Promotionsthemen

In this project, we employ state of the art high-energy, Yb:YAG thin disk lasers combined with broadband optical parametric amplifier and coherent field synthesis to generate sub-cycle pulses, or the so called light transients. High energy, optimised light transients allow to control nonlinear phenomena in sub-cycle regime (1,2). A good experience on developing lasers, handling ultrashort pulses, Labview and Matlab is beneficial for this position. 1."Multi-octave, CEP-stable source for high-energy field synthesis” A Alismail, H Wang, G Barbiero, …, H. Fattahi, Science advances 6 (7), eaax3408, 2020 2."Third-generation femtosecond technology", H Fattahi, HG Barros, M Gorjan, …, F. Krausz, Optica 1 (1), 45-63, 2014

Before particle acceleration comes transport. Transport through an accelerator structure means that a particle beam can be actively steered through the required elements needed to build a particle accelerator. This is what we have achieved through a nanophotonic channel (Nature 2021). Based on this, we are now ready to build the particle accelerator on a chip. Various topics have to be tackled for this, ranging from electron beam simulation driven with the optical nearfields, via structure optimization to the integration of the optics to power the structure.
* Building the accelerator on a naonphotonic chip
* Next to acceleration, these structures also allow generation of attosecond electron bunches, which we will use for various laser pump--electron probe experiments.

With phase-controlled laser fields, we have driven electrons inside of graphene fully coherently. We have demonstrated Landau-Zener-Stückelberg interferometry, so subsequent coherent Landau-Zener transitions, leading to a net current (Nature 2017). Based on this, we have further demonstrated more complex coherent current control driven on optical time scales. These insights allow us to now move towards electronics on optical timescales, i.e., petahertz electronics. Both the fundamental physics in various materials and at heterostructures as well as initial building blocks are on our research agenda.

We explore different ways to overcome the Manley-Rowe limit for THz generation with lasers through optical rectification through cascading in periodically poled nonlinear materials. Students will get familiar with the use of high energy short pulse lasers in driving strongly nonlinear processes. Eventually conversion beyond several percent from optical to THz is realized. The high energy THz pulses are used ifor THz acceleration.

We us sub-cycle parametric waveform synthesis to generate and control isolated attosecond pulses in the EUV to soft-x-ray region for attosecond transient absorption spectroscopy and apply this technique to various fundamental processes such as radiolysis in water, chemical processes in energy conversion and controlled attosecond electron wave packet generation.

We are interested in ultrafast correlated and collective electron dynamics that unfold when complex materials are exposed to intense, ultrashort laser fields. In particular this includes the attosecond control and tracing of strongly coupled electron-nuclear dynamics in quantum systems such as molecules, and of collective electron dynamics in nanostructured materials, and lightwave-driven petahertz (nano)electronics. We develop and use multi-dimensional imaging techniques in conjunction with (attosecond) pump-probe experiments to gain detailed insight into the dynamics from the interaction of these materials with near-single cycle laser fields of typically attosecond to femtosecond duration. We actively push the theoretical modeling of the rather complex multi-charge dynamics, which are partly carried out in our group using Monte-Carlo simulations with more intricate models within collaborations.

Volume Bragg gratings (VBGs) are a refractive index modulations within a transparent bulk material, having grating periods in the range of 1 µm, and act as a narrowband reflector. The gratings are inscribed by nonlinear absorption of femtosecond laser pulses, enabling the realization of VBGs in various types of glasses. Chirped VBGs, i.e. gratings with varying period over length, control the dispersive properties of the reflected signal, which can be utilized for pulse compression in chirped pulse amplification laser systems. The goal of this project is the realization of high power stable chirped VBGs by using femtosecond laser pulses to generate the refractive index modifications. The task is to further develop the inscription process from gratings with constant period to tailored profiles, providing excellent stability, ensuring the high power durability. Crucial is the control of the dispersive response for the successful application in pulse stretcher and compressor combinations.

In standard photonics light propagation is dictated by the refractive index gradient: photons behave as massive quantum particles under the action of a potential equal to the refractive index. Nonetheless, the behavior of photons can be far more complex, especially in the presence of exotic light-matter interaction. Here we aim to investigate the strong interaction between the spin (i.e., the light polarization) and the orbital (i.e., the wave vector) occurring when photons propagate in inhomogeneous anisotropic materials. In this regime light evolution recalls the propagation of charged particles in a magnetic field, and new effects are observed due to the gradients in the Pancharatnam-Berry phase. The fundamental nature and possible applications of spin-orbit phenomena will be both pursued, in the linear and nonlinear regime, including materials such as liquid crystals, light-written nanogratings in glass, two-photon polymerized structures and biased nanoguides in Lithium Niobate.

The world faces tremendous challenges in environmental technology, as the emission of greenhouse gases has to be reduced rapidly and significantly to achieve climate goals. One of the main unsolved problems is the lack of technology for energy storage. A promising approach is the conversion of exhaust gases (CH4, CO, CO2, H2) to storable and transportable resources. Catalytic processes are the method of choice for this purpose, but it is difficult to trigger these reactions with thermal activation energy only. The present project should investigate the control of chemical reactions to produce fuels and sustainable resources using innovative laser technologies. Among these, coherent control using few-cycle pulses and pulse shaping will be applied to drive chemical reactions in the desired direction.

This highly interdisciplinary project is a collaboration with physicians and engineers of the universities in Göttingen and Jena to evaluate treatment scenarios using fs-laser induced cross-linking of corneal tissue. Possible applications are the treatment of keratoconus, or even refractive correction of myopia. Currently, the method of UV (A)-riboflavin crosslinking is used to stabilize the cornea in the keratoconus. Riboflavin is applied locally and cross-linked by UV (A) radiation. Yet, the precision of this procedure is low, and the exclusive treatment of diseased areas is not possible. In this project, fs-lasers should be used for the cross-linking of corneal collagen fibres. Using a high-speed scanner system, the focal spot of the fs-laser can be precisely controlled on the corneal surface allowing for a selective treatment of diseased areas.

A central goal of molecular physics is to steer the outcome of chemical reactions. Ultrashort laser pulses provide a powerful tool to study and also control molecular bonds on ultrashort time scales. However, the interaction of molecules with laser pulses is governed by the excitation of bound or unbound electronic states, while most conventional chemistry proceeds on the electronic ground state via excitation of vibrational levels. Using ultrashort IR lasers, we strive to investigate the largely unexplored field of ground-state photochemistry. To this end, we use highly sophisticated laser technology as well as an advanced ion beam apparatus with time- and position-resolved coincidence detection. Available topics range from instrumentation and laser technology to data analysis and theoretical simulations.

XUV radiation has distinctive advantages for imaging, including resolution and element-specific contrast. We recently invented XUV coherence tomography (XCT), which enables non-destructive cross-sectional imaging with nanoscale resolution. Available topics include the combination with other lens-less imaging modalities, ultrafast nanoscale imaging, and multispectral imaging.

We have developed X-ray polarimeters providing extinction ratios of better than 1E-10, by far the best in the world and also by far beyond of what is possible in the optical regime. The ultimate goal is to measure the birefringence of vacuum polarized by a strong laser – and we believe that we have sufficiently advanced X-ray polarimetry to do such an experiment at the European XFEL in the next years. However, precision X-ray polarimetry offers a large range of other research opportunities. One of these is X-ray polarization microscopy. Another, in collaboration with Professor Röhlsberger, X-ray quantum optics.

The structure and dynamics of a molecule are governed by the forces between its nuclei and electrons. These forces give rise to the molecular potentials, on which nuclear motion and chemical reactions occur. Intense femtosecond laser fields can exert external forces comparable to the intermolecular ones. Thus, they can be used to manipulate molecular potentials on the timescales on the time scale of the nuclear dynamics. In this project, we will explore such routes to manipulate chemical reactions by means of tailored laser light.

The advances in the fields of short-pulse laser and x-ray free-electron laser pulse creation allows probing of correlated and collective electron dynamics at its inherent attosecond timescale. State-of-the art methods to treat the interaction of ultrashort laser pulses with many-electron systems often resort to single-active electron approaches that are inherently unable to treat correlated many-body dynamics. On the other hand, computationally tractable time-dependent many-body theories, such as time-dependent density functional theory, the time-dependent Hartree Fock method, or time-dependent configuration interaction can account for electron correlation only to some limiting extent. This project focuses on the development of an alternative method to describe time-dependent many-body quantum systems. Recently, our group developed a theoretical framework that allows for a stochastic phase-space description of the time-dependent density matrix of an open many-body quantum system. In the project, the successful student should extend this formalism to treat correlated electron systems in external laser fields. Within this framework electron correlation beyond the Hartree-Fock level will be captured by stochastic processes defined by an appropriate noise correlation function, that has to be derived. Numerically, this will result in solving coupled stochastic differential equations, to sample the two-particle reduced electronic density matrix. We are looking for candidates with a strong interest in the development of novel theoretical methods, strong analytical skills and an interest in numerical implementation (programming).

We study some of the fastest processes that accompany light-matter interaction. As our primary experimental approach, we investigate the light-controlled motion of charge carriers photoinjected within a time interval as short as a femtosecond. Combining our experimental and theoretical expertise, we develop novel methods for observing electron dynamics with attosecond temporal resolution. We also apply these methods to study processes that determine the fundamental speed limits of optoelectronics. For more information, see https://www.attoworld.de/atto-20.html

Before particle acceleration comes transport. Transport through an accelerator structure means that a particle beam can be actively steered through the required elements needed to build a particle accelerator. This is what we have achieved through a nanophotonic channel (Nature 2021). Based on this, we are now ready to build the particle accelerator on a chip. Various topics have to be tackled for this, ranging from electron beam simulation driven with the optical nearfields, via structure optimization to the integration of the optics to power the structure.
* Building the accelerator on a naonphotonic chip
* Next to acceleration, these structures also allow generation of attosecond electron bunches, which we will use for various laser pump--electron probe experiments.

We explore different ways to overcome the Manley-Rowe limit for THz generation with lasers through optical rectification through cascading in periodically poled nonlinear materials. Students will get familiar with the use of high energy short pulse lasers in driving strongly nonlinear processes. Eventually conversion beyond several percent from optical to THz is realized. The high energy THz pulses are used ifor THz acceleration.

The use of process-adapted intensity distributions and the design of optical systems for the realization of these intensity distributions is essential for digital photonics production. We are investigating new ways to generate flexible and highly dynamic intensity profiles. With the help of so-called Spatial Light Modulators (SLMs), among other devices, the phase front of the laser radiation can be manipulated digitally with high resolution which enables the dynamic control of the intensity distribution in the processing plane. Commercially available SLMs are already capable of shaping the phase front of a laser beam. However, there are significant limitations to this. For example, neighboring pixels influence each other in their phase deviation (crosstalk). For the efficient use of SLMs, a method has to be developed to circumvent, compensate or at least take into account the existing limitations in phase matching. In this project, optical systems with integrated SLMs will be developed and validated for selected applications in laser material processing.

In laser materials processing, the intensity distribution of the laser beam significantly influences the temperature profile that is induced in the workpiece. As this temperature profile on the other hand accounts for the obtained material modifications, the processing quality and efficiency can be influenced by adapting the intensity distribution to the respective process. However, the computation of such an application adapted intensity distribution is not straightforward as an inverse heat conduction problem must be solved. In our group, we develop numerical solution methods which combine FEM and FDM solutions of the heat conduction equation with state-of-the-art optimization strategies. Up to date, these methods have been applied to laser heat treatment. In the research project, the algorithms will be extended to more complex materials, geometries, and especially processes such as laser powder bed fusion, laser welding or laser structuring with ultra-short pulsed radiation.

The increasing digitization of products and production processes not only changes our everyday life and creates new possibilities almost daily, but also impacts the design process and operation of optical systems. The use of sensor technology during assembly (wavefront sensor) and the integration of miniaturized sensor technology in optical systems enables the feedback of the acquired data into the digital (optical) models. It becomes possible to create a digital optical twin (DOT), which accompanies an optical system over its entire life cycle. This makes it possible to generate service life forecasts, enable targeted adjustments and plan adapted maintenance intervals at an early stage. In addition, improvements for future product design upgrades can be derived from the obtained data. Another research topic in the context of digitization is the use of artificial intelligence and the exploration of quantum computing for optics design and multi-physics optimization problems. One aim of using AI algorithms is the acceleration and automation of the design process of optical systems. Currently, the use of reinforcement learning to automate the design process of optical systems is being investigated.

Laser Sintering (LS) is a powder bed based Additive Manufacturing (AM) technology for the processing of polymer powders. To generate polymer parts with LS, CO2 lasers with a wavelength of 10.6 µm are used as standard, as the CO2 laser radiation is strongly absorbed by the polymer powders. In contrast, polymers hardly absorb any radiation in the near infrared (IR) at about 1 µm wavelength (e.g. fiber and diode laser radiation). In order to deepen research in this field, a fundamental understanding of the interaction of the laser radiation with different wavelengths and polymer materials must be developed. For this purpose, both the material itself as well as the interaction with the laser source have to be fundamentally investigated. Material combinations of polymers and absorber materials increase the absorption capacity at wavelengths of approximately 1 µm and offer the possibility to process polymers, but at the same time affect the resulting component properties. In addition, the fragile heat balance in LS (maximum variations of ±2-4°C) is to be analyzed with improved heating concepts in order to extend the knowledge gained on the processing of e.g. high-performance polymers such as PEEK with different radiation sources.

Nowadays, laser technology is used in many cases to melt materials. Especially in the field of Additive Manufacturing (AM) this is one of the most common processes (e.g. in Laser Sintering – LS). However, the exact interaction between the laser radiation and the molten material particles is not yet well understood. In view of the research into the processing of polymers by LS using laser beam sources with wavelengths that are insufficiently absorbed by the polymers, the sintering process must be analyzed in detail. The wavelength absorption as well as the conversion of the introduced energy to form sintered and molten polymer particles have not been sufficiently investigated so far. As a result, the formation of the molten material in correlation with the laser radiation, the formation of macro- and microscopic defects and the thermal behavior in x-, y- and z-direction must be studied. Therefore, different simulation methods (e.g. DEM, FEM) and monitoring equipment (e.g. pyrometers) will be further developed and applied to gain a fundamental understanding of the interaction between laser radiation and melting behavior, which will be calibrated and validated by iterative experiments.

Laser sintering (LS) is a powder bed based additive manufacturing (AM) technology for the processing of polymers. Since LS is a layer-by-layer process in which the material is sintered and melted, the interconnection between the individual layers is of utmost importance to achieve the required stability of the component in the build-up direction. This could be achieved by increased crystallization of the material across the layers, requiring research into the adaption of the chemical composition of the polymer. Furthermore, to produce high-performance polymer components with LS, additives such as carbon or glass fibers are often added to the base material itself. During the melting process of the layers to each other, these fibers are interference bodies and reduce the layer bonding. Accordingly, the matrix material (base polymer) must achieve the required crystallization across the layers and therefore requires material adaptation which has to be investigated.

The methods of coherent diffractive imaging and ptychography replace a lens with an algorithm to obtain quantitative phase-sensitive images, and may provide advantages over conventional electron microscopy of macromolecules. This project will develop simulations of coherent electron diffraction under realistic conditions and compare with experiments, and investigate new algorithms to recover images from the diffraction data. We are looking for a candidate with a strong interest in imaging and Fourier optics, and who is capable of programming and performing data visualisation.

One aspect of in-line holography is that the reference wave greatly amplifies the weak scattered wave from a small object such as a virus. With revolutionary new X-ray optics it becomes possible to record highly magnified in-line holograms of nanometer-sized objects. This project will explore the potentials of this for imaging biological materials, investigate the dose requirements, and experimentally demonstrate 3D imaging. The successful candidate should have a strong background in optics and imaging, and strong programming skills.

This project will extend the method of serial femtosecond crystallography using X-ray free-electron lasers and synchrotron radiation facilities to investigate the dynamics of proteins induced by novel triggers, such as terahertz radiation or photo-acoustic waves. The candidate will have a unique opportunity to work on forefront research at advanced X-ray facilities as well as develop and test new experimental methods in the laboratory. The project combines experimental physics, structural biology, and computational analysis. We are therefore searching for a candidate with a strong interest in multidisciplinary research.

We are interested in ultrafast correlated and collective electron dynamics that unfold when complex materials are exposed to intense, ultrashort laser fields. In particular this includes the attosecond control and tracing of strongly coupled electron-nuclear dynamics in quantum systems such as molecules, and of collective electron dynamics in nanostructured materials, and lightwave-driven petahertz (nano)electronics. We develop and use multi-dimensional imaging techniques in conjunction with (attosecond) pump-probe experiments to gain detailed insight into the dynamics from the interaction of these materials with near-single cycle laser fields of typically attosecond to femtosecond duration. We actively push the theoretical modeling of the rather complex multi-charge dynamics, which are partly carried out in our group using Monte-Carlo simulations with more intricate models within collaborations.