Optically excited structural transition in atomic wires on surfaces at the quantum limit.

Transient control over the atomic potential-energy landscapes of solids could lead to new states of matter and to quantum control of nuclear motion on the timescale of lattice vibrations. Recently developed ultrafast time-resolved diffraction techniques combine ultrafast temporal manipulation with atomic-scale spatial resolution and femtosecond temporal resolution. These advances have enabled investigations of photo-induced structural changes in bulk solids that often occur on timescales as short as a few hundred femtoseconds. In contrast, experiments at surfaces and on single atomic layers such as graphene report timescales of structural changes that are orders of magnitude longer. This raises the question of whether the structural response of low-dimensional materials to femtosecond laser excitation is, in general, limited. Here we show that a photo-induced transition from the low- to high-symmetry state of a charge density wave in atomic indium (In) wires supported by a silicon (Si) surface takes place within 350 femtoseconds. The optical excitation breaks and creates In-In bonds, leading to the non-thermal excitation of soft phonon modes, and drives the structural transition in the limit of critically damped nuclear motion through coupling of these soft phonon modes to a manifold of surface and interface phonons that arise from the symmetry breaking at the silicon surface. This finding demonstrates that carefully tuned electronic excitations can create non-equilibrium potential energy surfaces that drive structural dynamics at interfaces in the quantum limit (that is, in a regime in which the nuclear motion is directed and deterministic). This technique could potentially be used to tune the dynamic response of a solid to optical excitation, and has widespread potential application, for example in ultrafast detectors.

A modular table-top setup for ultrafast x-ray diffraction.

We present a table-top setup for femtosecond time-resolved x-ray diffraction based on a Cu Kα (8.05 keV) laser driven plasma x-ray source. Due to its modular design, it provides high accessibility to its individual components (e.g., x-ray optics and sample environment). The Kα-yield of the source is optimized using a pre-pulse scheme. A magnifying multilayer x-ray mirror with Montel-Helios geometry is used to collect the emitted radiation, resulting in a quasi-collimated flux of more than 105 Cu Kα photons/pulse impinging on the sample under investigation at a repetition rate of 10 Hz. A gas ionization chamber detector is placed right after the x-ray mirror and used for the normalization of the diffraction signals, enabling the measurement of relative signal changes of less than 1% even at the given low repetition rate. Time-resolved diffraction experiments on laser-excited epitaxial Bi films serve as an example to demonstrate the capabilities of the setup. The setup can also be used for Debye-Scherrer type measurements on poly-crystalline samples.

Detection of nonthermal melting by ultrafast X-ray diffraction.

Using ultrafast, time-resolved, 1.54 angstrom x-ray diffraction, thermal and ultrafast nonthermal melting of germanium, involving passage through nonequilibrium extreme states of matter, was observed. Such ultrafast, optical-pump, x-ray diffraction probe measurements provide a way to study many other transient processes in physics, chemistry, and biology, including direct observation of the atomic motion by which many solid-state processes and chemical and biochemical reactions take place.

Optimized Kalpha x-ray flashes from femtosecond-laser-irradiated foils.

We investigate the generation of ultrashort Kalpha pulses from plasmas produced by intense femtosecond p-polarized laser pulses on Copper and Titanium targets. Particular attention is given to the interplay between the angle of incidence of the laser beam on the target and a controlled prepulse. It is observed experimentally that the Kalpha yield can be optimized for correspondingly different prepulse and plasma scale-length conditions. For steep electron-density gradients, maximum yields can be achieved at larger angles. For somewhat expanded plasmas expected in the case of laser pulses with a relatively poor contrast, the Kalpha yield can be enhanced by using a near-normal-incidence geometry. For a certain scale-length range (between 0.1 and 1 times a laser wavelength) the optimized yield is scale-length independent. Physically this situation arises because of the strong dependence of collisionless absorption mechanisms-in particular resonance absorption-on the angle of incidence and the plasma scale length, giving scope to optimize absorption and hence the Kalpha yield. This qualitative description is supported by calculations based on the classical resonance absorption mechanism and by particle-in-cell simulations. Finally, the latter simulations also show that even for initially steep gradients, a rapid profile expansion occurs at oblique angles in which ions are pulled back toward the laser by hot electrons circulating at the front of the target. The corresponding enhancement in Kalpha yield under these conditions seen in the present experiment represents strong evidence for this suprathermal shelf formation effect.

Space charge effects in photoemission electron microscopy using amplified femtosecond laser pulses.

Linear and nonlinear photoemission microscopy is used to study the origin of space charge effects that are frequently observed if amplified femtosecond lasers are used for generation of photoelectrons. Space charge effects are apparent in the width of the photoemission spectra, but also create image blur. The onset threshold for space charge effects is determined by recording the width of photoemission spectra and by finding the conditions under which spectral broadening is just less than the energy resolution of the microscope. The principal findings are independent if harmonics of the fundamental of the fs laser pulses are used, but the space charge effects are found to be more dominant at lower repetition rates. By inserting apertures into the electron path, the place at which space charge effects occur can be localized.

Decelerated lattice excitation and absence of bulk phonon modes at surfaces: Ultra-fast electron diffraction from Bi(111) surface upon fs-laser excitation.

Ultrafast reflection high-energy electron diffraction is employed to follow the lattice excitation of a Bi(111) surface upon irradiation with a femtosecond laser pulse. The thermal motion of the atoms is analyzed through the Debye-Waller effect. While the Bi bulk is heated on time scales of 2 to 4 ps, we observe that the excitation of vibrational motion of the surface atoms occurs much slower with a time constant of 12 ps. This transient nonequilibrium situation is attributed to the weak coupling between bulk and surface phonon modes which hampers the energy flow between the two subsystems. From the absence of a fast component in the transient diffraction intensity, it is in addition concluded that truncated bulk phonon modes are absent at the surface.

Ultrafast electron diffraction from a Bi(111) surface: Impulsive lattice excitation and Debye-Waller analysis at large momentum transfer.

The lattice response of a Bi(111) surface upon impulsive femtosecond laser excitation is studied with time-resolved reflection high-energy electron diffraction. We employ a Debye-Waller analysis at large momentum transfer of 9.3 Å-1 ≤ Δ k ≤ 21.8 Å-1 in order to study the lattice excitation dynamics of the Bi surface under conditions of weak optical excitation up to 2 mJ/cm2 incident pump fluence. The observed time constants τ int of decay of diffraction spot intensity depend on the momentum transfer Δk and range from 5 to 12 ps. This large variation of τ int is caused by the nonlinearity of the exponential function in the Debye-Waller factor and has to be taken into account for an intensity drop ΔI > 0.2. An analysis of more than 20 diffraction spots with a large variation in Δk gave a consistent value for the time constant τT of vibrational excitation of the surface lattice of 12 ± 1 ps independent on the excitation density. We found no evidence for a deviation from an isotropic Debye-Waller effect and conclude that the primary laser excitation leads to thermal lattice excitation, i.e., heating of the Bi surface.

A pulsed electron gun for ultrafast electron diffraction at surfaces.

The construction of a pulsed electron gun for ultrafast reflection high-energy electron diffraction experiments at surfaces is reported. Special emphasis is placed on the characterization of the electron source: a photocathode, consisting of a 10 nm thin Au film deposited onto a sapphire substrate. Electron pulses are generated by the illumination of the film with ultraviolet laser pulses of femtosecond duration. The photoelectrons are emitted homogeneously across the photocathode with an energy distribution of 0.1 eV width. After leaving the Au film, the electrons are accelerated to kinetic energies of up to 15 keV. Focusing is accomplished by an electrostatic lens. The temporal resolution of the experiment is determined by the probing time of the electrons traveling across the surface which is about 30 ps. However, the duration of the electron pulses can be reduced to less than 6 ps.

Phase-matching solutions for high-order harmonic generation in hollow-core photonic-crystal fibers.

Hollow-core photonic-crystal fibers are shown to allow phase-matched high-order harmonic generation by an isolated guided mode of pump radiation. Regimes of phase matching are analyzed for the fundamental guided mode of pump field with a wavelength around 800 nm, generating harmonics within the wavelength range of 25-50 nm in hollow photonic-crystal fibers filled with argon, krypton, and helium. Geometric parameters of the fiber structure and the pressure of the gas filling the fiber core are shown to serve as important, often orthogonal, control knobs, allowing a fine adjustment of the phase matching for high-order harmonic generation.

3/2 harmonic generation by femtosecond laser pulses in steep-gradient plasmas.

The onset of an electron parametric instability and 3/2 harmonic generation in variable-scale-length plasmas on solid surfaces using femtosecond pulses is observed. With the intensity approaching 10(18) W/cm(2), the instability threshold is already reached at plasma scale lengths of the order of the laser wavelength. A well-collimated harmonic emission with unusually broad spectrum is obtained.

Two-beam high-order harmonics from solids: coupling mechanisms.

The polarization of the two beam (driver-probe) high-order harmonic generation from solids is measured. The experiments, together with computer simulations, allow us to distinguish two different coupling mechanisms of the driver and the probe, resulting in different harmonic efficiencies and spectral slopes. We find that in the nonrelativistic regime the coupling is mostly due to the nonlinear plasma density modulation.

Toward ultrafast time-resolved Debye-Scherrer x-ray diffraction using a laser-plasma source.

An elliptical glass capillary has been used to focus ultrashort Cu K alpha x-ray pulses emitted from a femtosecond laser-produced plasma. Due to its high magnification (7x), the optic transforms the divergent x-ray emission of the plasma into a quasicollimated x-ray beam with a divergence of only 0.18 degrees. As an application we demonstrate the possibility to perform Debye-Scherrer diffraction experiments with the simultaneous detection of several diffraction orders. This will allow one to extend time-resolved x-ray diffraction with femtosecond laser-plasma x-ray sources to a much wider range of materials, which are not easily available as single crystals.

Carrier-density-dependent lattice stability in InSb.

The ultrafast decay of the x-ray diffraction intensity following laser excitation of an InSb crystal has been utilized to observe carrier dependent changes in the potential energy surface. For the first time, an abrupt carrier dependent onset for potential energy surface softening and the appearance of accelerated atomic disordering for a very high average carrier density have been observed. Inertial dynamics dominate the early stages of crystal disordering for a wide range of carrier densities between the onset of crystal softening and the appearance of accelerated atomic disordering.

Ultrafast bond softening in bismuth: mapping a solid's interatomic potential with X-rays.

Intense femtosecond laser excitation can produce transient states of matter that would otherwise be inaccessible to laboratory investigation. At high excitation densities, the interatomic forces that bind solids and determine many of their properties can be substantially altered. Here, we present the detailed mapping of the carrier density-dependent interatomic potential of bismuth approaching a solid-solid phase transition. Our experiments combine stroboscopic techniques that use a high-brightness linear electron accelerator-based x-ray source with pulse-by-pulse timing reconstruction for femtosecond resolution, allowing quantitative characterization of the interatomic potential energy surface of the highly excited solid.

Multiphoton ionization in dielectrics: comparison of circular and linear polarization.

Ionization mechanisms in bulk dielectrics irradiated by single intense 50-fs-laser pulses are investigated by ultrafast time-resolved imaging interferometry. Polarization-sensitive 6-photon ionization is shown to be the dominant ionization mechanism in fused silica and sapphire at intensities around 10 TW/cm2. For both materials the cross sections of 6-photon ionization are found to be significantly higher for linear polarization than for circular. Our experimental results corroborate an earlier theoretical prediction on the dominance of linear polarization in high-order multiphoton ionization.

Observation of structural anisotropy and the onset of liquidlike motion during the nonthermal melting of InSb.

The melting dynamics of laser excited InSb have been studied with femtosecond x-ray diffraction. These measurements observe the delayed onset of diffusive atomic motion, signaling the appearance of liquidlike dynamics. They also demonstrate that the root-mean-squared displacement in the [111] direction increases faster than in the [110] direction after the first 500 fs. This structural anisotropy indicates that the initially generated fluid differs significantly from the equilibrium liquid.

Atomic-scale visualization of inertial dynamics.

The motion of atoms on interatomic potential energy surfaces is fundamental to the dynamics of liquids and solids. An accelerator-based source of femtosecond x-ray pulses allowed us to follow directly atomic displacements on an optically modified energy landscape, leading eventually to the transition from crystalline solid to disordered liquid. We show that, to first order in time, the dynamics are inertial, and we place constraints on the shape and curvature of the transition-state potential energy surface. Our measurements point toward analogies between this nonequilibrium phase transition and the short-time dynamics intrinsic to equilibrium liquids.

Multipass dye-cell amplifier for high-repetition-rate femtosecond optical pulses.

We describe a dye-cell amplifier pumped by a copper-vapor laser at a 8.4-kHz repetition rate. This system avoids possible problems and restrictions arising from nozzle performance in commonly used jet-stream amplifiers. We obtained a nearly diffraction-limited beam with pulses of 60 fsec and energies of up to 50 microJ.

Self-channeling of subgigawatt femtosecond laser pulses in a ground-state waveguide induced in the hollow core of a photonic crystal fiber.

Femtosecond laser pulses with powers below the blowup threshold for self-focused beams are shown to experience spatial self-action in hollow-core photonic crystal fibers filled with argon, nitrogen, and atmospheric air. Regardless of the transverse field distribution at the input of the fiber, the output beam pattern in this regime tends to a circularly symmetric profile, corresponding to a ground-state waveguide induced by laser pulses inside a hollow fiber.