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<p class="MsoNormal">The next colloquium will be given by Prof Gerhard Paulus (Jena) this Wednesday. Coffee and tea will be served afterwards.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Speaker: Gerhard Paulus, Institut für Optik und Quantenelektronik, Universität Jena<o:p></o:p></p>
<p class="MsoNormal">Time/Date: 3pm Wednesday 28<sup>th</sup> January 2015<o:p></o:p></p>
<p class="MsoNormal">Venue: Room JA3.14<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Title: Relativistic surface high-harmonic generation<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Abstract:<o:p></o:p></p>
<p class="MsoNormal">Coherent radiation in the XUV spectral regime with attosecond pulse duration can not only be generated by high-harmonic generation in gases, but also by interaction of terawatt laser pulses with surfaces (surface high-harmonic generation,
SHHG). Under suitable conditions, an overdense plasma oscillating with relativistic velocities is generated. Therefore, the laser which has generated this plasma, is reflected and modulated (relativistically oscillating mirror, ROM). The Doppler effect in
conjunction with relativistic retardation results in the creation of high-order harmonics. More elaborate models predict the spectral shape of the harmonic emission as well as the efficiency. We have tested the present theoretical understanding of SHHG in
a series of experiments at the JETI terawatt laser facility. For the first time, the efficiency of surface harmonic generation was measured using an XUV spectrometer calibrated at a synchrotron. The efficiency at the 21st harmonic was found to be 10^-5, falling
short of expectations. Extremely short plasma scale lengths lead to low efficiencies, also contrary to expectations. The reason is that the strong restoring forces in the plasma inhibit large oscillation amplitudes and thus large velocities [1]. Methods and
results to overcome this problem will be presented.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">We also found a new effect that can be directly related to the attosecond time structure of surface harmonics: At plasma scale lengths of the order λ/5, the harmonics exhibit a distinct fine structure. Analytical and numerical modeling
have revealed that the fine structure in harmonic emission can be attributed to denting of the plasma by the radiation pressure of the incident laser pulse. The attosecond pulses emitted by the plasma therefore have a time-dependent temporal separation. In
frequency domain, this leads to the observed spectral features [2]. The effect can largely be cancelled by an appropriately pre-chirped laser pulse. High-harmonic spectra are regularly characterized by a cutoff beyond a certain harmonic order. The cutoff energy
scales with a parameter characteristic for the process responsible for high harmonic generation. In case of gas harmonics this is the ponderomotive energy, in the present case of surface harmonics the relativistic γ-factor. The intriguing observation we have
made is a strong enhancement of a particular harmonic beyond the SHHG roll-off. We show that the effect is due to frequency mixing mediated by a relativistic nonlinearity [3].<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">[1] C. Roedel et al., Phys. Rev. Lett. 109, 125002 (2012).<o:p></o:p></p>
<p class="MsoNormal">[2] M. Behmke et al., Phys. Rev. Lett. 106, 185002 (2011).<o:p></o:p></p>
<p class="MsoNormal">[3] C. Roedel et al., (submitted for publication).<o:p></o:p></p>
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