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27 W 2.1 µm OPCPA system for coherent soft X-ray generation operating at 10 kHz
We developed a high power optical parametric chirped-pulse amplification (OPCPA) system at 2.1 µm harnessing a 500 W Yb:YAG thin disk laser as the only pump and signal generation source. The OPCPA system operates at 10 kHz with a single pulse energy of up to 2.7 mJ and pulse duration of 30 fs. The maximum average output power of 27 W sets a new record for an OPCPA system in the 2 µm wavelength region. The soft X-ray continuum generated through high harmonic generation with this driver laser can extend to around 0.55 keV, thus covering the entire water window (284 eV - 543 eV). With a repetition rate still enabling pump-probe experiments on solid samples, the system can be used for many applications. © 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Prospects of target nanostructuring for laser proton acceleration
In laser-based proton acceleration, nanostructured targets hold the promise to allow for significantly boosted proton energies due to strong increase of laser absorption. We used laser-induced periodic surface structures generated in-situ as a very fast and economic way to produce nanostructured targets capable of high-repetition rate applications. Both in experiment and theory, we investigate the impact of nanostructuring on the proton spectrum for different laser-plasma conditions. Our experimental data show that the nanostructures lead to a significant enhancement of absorption over the entire range of laser plasma conditions investigated. At conditions that do not allow for efficient laser absorption by plane targets, i.e. too steep plasma gradients, nanostructuring is found to significantly enhance the proton cutoff energy and conversion efficiency. In contrast, if the plasma gradient is optimized for laser absorption of the plane target, the nanostructure-induced absorption increase is not reflected in higher cutoff energies. Both, simulation and experiment point towards the energy transfer from the laser to the hot electrons as bottleneck.
Monitoring of evolving laser induced periodic surface structures
Laser induced periodic surface structures (LIPSS) are generated on titanium and silicon nitride surfaces by multiple femtosecond laser pulses. An optical imaging system is used to observe the backscattered light during the patterning process. A characteristic fringe pattern in the backscattered light is observed and evidences the surface modification. Experiments are complemented by finite difference time domain numerical simulations which clearly show that the periodic surface modulation leads to characteristic modulations in the coherently scattered light field. It is proposed that these characteristic fringe pattern can be used as a very fast and low-cost monitor of LIPSS formation formation during the manufacturing process. © 2019 by the authors.
Tracing dynamics of laser-induced fields on ultra-thin foils using complementary imaging with streak deflectometry
We present a detailed study of the electric and magnetic fields, which are created on plasma vacuum interfaces as a result of highly intense laser-matter interactions. For the field generation ultrathin polymer foils (30–50 nm) were irradiated with high intensity femtosecond (1019–1020 W/cm2) and picosecond (∼1017 W/cm2) laser pulses with ultrahigh contrast (1010–1011). To determine the temporal evolution and the spatial distribution of these fields the proton streak deflectometry method has been developed further and applied in two different imaging configurations. It enabled us to gather complementary information about the investigated field structure, in particular about the influence of different field components (parallel and normal to the target surface) and the impact of a moving ion front. The applied ultrahigh laser contrast significantly increased the reproducibility of the experiment and improved the accuracy of the imaging method. In order to explain the experimental observations, which were obtained by applying ultrashort laser pulses, two different analytical models have been studied in detail. Their ability to reproduce the streak deflectometry measurements was tested on the basis of three-dimensional particle simulations. A modification and combination of the two models allowed for an extensive and accurate reproduction of the experimental results in both imaging configurations. The controlled change of the laser pulse duration from 50 femtoseconds to 2.7 picoseconds led to a transition of the dominating force acting on the probing proton beam at the rear side of the polymer foil. In the picosecond case the (⇀vx⇀B)-term of the Lorentz force dominated over the counteracting ⇀E-field and was responsible for the direction of the net force. The applied proton deflectometry method allowed for an unambiguous determination of the magnetic field polarity at the rear side of the ultrathin foil.