Dynamics of relativistic transparency and optical shuttering in expanding overdense plasmas

Overdense plasmas are usually opaque to laser light. However, when the light is of sufficient intensity to drive electrons in the plasma to near light speeds, the plasma becomes transparent. This process—known as relativistic transparency—takes just a tenth of a picosecond. Yet all studies of relativistic transparency so far have been restricted to measurements collected over timescales much longer than this, limiting our understanding of the dynamics of this process. Here we present time-resolved electric field measurements (with a temporal resolution of ~ 50 fs) of the light, initially reflected from, and subsequently transmitted through, an expanding overdense plasma. Our result provides insight into the dynamics of the transparent-overdense regime of relativistic plasmas, which should be useful in the development of laser-driven particle accelerators, X-ray sources and techniques for controlling the shape and contrast of intense laser pulses.

Increasing recoverable energy storage in electroceramic capacitors using “dead-layer” engineering

The manner in which ultrathin films of alumina, deposited at the dielectric-electrode interface, affect the recoverable energy density associated with (BiFeO3)0.6–(SrTiO3)0.4 (BFST) thin film capacitors has been characterised. Approximately 6 nm of alumina on 400 nm of BFST increases the maximum recoverable energy of the system by around 30% from 13 Jcc1 to 17 Jcc1.
Essentially, the alumina acts in the same way as a naturally present parasitic “dead-layer,” distorting the polarisation-field response such that the ultimate polarisation associated with the BFST is pushed to higher values of electric field. The work acts as a proof-of-principle to illustrate how the design of artificial interfacial dielectric “dead-layers” can increase energy densities in simple dielectric capacitors, allowing them to compete more generally with other energy storage technologies.

Measuring oxygen reduction/evolution reactions on the nanoscale

The efficiency of fuel cells and metal-air batteries is significantly limited by the activation of oxygen reduction and evolution reactions. Despite the well-recognized role of oxygen reaction kinetics on the viability of energy technologies, the governing mechanisms remain elusive and until now have been addressable only by macroscopic studies. This lack of nanoscale understanding precludes optimization of material architecture. Here, we report direct measurements of oxygen reduction/evolution reactions and oxygen vacancy diffusion on oxygen-ion conductive solid surfaces with sub-10 nm resolution. In electrochemical strain microscopy, the biased scanning probe microscopy tip acts as a moving, electrocatalytically active probe exploring local electrochemical activity. The probe concentrates an electric field in a nanometre-scale volume of material, and bias-induced, picometre-level surface displacements provide information on local electrochemical processes. Systematic mapping of oxygen activity on bare and platinum-functionalized yttria-stabilized zirconia surfaces is demonstrated. This approach allows direct visualization of the oxygen reduction/evolution reaction activation process at the triple-phase boundary, and can be extended to a broad spectrum of oxygen-conductive and electrocatalytic materials.

A Strain-Driven Morphotropic Phase Boundary in BiFeO3

Piezoelectric materials, which convert mechanical to electrical energy and vice versa, are typically characterized by the intimate coexistence of two phases across a morphotropic phase boundary. Electrically switching one to the other yields large electromechanical coupling coefficients. Driven by global environmental concerns, there is currently a strong push to discover practical lead-free piezoelectrics for device engineering. Using a combination of epitaxial growth techniques in conjunction with theoretical approaches, we show the formation of a morphotropic phase boundary through epitaxial constraint in lead-free piezoelectric bismuth ferrite (BiFeO3) films. Electric field-dependent studies show that a tetragonal-like phase can be reversibly converted into a rhombohedral-like phase, accompanied by measurable displacements of the surface, making this new lead-free system of interest for probe-based data storage and actuator applications.

Probing Ferroelectrics Using Optical Second Harmonic Generation

Nonlinear optics is an essential component of modern laser systems and optoelectronic devices. It has also emerged as an important tool in probing the electronic, vibrational, magnetic, and crystallographic structure of materials ranging from oxides and metals, to polymers and biological samples. This review focuses on the specific technique of optical second harmonic generation (SHG), and its application in probing ferroelectric complex oxide crystals and thin films. As the dominant SHG interaction mechanism exists only in materials that lack inversion symmetry, SHG is a sensitive probe of broken inversion symmetry, and thus also of bulk polar phenomena in materials. By performing in-situ SHG polarimetry experiments in different experimental conditions such as sample orientation, applied electric field, and temperature, one can probe ferroelectric hysteresis loops and phase transitions. Careful modeling of the polarimetry data allows for the determination of the point group symmetry of the crystal. In epitaxial thin films with a two-dimensional arrangement of well-defined domain orientations, one can extract information about intrinsic material properties such as nonlinear coefficients, as well as microstructural information such as the local statistics of the different domain variants being probed. This review presents several detailed examples of ferroelectric systems where such measurements and modeling are performed. The use of SHG microscopic imaging is discussed, and its ability to reveal domain structures and phases not normally visible with linear optics is illustrated.

Controlling magnetoelectric coupling by nanoscale phase transformation in strain engineered bismuth ferrite

The magnetoelectric coupling in multiferroic materials is promising for a wide range of applications, yet manipulating magnetic ordering by electric field proves elusive to obtain and difficult to control. In this paper, we explore the prospect of controlling magnetic ordering in misfit strained bismuth ferrite (BiFeO3, BFO) films, combining theoretical analysis, numerical simulations, and experimental characterizations. Electric field induced transformation from a tetragonal phase to a distorted rhombohedral one in strain engineered BFO films has been identified by thermodynamic analysis, and realized by scanning probe microscopy (SPM) experiment. By breaking the rotational symmetry of a tip-induced electric field as suggested by phase field simulation, the morphology of distorted rhombohedral variants has been delicately controlled and regulated. Such capabilities enable nanoscale control of magnetoelectric coupling in strain engineered BFO films that is difficult to achieve otherwise, as demonstrated by phase field simulations.

Ultrafast nonlinearities of minibands in metallodielectric Bragg resonators

The nonlinear properties of metallodielectric DBRs are investigated via optical pump-probe techniques using a widely tunable, dual-colour, high-repetition rate, ultrafast setup. As a consequence of the Bragg-arranged multilayers, the electric field penetrates to different depths of the nanostructure at different excitation resonances, strongly enhancing the intrinsic nonlinear response of the metal in comparison with bulk films. The analyzed spectral response of these structures reveals how their nonlinear behavior is dominated by the pump-induced modification of the metal dielectric function. Fitting the simulated changes of the optical resonances using transfer-matrix methods matches experiment well, and shows the key effects of the spectral dependence of the spatial mode profiles.

Tip-enhanced Raman microscopy: practicalities and limitations

The feasibility of apertureless scanning near-field Raman microscopy, exploiting the local enhancement in Raman scattering in the vicinity of a silver or gold tip, was investigated. Using the finite difference time domain method we calculated the enhancement of electric field strength, and hence Raman scattering, achieved through the resonant excitation of local modes in the tip. By modelling the frequency-dependent dielectric response of the metal tip we were able to highlight the resonant nature of the tip-enhancement and determine the excitation wavelength required for the strongest electric field enhancement, and hence Raman scattering intensity, which occurs for the excitation of modes localized at the tip apex. It is demonstrated that a peak Raman enhancement of 10(7)-fold should be achievable with <5 nm spatial resolution. We show that surface-enhanced Raman scattering from carbon contamination on a silver or gold tip can be significant. However, we find for a tip of radius of curvature 20 nm that the Raman enhancement should decay totally within 20 nm from the tip. Hence withdrawal of the tip by this distance should lead to the disappearance of the tip-enhanced signal, leaving only that from carbon contamination on the tip itself and the intrinsic signal from the sample. Copyright (C) 2003 John Wiley Sons, Ltd.

PIC simulation of a thermal anisotropy-driven Weibel instability in a circular rarefaction wave

The expansion of an initially unmagnetized planar rarefaction wave has recently been shown to trigger a thermal anisotropy-driven Weibel instability (TAWI), which can generate magnetic fields from noise levels. It is examined here whether the TAWI can also grow in a curved rarefaction wave. The expansion of an initially unmagnetized circular plasma cloud, which consists of protons and hot electrons, into a vacuum is modelled for this purpose with a two-dimensional particle-in-cell (PIC) simulation. It is shown that the momentum transfer from the electrons to the radially accelerating protons can indeed trigger a TAWI. Radial current channels form and the aperiodic growth of a magnetowave is observed, which has a magnetic field that is oriented orthogonal to the simulation plane. The induced electric field implies that the electron density gradient is no longer parallel to the electric field. Evidence is presented here that this electric field modification triggers a environments, which are needed to explain the electromagnetic emissions by astrophysical jets. It is outlined how this instability could be examined experimentally.second magnetic instability, which results in a rotational low-frequency magnetowave. The relevance of the TAWI is discussed for the growth of small-scale magnetic fields in astrophysical

Fragmentation of Neutral Amino Acids and Small Peptides by Intense, Femtosecond Laser Pulses

High power femtosecond laser pulses have unique properties that could lead to their application as ionization or activation sources in mass spectrometry. By concentrating many photons into pulse lengths approaching the timescales associated with atomic motion, very strong electric field strengths are generated, which can efficiently ionize and fragment molecules without the need for resonant absorption. However, the complex interaction between these pulses and biomolecular species is not well understood. To address this issue, we have studied the interaction of intense, femtosecond pulses with a number of amino acids and small peptides. Unlike previous studies, we have used neutral forms of these molecular targets, which allowed us to investigate dissociation of radical cations without the spectra being complicated by the action of mobile protons. We found fragmentation was dominated by fast, radical-initiated dissociation close to the charge site generated by the initial ionization or from subsequent ultrafast migration of this charge. Fragments with lower yields, which are useful for structural determinations, were also observed and attributed to radical migration caused by hydrogen atom transfer within the molecule.

LINEAR ACCELERATION EMISSION IN PULSAR MAGNETOSPHERES

Linear acceleration emission occurs when a charged particle is accelerated parallel to its velocity. We evaluate the spectral and angular distribution of this radiation for several special cases, including constant acceleration (hyperbolic motion) of finite duration. Based on these results, we find the following general properties of the emission from an electron in a linear accelerator that can be characterized by an electric field E acting over a distance L: (1) the spectrum extends to a cutoff frequency (h) over bar omega(c)/mc(2) approximate to L(E/E(Schw))(2)/(lambda) over bar (C), where E(Schw) = 1.3 x 10(18) V m(-1) is the Schwinger critical field and (lambda) over bar (C) = (h) over bar /mc = 3.86 x 10(-13) m is the Compton wavelength of the electron, (2) the total energy emitted by a particle traversing the accelerator is 4/3 alpha(f)(h) over bar omega(c) in accordance with the standard Larmor formula where alpha(f) is the fine-structure constant, and (3) the low frequency spectrum is flat for hyperbolic trajectories, but in general depends on the details of the accelerator. We also show that linear acceleration emission complements curvature radiation in the strongly magnetized pair formation regions in pulsar magnetospheres. It dominates when the length L of the accelerator is less than the formation length rho/gamma of curvature photons, where rho is the radius of curvature of the magnetic field lines and gamma the Lorentz factor of the emitting particle. In standard static models of pair creating regions linear acceleration emission is negligible, but it is important in more realistic dynamical models in which the accelerating field fluctuates on a short length scale.

Dust-ion-acoustic supersolitons in dusty plasmas with nonthermal electrons

Supersolitons are a recent addition to the literature on large-amplitude solitary waves in multispecies plasmas. They are distinguished from the usual solitons by their associated electric field profiles which are inherently distinct from traditional bipolar structures. In this paper, dust-ion-acoustic modes in a dusty plasma with stationary negative dust, cold fluid protons, and nonthermal electrons are investigated through a Sagdeev pseudopotential approach to see where supersolitons fit between ranges of ordinary solitons and double layers, as supersolitons always have finite amplitudes. They therefore cannot be described by reductive perturbation treatments, which rely on a weak amplitude assumption. A systematic methodology and discussion is given to distinguish the existence domains in solitary wave speed and amplitude for the different solitons, supersolitons and double layers, in terms of compositional parameters for the plasma model under consideration. © 2013 American Physical Society.

Electrostatic supersolitons in three-species plasmas

Superficially, electrostatic potential profiles of supersolitons look like those of traditional solitons. However, their electric field profiles are markedly different, having additional extrema on the wings of the standard bipolar structure. This new concept was recently pointed out in the literature for a plasma model with five species. Here, it is shown that electrostatic supersolitons are not an artefact of exotic, complicated plasma models, but can exist even in three-species plasmas and are likely to occur in space plasmas. Further, a methodology is given to delineate their existence domains in a systematic fashion by determining the specific limiting factors. © 2013 American Institute of Physics.

Ferroelectric Domain Wall Injection

Ferroelectric domain wall injection has been demonstrated by engineering of the
local electric field, using focused ion beam milled defects in thin single crystal lamellae of KTiOPO4 (KTP). The electric field distribution (top) displays localized field hot-spots, which correlate with nucleation events (bottom). Designed local field variations can also dictate subsequent domain wall mobility, demonstrating a new paradigm in ferroelectric domain wall control.

Manipulating Ferroelectric Domains in Nanostructures Under Electron Beams

Freestanding BaTiO3 nanodots exhibit domain structures characterized by distinct quadrants of ferroelastic 90 domains in transmission electron microscopy (TEM) observations. These differ significantly from flux-closure domain patterns in the same systems imaged by piezoresponse force microscopy. Based upon a series of phase field simulations of BaTiO3 nanodots, we suggest that the TEM patterns result from a radial electric field arising from electron beam charging of the nanodot. For sufficiently large charging, this converts flux-closure domain patterns to quadrant patterns with radial net polarizations. Not only does this explain the puzzling patterns that have been observed in TEM studies of ferroelectric nanodots, but also suggests how to manipulate ferroelectric domain patterns via electron beams.