Miscellaneous publications

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In modern laser-based ion acceleration systems, the field distribution of the focused laser beam at the position of the target strongly influences the overall characteristics of the resulting ion beam. To obtain an unidirectional and quasi mono-energetic ion beam, a flat-top field distribution of the focused laser beam is optimal. This can only be achieved, by using a beam-profiling system that reshapes the incident laser beam into an Airy-shaped field distribution in the far field. Here, we present an extensive design study of such a beam-profiling system based on two free-form mirrors. In order to realize the rings of zero intensity, corresponding to the roots of the Airy-function, strong curvature peaks on the first mirror are necessary. Additionally, the alternating phase in between these rings can only be achieved with grooves on the second mirror. These aspects actually raise the question, if the used purely geometric optical modeling approach is still valid. Therefore, our design study is entirely accompanied with wave-optical simulations to identify influences of diffraction within the beam profiling system. We find that especially the grooves on the second mirror are mandatory, not only to ensure the alternating phase, but also to realize the roots of zero intensity of the Airy-function. On the other hand, these grooves cause diffraction effects in the beam-profiling system that slightly degrade the at-top focal field. These influences are in the range of a few percent and cannot be further avoided.

The photoelectric effect, the emission of electrons from a metal surface after absorbing light, was explained by Einstein's model, where light particles (photons) must have a minimum energy (frequency) to ionize atoms. The number of excited atoms is proportional to the intensity (the number of photons delivered). However, when the light is supplied by very intense, very fast pulses from lasers, the number of ionized atoms will depend on the electric field strength - the amplitude of the light seen as an electromagnetic wave. This change occurs because ionization occurs via quantum tunneling through the relevant energy barrier during a short time window near the maxima of the electric field. Isolated attosecond pulses recently enabled studies of the dynamics of tunneling ionization of atoms in gases. On page 1348 of this issue, Schultze et al. experimentally show that atoms in a solid are also excited via the tunneling process.

Hochleistungslaser ermöglichen es inzwischen, relativistische Elektronenpulse mit bemerkenswerten Eigenschaften zu erzeugen. Neben der äußerst kurzen Beschleunigungslänge sind vor allem die kleine Quellgröße und auch die kurze Pulsdauer interessant. Mit diesen Elektronenpulsen lässt sich zudem elektromagnetische Sekundärstrahlung im Kiloelektronenvolt-Bereich erzeugen. Mit dieser Röntgenstrahlung würden auch Universitäten relativ kompakte Röntgenquellen zur Verfügung stehen, mit denen sich Effekte beobachten lassen, die auf äußerst kurzen räumlichen und zeitlichen Skalen ablaufen. Momentan ist diese Art von Forschung nur an großen, konventionellen Synchrotron-Beschleunigern möglich.

In cancer treatment it is highly desirable to identify and /or classify individual cancer cells in real time. Nowadays, the standard method is PCR which is costly and time-consuming. Here we present a different approach to rapidly classify cell types: we measure the pattern of coherently diffracted extreme ultraviolet radiation (XUV radiation at 38nm wavelength), allowing to distinguish different single breast cancer cell types. The output of our laser driven XUV light source is focused onto a single unstained and unlabeled cancer cell, and the resulting diffraction pattern is measured in reflection geometry. As we will further show, the outer shape of the object can be retrieved from the diffraction pattern with sub-micron resolution. For classification it is often not necessary to retrieve the image, it is only necessary to compare the diffraction patterns which can be regarded as a spatial fingerprint of the specimen. For a proof-of-principle experiment MCF7 and SKBR3 breast cancer cells were pipetted on gold-coated silica slides. From illuminating each single cell and measuring a diffraction pattern we could distinguish between them. Owing to the short bursts of coherent soft x-ray light, one could also image temporal changes of the specimen, i.e. studying changes upon drug application once the desired specimen is found by the classification method. Using a more powerful laser, even classifying circulating tumor cells (CTC) at a high throughput seems possible. This lab-sized equipment will allow fast classification of any kind of cells, bacteria or even viruses in the near future.

We present a novel approach for the construction of a high energy, high power burst mode laser system, based on diode pumped cryogenically cooled Yb:CaF2. The system consists of a frontend producing pulses of 300 fs duration with 1 MHz. Bursts of 1000 subsequent pulses are cut from the continuous train by an electro optical modulator. Afterwards the duration of the individual pulses is stretched to 50 ps. The amplifier system consists of two amplifiers. Both amplifiers utilize mirror based relay imaging schemes to allow for a sufficient number of extraction passes for achieving efficient energy extraction. The goal parameters of the system are to achieve a total energy of 5 J per burst with a repetition rate of 10Hz. Amplification results for the first of two amplifiers are demonstrated. A total output energy of 480 mJ was achieved corresponding to an optical to optical efficiency from absorbed pump energy to extracted energy of more than 17%. Single pulse energies of up to 7.5mJ are generated when changing to less pulses per burst. To achieve a constant energy from pulse to pulse during the burst we present a technique based on the modulation of the laser diode current during one pulse. With this technique the gain variation during the burst was than 5% peak to peak.

Advanced high intensity laser matter interaction experiments always call for optimized laser performance. In order to further enhance the POLARIS laser system, operational at the University of Jena and the Helmholtz-Institute Jena, in particular its energy, bandwidth and focusability, new amplifier technologies have been developed and are reported here. Additionally, existing sections were considerably improved. A new multi-pass amplification stage, which is able to replace two currently used ones, was developed in close collaboration with the MPQ (Garching). The new basic elements of this amplifier are well homogenized pump modules and the application of a successive imaging principle. By operating the amplifier under vacuum conditions a top hat beam profile with an output energy of up to 1.5 J per pulse is foreseen. The already implemented POLARIS amplifier A4 was further improved by adapting an advanced method for the homogenization of the multi-spot composed pump profile. The new method comprises a computer-based evolutionary algorithm which optimizes the position of the different spots regarding its individual size, shape and intensity. The latter allowed a better homogenization of the POLARIS near field profile.

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Using state-of-the-art high-power laser systems, we are able to routinely generate extreme energy densities and focused light intensities in a controlled laboratory environment. During the interaction of these laser pulses with matter a plasma is generated that can both sustain and support huge electric fields. These can be used as a novel type of accelerator structure for electrons and ions having properties favorable for a large number of future applications.

Energy-loss straggling measurements performed at the high-momentum-resolution magnetic spectrometer FRS with bare and highly charged O-8, Xe-54, (79)AU, and U-92 ions with specific kinetic energies (700-1000) MeV/u are reported. The results are in good agreement with rigorous calculations recently reported by Lindhard and Sorensen and reveal systematic deviations from the well-known relativistic Bohr formula, which was obtained within the framework of the first-order Born approximation.