Heinrich Schwoerer

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.

Coherent octave spanning near-infrared and visible supercontinuum generation in all-normal dispersion photonic crystal fibers.

We present the first detailed demonstrations of octave-spanning SC generation in all-normal dispersion photonic crystal fibers (ANDi PCF) in the visible and near-infrared spectral regions. The resulting spectral profiles are extremely flat without significant fine structure and with excellent stability and coherence properties. The key benefit of SC generation in ANDi PCF is the conservation of a single ultrashort pulse in the time domain with smooth and recompressible phase distribution. For the first time we confirm the exceptional temporal properties of the generated SC pulses experimentally and demonstrate their applicability in ultrafast transient absorption spectroscopy. The experimental results are in excellent agreement with numerical simulations, which are used to illustrate the SC generation dynamics by self-phase modulation and optical wave breaking. To our knowledge, we present the broadest spectra generated in the normal dispersion regime of an optical fiber.

Achromatic reflectron compressor design for bright pulses in femtosecond electron diffraction

We have designed a femtosecond electron gun suitable for ultrafast electron diffraction experiments, operating in the 30–100 kV regime. The concept is based on recompression of chirped expanding electron pulses emitted from a direct current photogun using a novel dispersion-corrected reflectron concept. We show, using detailed numerical simulations, that our design is capable of producing electron pulses containing 200 000 electrons with a full width at half maximum pulse duration of 130 fs, a root mean squared (rms) pulse radius of 140 μm, and transverse coherence length of 1.5 nm at 100 kV. Our analysis includes the bunch properties at the sample, as well as interactions of the main pulse of high charge density with diffracted electrons. Since our design employs only static electron optics, we believe that it will be easier to implement than concepts based on radio frequency compression.

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.

Selective generation and control of excited vibrational wave packets in the electronic ground state of K2

We investigate the generation and real-time monitoring of coherent vibrational wave packets in the electronic ground state of supersonic jet-cooled potassium dimers. Vibrationally excited wave packets with mean quantum numbers v=6 and v=11 are generated by a stimulated Raman process which is enhanced by an electronic resonance. Two ultrashort laser pulses of different wavelengths induce the pump and the dump process. The population of the final hot ground-state wave packets is successfully controlled by a variable time delay between the pump and the dump process, which enables us to wait with the dumping for the optimal Franck–Condon overlap between the intermediate and the predicted final vibrational wave packet in the electronic ground state.

Spectral shaping of laser generated proton beams

The rapid progress in the field of laser particle acceleration has stimulated a debate about the promising perspectives of laser based ion beam sources. For a long time, the beams produced exhibited quasi-thermal spectra. Recent proof-of-principle experiments demonstrated that ion beams with narrow energy distribution can be generated from special target geometries. However, the achieved spectra were strongly limited in terms of monochromacity and reproducibility. We show that microstructured targets can be used to reliably produce protons with monoenergetic spectra above 2 MeV with less than 10% energy spread. Detailed investigations of the effects of laser ablation on the target resulted in a significant improvement of the reproducibility. Based on statistical analysis, we derive a scaling law between proton peak position and laser energy, underlining the suitability of this method for future applications. Both the quality of the spectra and the scaling law are well reproduced by numerical simulations.

Femtosecond time-resolved two-photon ionization spectroscopy of K2

We investigated the coherent motion of vibrational wave packets in the |B〉 1Πu state of the potassium dimer applying two color pump/probe spectroscopy with a sub 100 fs time resolution. Special interest was paid to the ionization probe step which was analyzed carefully by varying the probe energy over a wide range. Time-dependent quantum calculations explain the experimental outcomes by introducing a nonconstant transition dipole moment between the |B〉 and the ionic state |X+〉 and by taking into account the excitation of long lived autoionizing Rydberg states.

Fission of actinides with tabletop lasers

We report the first laser induced fission of actinide nuclei with high intensity tabletop lasers. The results are discussed in the context of laser plasma diagnostics and nuclear waste handling.

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.

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.