Bastian Aurand

High-harmonic generation and parametric amplification in the soft X-rays from extended electron trajectories

We report, for the first time, the generation of high-order harmonics in a spectral range between 200 eV and 1 keV with an unusual spectral property: only every 4th (4i + 1, i∈ℵ) harmonic line appears, whereas the usual high-harmonic spectra consist of every odd (2i + 1) harmonic. We attribute this unique property to the quantum path interference of two extended electron trajectories that experience multiple re-scattering. In the well-established theory, electrons emitted via tunnel ionisation are accelerated by a laser field, return to the ion and recombine. The acceleration typically lasts for less than one optical cycle, and the electrons radiate in the extreme ultraviolet range at recombination. In contrast, for extended trajectories, electrons are accelerated over two or more optical cycles. Here, we demonstrate that two sets of trajectories dominate and provide substantial contributions to the generated soft X-ray radiation because they fulfil the resonance condition for X-ray parametric amplification.

Parametric amplification of attosecond pulse trains at 11 nm

We report the first experimental demonstration of the parametric amplification of attosecond pulse trains at around 11 nm. The helium amplifier is driven by intense laser pulses and seeded by high-order harmonics pulses generated in a neon gas jet. Our measurements suggest that amplification takes place only if the seed pulse-trains are perfectly synchronized in time with the driving laser field in the amplifier. Varying the delay, we estimate the durations of the individual extreme ultraviolet pulses within the train to be on the order of 0.2 fs. Our results demonstrate that strong-field parametric amplification can be a suitable tool to amplify weak attosecond pulses from non-destructive pump-probe experiments and it is an important step towards designing amplifiers for realization of energetic XUV pulses with sub-femtosecond duration using compact lasers fitting in university laboratories.

Creating circularly polarized light with a phase-shifting mirror.

We report on the performance of a system employing a multi-layer coated mirror creating circularly polarized light in a fully reflective setup. With one specially designed mirror we are able to create laser pulses with an ellipticity of more than ε = 98% over the entire spectral bandwidth from initially linearly polarized Titanium:Sapphire femtosecond laser pulses. We tested the homogeneity of the polarization with beam sizes of the order of approximately 10 cm. The damage threshold was determined to be nearly 400 times higher than for a transmissive quartz-wave plate which suggests applications in high intensity laser experiments. Another advantage of the reflective scheme is the absence of nonlinear effects changing the spectrum or the pulse-form and the scalability of coating fabrication to large aperture mirrors.

X-RAY LASER DEVELOPMENTS AT PHELIX

Development of x-ray lasers using the PHELIX laser at the GSI Helmholtz center for heavy-ion research [1] is targeting a number of applications of novel x-ray sources in combination with energetic heavy-ion beams. This includes Thomson scattering diagnostics of heavy-ion driven plasmas, x-ray opacity measurements, and x-ray laser spectroscopy of highly-charged ions. Developments centered on the application of a novel double-pulse GRIP-like pumping scheme, DGRIP, where nonnormal incidence geometry is used for both the pre- and the main pulse for transient pumped Ni-like x-ray lasers [2,3]. This scheme was used at lower energy levels to pump soft x-ray lasers in the 50 – 100 eV regime as well as for pulse energies above 100 J for the pumping of shorter wavelength soft x-ray lasers [4].

Note: A large aperture four-mirror reflective wave-plate for high-intensity short-pulse laser experiments.

We report on a four-mirror reflective wave-plate system based on a phase-shifting mirror (PSM) for a continuous variation of elliptical polarization without changing the beam position and direction. The system presented and characterized here can replace a conventional retardation plate providing all advantages of a PSM, such as high damage-threshold, large scalability, and low dispersion. This makes reflective wave-plates an ideal tool for ultra-high power laser applications.

Avalanche of stimulated forward scattering in high harmonic generation.

Optical amplifiers in all ranges of the electromagnetic spectrum exhibit an essential characteristic, namely the input signal during the propagation in the amplifier medium is multiplied by the avalanche effect of the stimulated emission to produce exponential growth. We perform a theoretical study motivated and supported by experimental data on a He gas amplifier driven by intense 30-fs-long laser pulses and seeded with attosecond pulse trains generated in a separated Ne gas jet. We demonstrate that the strong-field theory in the frame of high harmonic generation fully supports the appearance of the avalanche effect in the amplification of extreme ultraviolet attosecond pulse trains. We theoretically separate and identify different physical processes taking part in the interaction and we demonstrate that X-ray parametric amplification dominates over others. In particular, we identify strong-field mediated intrapulse X-ray parametric processes as decisive for amplification at the single-atom level. We confirm that the amplification takes place at photon energies where the amplifier is seeded and when the seed pulses are perfectly synchronized with the driving strong field in the amplifier. Furthermore, propagation effects, phase matching and seed synchronization can be exploited to tune the amplified spectral range within the seed bandwidth.

Experimental demonstration of a collinear triple pulse grazing-incidence pumping scheme for a transient collisional pumped x-ray laser

We present a nickel-like Molybdenum x-ray laser (XRL) based on the transient collisional excitation scheme. Extending the double grazing-incidence pumping scheme to a triple pulse scheme with a nanosecond pre-pulse and two consecutive 3 ps pulses the output power of the XRL can be ~6 times increased compared to existing configurations, while using a relatively low total pump energy of 0.7 J. Additionally we show that this setup can be extended to a double stage, oscillator and amplifier stage, XRL.

Laser-driven parametric amplification of soft X-rays

For time-resolved spectroscopy, ultra-short pulses can be directly generated with lasers and the wavelength range can be extended with nonlinear frequency conversion techniques. For covering the shorter wavelength range, XFELs and plasma based x-ray lasers are attractive candidates. However their stability, coherence and beam quality is limited and can be improved e.g. by seeding with coherent high harmonic radiation [1]. Another route is the nonlinear frequency conversion of laser pulses into the XUV range by high order harmonic generation resulting very short XUV or soft-x-ray pulses in a laser like beam. The major drawback of high harmonics is their low conversion efficiency.