E. Seres

Development of an intracavity EUV Source based on a high power Ti:sapphire oscillator

We report on the realization towards a compact, pulsed XUV source for high temporal and spatial resolution pumpprobe spectroscopy. The system will be based on intracavity high harmonic generation in a Ti:sapphire oscillator. An oscillator with repetition rate of 20 MHz has been realized, which operates in the net negative (near zero) dispersion regime with intracavity pulse energy up to 280 nJ. The cavity has been extended with a secondary focus, where the high harmonic generation can take place. In the recent state, the oscillator is capable to generate XUV harmonics up to 35 eV.

Generation of sub-30-fs pulses from a scaleable high-energy oscillator

We report on the generation of sub-30fs pulses from a mirror-dispersion-controlled (MDC) Ti:sapphire oscillator, containing a multiple-pass Herriott-cell for increasing the cavity length. Using that scheme, repetition rates down to some few MHz could be achieved. To avoid multiple pulsing instabilities, we operate the laser in a regime of slight positive group-delay dispersion (GDD) over a very broad wavelength range. This results in the formation of strongly chirped light pulses, reducing the otherwise very high peak-intensity inside the laser crystal, which would limit the maximum output energy. We have investigated the spectral phase associated with these pulses with the help of the well known SPIDER-technique, and, based on the results, have constructed an optimized compressor. When pumped with the full 10 W of a frequency-doubled Nd:YVO4 laser (Coherent Verdi V10), output energies well above 200 nJ could be obtained. As no signs of instabilities were observed, we believe, that our approach is scaleable to even higher energies if more powerful pump lasers are used. Thanks to the excellent beam profile, high-resolution micromachining of various materials, including transparent dielectrica could be demonstrated. Results on sub-micrometer surface modification of transparent materials will be presented.

Optimizing phase matching of high-order harmonic radiation in the range up to 1 keV

Phase matching is indispensible for the efficient generation high-order harmonic radiation (HHG), especially in the range of several-100-eV. Recently it has been theoretically predicted that perfect pressure induced phase matching cannot be realized in helium in the 0.2-1 keV spectral range using 800 nm light pulses. In this contribution, a detailed experimental study of the phase mismatch is presented for the first time and guidelines for maximizing the short wavelength signal without perfect phase matching are derived.

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.

Strong field amplification of XUV: phase matching aspects

The dependence of the yield of high-order harmonic generation (HHG) on several important experimental parameters has been successfully modeled in the last 20 years by taking into account the single atom response and propagation effects. We extended this description by adding a stimulated emission process and named it x-ray parametric amplification (XPA). Beyond the super-quadratic increase of the XUV signal, which can be explained only in a limited pressure range by HHG theory, other observed characteristics like exponential growth, gain narrowing, strong blue-shift, beam divergence, etc. and their dependence on laser intensity and gas pressure can be explained accurately only by the new XPA model. We experimentally demonstrated XPA in Argon in the spectral range of 40-50 eV in excellent agreement with the theory. XPA holds the promise to realize a new class of bright x-ray sources for spectroscopy.

Laser driven parametric amplification in the xuv and soft-x-ray spectral range

We present the first experimental realization of a new x-ray laser scheme based on strong-field parametric amplification of high-order harmonic radiation. With a simple semi-classical model, we can identify the most important experimental parameters, the spectral range and the small signal gain in gases. Using a single amplifier stage a small signal gain of 8000 has been obtained in Argon for the spectral range of 40-50 eV, using 350 fs, 7 mJ pulses at 1.05 µm. In Helium, we observed a small signal gain of 280 around 300 eV using 6 fs, 1.5 mJ pulses at 800 nm.

Brilliant high harmonic sources with extended cut‐off

The most challenging application of time resolved spectroscopy is to directly watch the structural and electronic dynamics. Here we present several ways for realizing laser driven x‐ray sources, offering atomic spatial and temporal resolution. Our approaches are based on high harmonic generation and include quasi‐phase matching in two successive gas jets, extending the cut‐off by high harmonic generation in an ion channel, and amplification of HHG in a plasma based amplifier.