Search Results (10 total)

The initial stage of polarization switching in ferroelectric thin films depends on phenomena that occur at characteristic time scales of tens to hundreds of nanoseconds, including the nucleation polarization domains and the propagation of domain walls. These long intrinsic times allow short-duration electric fields with magnitudes far above the low-frequency coercive electric field to be applied across capacitor devices without inducing switching. Using time-resolved x-ray microdiffraction, we have found that a series of 50 ns duration electric field pulses switches the polarization of a 35-nm-thick ferroelectric Pb(Zr,Ti)O₃ film only at electric fields greater than 1.5 MV/cm, a factor of three higher than the low-frequency coercive field. There is no switching in response to a large number of short pulses with amplitudes lower than 1.5 MV/cm, even when the total duration reaches several milliseconds. In comparison, a series of microsecond-duration pulses causes cumulative changes in the area of switched polarization and eventually switches the entire capacitor. The difference between long- and short-duration electric field pulses arises from effects linked to domain nucleation and charge transport in the ferroelectric film. A phase-field model shows that the shrinking of the switched domain in the interval between pulses is a less important effect. This opportunity to apply large fields for short times without inducing switching by domain-wall motion raises the possibility that future experiments could reach the intrinsic coercive field of ferroelectric layers and provides a way to study the properties of materials under high electric fields.

We report studies of thermal transport across the interface of a semiconductor heterostructure using x-ray diffraction to measure the time-dependent lattice expansion after ultrafast laser excitation. Femtosecond laser pulses are used to rapidly and locally heat the substrate at the buried interface of an Al_0.3Ga_0.7As/GaAs heterostructure grown by molecular-beam epitaxy. High-resolution time-resolved x-ray diffraction is used to study the heating and cooling of the film and substrate independently. The data are compared with a simple model for the thermal transport incorporated into dynamical diffraction calculations allowing us to extract the room-temperature cross-plane film thermal conductivity. The value is 40% lower than that extrapolated from prior results on liquid-phase epitaxy grown samples of varying concentrations.

Nonlinear effects in the coupling of polarization with elastic strain have been predicted to occur in ferroelectric materials subjected to high electric fields. Such predictions are tested here for a PbZr_0.2Ti_0.8O₃ ferroelectric thin film at electric fields in the range of several hundred MV/m and strains reaching up to 2.7%. The piezoelectric strain exceeds predictions based on constant piezoelectric coefficients at electric fields from approximately 200 to 400  MV/m, which is consistent with a nonlinear effect predicted to occur at corresponding piezoelectric distortions.

Tunable, polarized, microfocused x-ray pulses were used to record x-ray absorption spectra across the K edges of Kr⁺ and Kr²⁺ produced by laser ionization of Kr. Prominent 1s→4p and 5p excitations are observed below the 1s ionization thresholds in accord with calculated transition energies and probabilities. Due to alignment of 4p hole states in the laser-ionization process, the Kr⁺ 1s→4p cross section varies with respect to the angle between the laser and x-ray polarization vectors. This effect is used to determine the Kr⁺ 4p_3∕2 and 4p_1∕2 quantum state populations, and these are compared with results of an adiabatic strong-field ionization theory that includes spin-orbit coupling.

We use time-resolved measurements of the evolution of surface and buried layer temperatures to quantify the contribution of ballistic phonons to heat transport on nanometer length scales. A laser pulse heats a 100  nm thick Al film which cools by conduction into a GaAs substrate. The top 120–250  nm of the GaAs substrate is doped with In to create a buried layer with a distinct lattice constant. The cooling of the Al film is monitored by time-domain thermoreflectance and, in the second set of experiments, the heating and cooling of the GaAs:In buried layer are monitored by time-resolved x-ray diffraction. The combination of these data shows that thermal transport by ballistic phonons accounts for nearly 20% of the heat flow across the buried layer on nanosecond time scales.

We observe the time evolution of ground-state ion alignment in a laser-produced plasma. Krypton ions produced in a strong, linearly polarized optical laser field (10¹⁴–10¹⁵  W∕cm²) are aligned along the field polarization axis. Using microfocused, tunable x rays from Argonne’s Advanced Photon Source, we measure orbital alignment as a function of time. For plasma densities of the order of 10¹⁴  cm⁻³, the alignment decays within a few nanoseconds. A quantitative model explains the decay in terms of electron-ion collisions in the plasma. By applying an external magnetic field, we are able to suppress the disalignment and induce coherent spin precession of the Kr ions, thus providing an in situ monitor of magnetic fields in a plasma.

We have developed a synchrotron-based, time-resolved x-ray microprobe to investigate optical strong-field processes at intermediate intensities (10¹⁴–10¹⁵  W/cm²). This quantum-state specific probe has enabled the direct observation of orbital alignment in the residual ion produced by strong-field ionization of krypton atoms via resonant, polarized x-ray absorption. We found strong alignment to persist for a period long compared to the spin-orbit coupling time scale (6.2 fs). The observed degree of alignment can be explained by models that incorporate spin-orbit coupling. The methodology is applicable to a wide range of problems.

Detailed experimental studies of the first operation of an X-band (8.547 GHz) rf photoinjector are reported. The rf characteristics of the device are first described, as well as the tuning technique used to ensure operation of the 11 / 2-cell rf gun in the balanced π-mode. The characterization of the photoelectron beam produced by the rf gun includes: measurements of the bunch charge as a function of the laser injection phase, yielding information about the quantum efficiency of the Cu photocathode ( 2×10^-5 for a surface field of 100 MV/m); measurements of the beam energy (1.5–2 MeV) and relative energy spread ( Δγ/γ₀  =  1.8±0.2%) using a magnetic spectrometer; measurements of the beam 90% normalized emittance, which is found to be ɛ_n  =  1.65π mm mrad for a charge of 25 pC; and measurements of the bunch duration ( <2 ps). Coherent synchrotron radiation experiments at Ku-band and Ka-band confirm the extremely short duration of the photoelectron bunch and a peak power scaling quadratically with the bunch charge.

The validity of the concept of laser-driven vacuum acceleration has been questioned, based on an extrapolation of the well-known Lawson-Woodward theorem, which stipulates that plane electromagnetic waves cannot accelerate charged particles in vacuum. To formally demonstrate that electrons can indeed be accelerated in vacuum by focusing or diffracting electromagnetic waves, the interaction between a point charge and coherent dipole radiation is studied in detail. The corresponding four-potential exactly satisfies both Maxwell’s equations and the Lorentz gauge condition everywhere, and is analytically tractable. It is found that in the far-field region, where the field distribution closely approximates that of a plane wave, we recover the Lawson-Woodward result, while net acceleration is obtained in the near-field region. The scaling of the energy gain with wave-front curvature and wave amplitude is studied systematically.

The relativistic dynamics of an electron submitted to the three-dimensional field of a focused, ultrahigh-intensity laser pulse are studied numerically. The diffracting field in vacuum is modeled by the paraxial propagator and exactly satisfies the Lorentz gauge condition everywhere. In rectangular coordinates, the electromagnetic field is Fourier transformed into transverse and longitudinal wave packets, and diffraction is described through the different phase shifts accumulated by the various Fourier components, as constrained by the dispersion relation. In cylindrical geometry, the radial dependence of the focusing wave is described as a continuous spectrum of Bessel functions and can be obtained by using Hankel’s integral theorem. To define the boundary conditions for this problem, the beam profile is matched to a Gaussian-Hermite distribution at focus, where the wave front is planar. Plane-wave dynamics are verified for large f numbers, including canonical momentum invariance, while high-energy scattering is predicted for smaller values of f at relativistic laser intensities.