al. Electron Diffraction from Free-Standing, Metal-Coated Transmission Gratings http://digitalcommons.unl.edu/physicsfacpub/117 http://digitalcommons.unl.edu/physicsfacpub/117 Mon, 05 Nov 2012 09:31:12 PST Electron diffraction from a free-standing nanofabricated transmission grating was demonstrated, with energies ranging from 125 eV to 25 keV. Observation of 21 diffraction orders highlights the quality of the gratings. The image charge potential due to one electron was measured by rotating the grating. These gratings may pave the way to low-energy electron interferometry.

]]> Glen Gronniger et al. Focused-Laser Interferometric Position Sensor http://digitalcommons.unl.edu/physicsfacpub/116 http://digitalcommons.unl.edu/physicsfacpub/116 Mon, 05 Nov 2012 09:16:17 PST We describe a simple method to measure the position shifts of an object with a range of tens of micrometers using a focused-laser (FL) interferometric position sensor. In this article we examine the effects of mechanical vibration on FL and Michelson interferometers. We tested both interferometers using vibration amplitudes ranging from 0 to 20 μm. Our FL interferometer has a resolution much better than the diffraction grating periodicities of 10 and 14 μm used in our experiments. A FL interferometer provides improved mechanical stability at the expense of spatial resolution. Our experimental results show that Michelson interferometers cannot be used when the vibration amplitude is more than an optical wavelength. The main purpose of this article is to demonstrate that a focused-laser interferometric position sensor can be used to measure the position shifts of an object on a less sensitive, micrometer scale when the vibration amplitude is too large to use a Michelson interferometer.

]]> Stephen J. Friedman et al. A Macroscopic Test of the Aharonov-Bohm Effect http://digitalcommons.unl.edu/physicsfacpub/115 http://digitalcommons.unl.edu/physicsfacpub/115 Mon, 05 Nov 2012 09:01:19 PST The Aharonov-Bohm (AB) effect is a purely quantum mechanical effect. The original (classified as Type-I) AB-phase shift exists in experimental conditions where the electromagnetic fields and forces are zero. It is the absence of forces that makes the ABeffect entirely quantum mechanical. Although the AB-phase shift has been demonstrated unambiguously, the absence of forces in Type-I AB-effects has never been shown. Here, we report the observation of the absence of time delays associated with forces of the magnitude needed to explain the AB-phase shift for a macroscopic system.

]]> Adam Caprez et al. Illuminating the Kapitza-Dirac Effect with Electron Matter Optics [Colloquium] http://digitalcommons.unl.edu/physicsfacpub/114 http://digitalcommons.unl.edu/physicsfacpub/114 Mon, 05 Nov 2012 08:51:07 PST The observation of the Kapitza-Dirac effect raises conceptual, theoretical, and experimental questions. The Kapitza-Dirac effect is often described as diffraction of free electrons from a standing wave of light or stimulated Compton scattering. However, for the two-color Kapitza-Dirac effect these two interpretations appear to lead to paradoxical conclusions. The discussion of this paradox deepens our understanding of both of these versions of the Kapitza-Dirac effect.

]]> Herman Batelaan The Aharonov-Bohm Effects: Variations on a Subtle Theme http://digitalcommons.unl.edu/physicsfacpub/113 http://digitalcommons.unl.edu/physicsfacpub/113 Mon, 05 Nov 2012 08:35:59 PST The notion, introduced 50 years ago, that electrons could be affected by electromagnetic potentials without coming in contact with actual force fields was received with a skepticism that has spawned a flourishing of experimental tests and expansions of the original idea.

]]> Herman Batelaan et al. More Variations on Aharonov-Bohm, Peter A. Sturock, Timothy R. Groves, Alexander Ershkovich, C. Alden Mead, Herman Batelaan and Akira Tonomura: Batelaan and Tonomura Reply http://digitalcommons.unl.edu/physicsfacpub/112 http://digitalcommons.unl.edu/physicsfacpub/112 Mon, 05 Nov 2012 08:31:21 PST First paragraph: Werner Ehrenberg and Raymond Siday did propose the magnetic version of what is now called the Aharonov-Bohm (AB) effect, as Peter Sturrock and Timothy Groves point out. We had included this reference in the early versions of our manuscript. However, limited space directed the focus of the paper to the "effect without a force" discussion, rather than a historic perspective.

]]> Herman Batelaan et al. Quantum Description and Properties of Electrons Emitted from Pulsed Nanotip Electron Sources http://digitalcommons.unl.edu/physicsfacpub/111 http://digitalcommons.unl.edu/physicsfacpub/111 Mon, 05 Nov 2012 08:16:30 PST We present a quantum calculation of the electron degeneracy for electron sources. We explore quantum interference of electrons in the temporal and spatial domain and demonstrate how it can be utilized to characterize a pulsed electron source. We estimate effects of Coulomb repulsion on two-electron interference and show that currently available nano tip pulsed electron sources operate in the regime where the quantum nature of electrons can be made dominant.

]]> Pavel Lougovski et al. Measurement of the Ultrafast Temporal Response of a Plasmonic Antenna http://digitalcommons.unl.edu/physicsfacpub/110 http://digitalcommons.unl.edu/physicsfacpub/110 Wed, 31 Oct 2012 11:56:14 PDT We report a measurement on the temporal response of a plasmonic antenna at the femtosecond time scale. The antenna consists of a square array of nanometer-size gold rods. We find that the far-field dispersion of light reflected from the plasmonic antenna is less than that of a 1.2 mm thick glass slide. Assuming a simple oscillating dipole model this implies that the near-field of the antenna may be used as an electron switch that responds faster than 20 f s. Alternatively, ultrafast electron di ffraction may be used to investigate the near-field dynamics of the plasmonic antenna.

]]> Maria Becker et al. Dynamics Underlying the Gaussian Distribution of the Classical Harmonic Oscillator in Zero-Point Radiation http://digitalcommons.unl.edu/physicsfacpub/109 http://digitalcommons.unl.edu/physicsfacpub/109 Wed, 31 Oct 2012 11:42:01 PDT In the past decades, Random Electrodynamics (also called Stochastic Electrodynamics) has been used to study the classical harmonic oscillator immersed in the classical electromagnetic zero-point radiation. Random Electrodynamics (RED) predicts an identical probability distribution for the harmonic oscillator compared to the quantum mechanical prediction for the ground state. Moreover, the Heisenberg minimum uncertainty relation is also recovered with RED. To understand the dynamics that gives rise to this probability distribution, we perform an RED simulation and follow the motion of the oscillator. This simulation provides insight in the relation between the striking di erent double-peak probability distribution of the classical harmonic oscillator and the Gaussian probability distribution of the RED harmonic oscillator.

A main objective for RED research is to establish to what extent the results of quantum mechanics can be obtained. The present simulation method can be applied to other physical systems, and it may assist in evaluating the validity range of RED.

]]> Wayne Cheng-Wei Huang et al. Quantized Excitation Spectrum of the Classical Harmonic Oscillator in Zero-Point Radiation http://digitalcommons.unl.edu/physicsfacpub/108 http://digitalcommons.unl.edu/physicsfacpub/108 Fri, 26 Oct 2012 13:46:21 PDT We report that upon excitation by a single pulse, the classical harmonic oscillator immersed in classical electromagnetic zero-point radiation, as described by random electrodynamics, exhibits a quantized excitation spectrum in agreement to that of the quantum harmonic oscillator. This numerical result is interesting in view of the generally accepted idea that classical theories do not support quantized energy spectra.

]]> Wayne Cheng-Wei Huang et al. On the Relation between the Feynman Paradox and the Aharonov–Bohm Effects http://digitalcommons.unl.edu/physicsfacpub/107 http://digitalcommons.unl.edu/physicsfacpub/107 Fri, 26 Oct 2012 12:45:59 PDT The magnetic Aharonov–Bohm (A–B) effect occurs when a point charge interacts with a line of magnetic flux, while its reciprocal, the Aharonov–Casher (A–C) effect, occurs when a magnetic moment interacts with a line of charge. For the two interacting parts of these physical systems, the equations of motion are discussed in this paper. The generally accepted claim is that both parts of these systems do not accelerate, while Boyer has claimed that both parts of these systems do accelerate. Using the Euler–Lagrange equations we predict that in the case of unconstrained motion, only one part of each system accelerates, while momentum remains conserved. This prediction requires a time-dependent electromagnetic momentum. For our analysis of unconstrained motion, the A–B effects are then examples of the Feynman paradox. In the case of constrained motion, the Euler–Lagrange equations give no forces, in agreement with the generally accepted analysis. The quantum mechanical A–B and A–C phase shifts are independent of the treatment of constraint. Nevertheless, experimental testing of the above ideas and further understanding of the A–B effects that are central to both quantum mechanics and electromagnetism could be possible.

]]> Scot McGregor et al.