Ultrafast dynamics of charge density waves in 4H(b)-TaSe2 probed by femtosecond electron diffraction.

The dynamics of the photoinduced commensurate-to-incommensurate charge density wave (CDW) phase transition in 4H(b)-TaSe(2) are investigated by femtosecond electron diffraction. In the perturbative regime, the CDW re-forms on a 150-ps time scale, which is two orders of magnitude slower than in other transition-metal dichalcogenides. We attribute this to a weak coupling between the CDW carrying T layers and thus demonstrate the importance of three-dimensionality for the existence of CDWs. With increasing optical excitation, the phase transition is achieved, showing a second-order character, in contrast to the first-order behavior in thermal equilibrium.

A compact streak camera for 150 fs time resolved measurement of bright pulses in ultrafast electron diffraction.

We have developed a compact streak camera suitable for measuring the duration of highly charged subrelativistic femtosecond electron bunches with an energy bandwidth in the order of 0.1%, as frequently used in ultrafast electron diffraction (UED) experiments for the investigation of ultrafast structural dynamics. The device operates in accumulation mode with 50 fs shot-to-shot timing jitter, and at a 30 keV electron energy, the full width at half maximum temporal resolution is 150 fs. Measured durations of pulses from our UED gun agree well with the predictions from the detailed charged particle trajectory simulations.

Measurement of magnetic-field structures in a laser-wakefield accelerator.

Experimental measurements of magnetic fields generated in the cavity of a self-injecting laser-wakefield accelerator are presented. Faraday rotation is used to determine the existence of multimegagauss fields, constrained to a transverse dimension comparable to the plasma wavelength ∼λp and several λp longitudinally. The fields are generated rapidly and move with the driving laser. In our experiment, the appearance of the magnetic fields is correlated with the production of relativistic electrons, indicating that they are inherently tied to the growth and wave breaking of the nonlinear plasma wave. This evolution is confirmed by numerical simulations, showing that these measurements provide insight into the wakefield evolution with high spatial and temporal resolution.

Generation of quasimonoenergetic electron bunches with 80-fs laser pulses.

Highly collimated, quasimonoenergetic multi-MeV electron bunches were generated by the interaction of tightly focused, 80-fs laser pulses in a high-pressure gas jet. These monoenergetic bunches are characteristic of wakefield acceleration in the highly nonlinear wave breaking regime, which was previously thought to be accessible only by much shorter laser pulses in thinner plasmas. In our experiment, the initially long laser pulse was modified in underdense plasma to match the necessary conditions. This picture is confirmed by semianalytical scaling laws and 3D particle-in-cell simulations. Our results show that laser-plasma interaction can drive itself towards this type of laser wakefield acceleration even if the initial laser and plasma parameters are outside the required regime.

Thomson-backscattered x rays from laser-accelerated electrons.

We present the first observation of Thomson-backscattered light from laser-accelerated electrons. In a compact, all-optical setup, the "photon collider," a high-intensity laser pulse is focused into a pulsed He gas jet and accelerates electrons to relativistic energies. A counterpropagating laser probe pulse is scattered from these high-energy electrons, and the backscattered x-ray photons are spectrally analyzed. This experiment demonstrates a novel source of directed ultrashort x-ray pulses and additionally allows for time-resolved spectroscopy of the laser acceleration of electrons.

Laser-plasma acceleration of quasi-monoenergetic protons from microstructured targets.

Particle acceleration based on high intensity laser systems (a process known as laser-plasma acceleration) has achieved high quality particle beams that compare favourably with conventional acceleration techniques in terms of emittance, brightness and pulse duration. A long-term difficulty associated with laser-plasma acceleration--the very broad, exponential energy spectrum of the emitted particles--has been overcome recently for electron beams. Here we report analogous results for ions, specifically the production of quasi-monoenergetic proton beams using laser-plasma accelerators. Reliable and reproducible laser-accelerated ion beams were achieved by intense laser irradiation of solid microstructured targets. This proof-of-principle experiment serves to illuminate the role of laser-generated plasmas as feasible particle sources. Scalability studies show that, owing to their compact size and reasonable cost, such table-top laser systems with high repetition rates could contribute to the development of new generations of particle injectors that may be suitable for medical proton therapy.

Spatial characteristics of Kalpha x-ray emission from relativistic femtosecond laser plasmas.

The spatial structure of the Kalpha emission from Ti targets irradiated with a high intensity femtosecond laser has been studied using a two-dimensional monochromatic imaging technique. For laser intensities I<5 x 10(17) W/cm(2), the observed spatial structure of the Kalpha emission can be explained by the scattering of the hot electrons inside the solid with the help of a hybrid particle-in-cell/Monte Carlo model. By contrast, at the maximum laser intensity I=7 x 10(18) W/cm(2) the half-width of the Kalpha emission was 70 microm compared to a laser-focus half-width of 3 microm. Moreover, the main Kalpha peak was surrounded by a halo of weak Kalpha emission with a diameter of 400 microm and the Kalpha intensity at the source center did not increase with increasing laser intensity. These three features point to the existence of strong self-induced fields, which redirect the hot electrons over the target surface.

High-intensity laser induced ion acceleration from heavy-water droplets.

Fusion neutrons from a heavy water droplet target irradiated with laser pulses of 3 x 10(19) W/cm(2) and from a deuterated secondary target are observed by a time-of-flight (TOF) neutron spectrometer. The observed TOF spectrum can be explained by fusion of deuterium ions simultaneously originating from two different sources: ion acceleration in the laser focus by ponderomotively induced charge separation and target-normal sheath acceleration off the target rear surface. The experimental findings agree well with 3D particle-in-cell simulations.

Optimization of Kalpha bursts for photon energies between 1.7 and 7 keV produced by femtosecond-laser-produced plasmas of different scale length.

The conversion efficiency of a 90 fs high-power laser pulse focused onto a solid target into x-ray Kalpha line emission was measured. By using three different elements as target material (Si, Ti, and Co), interesting candidates for fast x-ray diffraction applications were selected. The Kalpha output was measured with toroidally bent crystal monochromators combined with a GaAsP Schottky diode. Optimization was performed for different laser intensities as well as for different density scale lengths of a preformed plasma. These different scale lengths were realized by prepulses of different intensities and delay times with respect to the main pulse. Whereas the Kalpha yield varied by a factor of 1.8 for different laser intensities, the variation of the density scale length could provide a gain factor up to 4.6 for the Kalpha output.

Femtosecond silicon K alpha pulses from laser-produced plasmas.

Ultrashort bursts of silicon K alpha x-ray radiation from femtosecond-laser-produced plasmas have been generated. A cross-correlation measurement employing a laser-triggered ultrafast structural change of a CdTe crystal layer (320 nm) shows a K alpha pulse duration between 200 fs and 640 fs. This result is corroborated by particle in cell simulations combined with a Monte-Carlo electron stopping code and calculations on the structural changes of the crystal lattice.

Mev X rays and photoneutrons from femtosecond laser-produced plasmas.

We demonstrate a novel method to monitor the total angular distribution of the spectrum of hard x-ray emission from a plasma generated with femtosecond laser pulses with an intensity of 5 x 10(18) W/cm2 on a solid target. Measured and calculated angular distributions of x rays show a pronounced anisotropy for MeV photon energies. We complemented the spectral information by demonstrating a (gamma,n) nuclear reaction with a tabletop laser system.

Tryptophan hydroxylase antibodies used in the diagnosis of carcinoid.

The expression of tryptophan hydroxylase, the rate-limiting enzyme in the biosynthesis of serotonin, is described in a case of a 35 year-old patient with metastatic jejunal carcinoid. Immunohistochemically, monoclonal anti-tryptophan hydroxylase antibodies positively identified liver metastases of a neuroendocrine tumor. The cellular distribution of tryptophan hydroxylase was restricted exclusively to the cytoplasm of carcinoid cells, where it was found in large amounts. By means of immunoblotting, anti-tryptophan hydroxylase antibodies detected in samples from carcinoid tissue two closely migrating polypeptide bands with molecular weights of 26 kDa and 29 kDa, respectively. These two protein bands appear to represent proteolytically degraded polypeptides, since tryptophan hydroxylase is known for its extreme unstability in vitro. In our case, the immunohistochemical and biochemical identification of tryptophan hydroxylase in liver lesions of a neuroendocrine tumor permitted the correct diagnosis of a metastatic carcinoid.

Second-harmonic generation of amplified femtosecond Ti:sapphire laser pulses.

We study theoretically and experimentally second-harmonic generation (SHG) of 150-fs-duration amplified Ti:sapphire laser pulses at a wavelength of 780 nm in the nonlinear crystal KDP of different lengths and at different power densities as high as 150 GW cm(-2). The experimentally observed SHG conversion efficiency does not exceed 50%. It is shown theoretically that one possible process limiting the SHG efficiency at low as well as at high intensities is the modulation of the phase of the fundamental wave. In addition, continuum generation is observed at high intensities and can decrease the SHG efficiency.