Stephen B. Libby

Theory of the quantized hall effect (II)

Abstract We review the derivation that the effective lagrangian describing the critical fluctuations in a two-dimensional disordered electronic system in a transverse magnetic field contains a novel, topological term. We extend this result in several directions. We show how the importance of topological concepts can be seen by examining in detail the nature of the boundary current whenever the Fermi energy lies within a localized state region. This insight allows us to construct a field theoretic quantization argument. Our argument is reminiscent of the Laughlin-Halperin quantization approach, in that we make use of the response of the system to sources with nontrivial gauge topology. This then leads to a discussion of how to use the effective field theory to actually compute the response, and of why localization must break down somewhere within the Landau band. Our methodology unifies the results of Laughlin, Halperin and Thouless with the field theoretic approach to localization pioneered by Wegner.

Ultraviolet surprise: Efficient soft x-ray high-harmonic generation in multiply ionized plasmas.

Short wavelengths birth shorter ones The shortest laser pulses—with durations measured in attoseconds—arise from a process termed high-harmonic generation (HHG). Essentially, a longer, “driving” pulse draws electrons out of gaseous atoms like a slingshot, and, when they ricochet back, light emerges at shorter wavelengths. Most HHG has been carried out using light near the visible/infrared boundary for the driving pulse. Popmintchev et al. used an ultraviolet driving pulse instead, which yielded an unexpectedly efficient outcome. These results could presage a more generally efficient means of creating x-ray pulses for fundamental dynamics studies as well as technological applications. Science, this issue p. 1225 Ultraviolet pulses show unexpected efficiency in generating the higher-frequency emission underlying attosecond spectroscopy. High-harmonic generation is a universal response of matter to strong femtosecond laser fields, coherently upconverting light to much shorter wavelengths. Optimizing the conversion of laser light into soft x-rays typically demands a trade-off between two competing factors. Because of reduced quantum diffusion of the radiating electron wave function, the emission from each species is highest when a short-wavelength ultraviolet driving laser is used. However, phase matching—the constructive addition of x-ray waves from a large number of atoms—favors longer-wavelength mid-infrared lasers. We identified a regime of high-harmonic generation driven by 40-cycle ultraviolet lasers in waveguides that can generate bright beams in the soft x-ray region of the spectrum, up to photon energies of 280 electron volts. Surprisingly, the high ultraviolet refractive indices of both neutral atoms and ions enabled effective phase matching, even in a multiply ionized plasma. We observed harmonics with very narrow linewidths, while calculations show that the x-rays emerge as nearly time-bandwidth–limited pulse trains of ~100 attoseconds.

Pulsed laser interactions with space debris: Target shape effects

Abstract Among the approaches to the proposed mitigation and remediation of the space debris problem is the de-orbiting of objects in low Earth orbit through irradiation by ground-based high-intensity pulsed lasers. Laser ablation of a thin surface layer causes target recoil, resulting in the depletion of orbital angular momentum and accelerated atmospheric re-entry. However, both the magnitude and direction of the recoil are shape dependent, a feature of the laser-based remediation concept that has received little attention. Since the development of a predictive capability is desirable, we have investigated the dynamical response to ablation of objects comprising a variety of shapes. We derive and demonstrate a simple analytical technique for calculating the ablation-driven transfer of linear momentum, emphasizing cases for which the recoil is not exclusively parallel to the incident beam. For the purposes of comparison and contrast, we examine one case of momentum transfer in the low-intensity regime, where photon pressure is the dominant momentum transfer mechanism, showing that shape and orientation effects influence the target response in a similar, but not identical, manner. We address the related problem of target spin and, by way of a few simple examples, show how ablation can alter the spin state of a target, which often has a pronounced effect on the recoil dynamics.

Observation and control of shock waves in individual nanoplasmas.

Using an apparatus that images the momentum distribution of individual, isolated 100-nm-scale plasmas, we make the first experimental observation of shock waves in nanoplasmas. We demonstrate that the introduction of a heating pulse prior to the main laser pulse increases the intensity of the shock wave, producing a strong burst of quasimonoenergetic ions with an energy spread of less than 15%. Numerical hydrodynamic calculations confirm the appearance of accelerating shock waves and provide a mechanism for the generation and control of these shock waves. This observation of distinct shock waves in dense plasmas enables the control, study, and exploitation of nanoscale shock phenomena with tabletop-scale lasers.

Renormalization of the θ angle, the quantum Hall effect and the strong CP problem

Abstract We discuss the need for and consequences of non-perturbative renormalizations of couplings including the vacuum θ angle. An explicit example of these effects is found in the recently proposed scaling theory of the integral quantum Hall effect. In that theory the θ angle has a precise parallel in the value of the Hall conductivity σ xy which flows from its CPA (short distance) value to quantization at macroscopic distances. We further comment that such θ scaling phenomena provide a mechanism for strong CP conservation.

Mapping Nanoscale Absorption of Femtosecond Laser Pulses Using Plasma Explosion Imaging

We make direct observations of localized light absorption in a single nanostructure irradiated by a strong femtosecond laser field, by developing and applying a technique that we refer to as plasma explosion imaging. By imaging the photoion momentum distribution resulting from plasma formation in a laser-irradiated nanostructure, we map the spatial location of the highly localized plasma and thereby image the nanoscale light absorption. Our method probes individual, isolated nanoparticles in vacuum, which allows us to observe how small variations in the composition, shape, and orientation of the nanostructures lead to vastly different light absorption. Here, we study four different nanoparticle samples with overall dimensions of ∼100 nm and find that each sample exhibits distinct light absorption mechanisms despite their similar size. Specifically, we observe subwavelength focusing in single NaCl crystals, symmetric absorption in TiO2 aggregates, surface enhancement in dielectric particles containing a single gold nanoparticle, and interparticle hot spots in dielectric particles containing multiple smaller gold nanoparticles. These observations demonstrate how plasma explosion imaging directly reveals the diverse ways in which nanoparticles respond to strong laser fields, a process that is notoriously challenging to model because of the rapid evolution of materials properties that takes place on the femtosecond time scale as a solid nanostructure is transformed into a dense plasma.

Momentum Transfer by Laser Ablation of Irregularly Shaped Space Debris

Proposals for ground‐based laser remediation of space debris rely on the creation of appropriately directed ablation‐driven impulses to either divert the fragment or drive it into an orbit with a perigee allowing atmospheric capture. For a spherical fragment, the ablation impulse is a function of the orbital parameters and the laser engagement angle. If, however, the target is irregularly shaped and arbitrarily oriented, new impulse effects come into play. Here we present an analysis of some of these effects.

Non-abelian monopoles in the three-dimensional chiral spin liquid

Abstract We show by explicit numerical calculation that the charged spinless excitations of the three-dimensional chiral spin liquid state have nonzero Berry phases analogous to the 1 2 fractional statistics seen in two dimensions. These reduce, in the limit of large separation and low velocities, to vector potentials of magnetic monopoles coupling to the four internal degrees of freedom of the moving particle through generators of SU(4).

Application of x-ray-laser interferometry to study high-density laser-produced plasmas

Collisionally pumped soft-x-ray lasers now operate over a wavelength range extending from 4 to 40 nm. With the recent advances in the development of multilayer mirrors and beam splitters in the soft-x-ray regime, we can utilize the unique properties of x-ray lasers to study large, rapidly evolving laser-driven plasmas with high electron densities. Using a neonlike yttrium x-ray laser, which operates at a wavelength of 15.5 nm, we have performed a series of radiography, moire deflectometry, and interferometry experiments to characterize plasmas relevant to inertial confinement fusion. We describe experiments using a soft-x-ray laser interferometer, operated in the Mach–Zehnder configuration, to study CH plasmas. The two-dimensional density profiles obtained from the interferograms allow us to validate and benchmark our numerical models used to study the physics of laser–plasma interactions.

Bayesian inference of inaccuracies in radiation transport physics from inertial confinement fusion experiments

First principles microphysics models are essential to the design and analysis of high energy density physics experiments. Using experimental data to investigate the underlying physics is also essential, particularly when simulations and experiments are not consistent with each other. This is a difficult task, due to the large number of physical models that play a role, and due to the complex (and as a result, noisy) nature of the experiments. This results in a large number of parameters that make any inference a daunting task; it is also very important to consistently treat both experimental and prior understanding of the problem. In this paper we present a Bayesian method that includes both these effects, and allows the inference of a set of modifiers which have been constructed to give information about microphysics models from experimental data. We pay particular attention to radiation transport models. The inference takes into account a large set of experimental parameters and an estimate of the prior knowledge through a modified