DFG Priority Programme 1840 QUTIF Quantum Dynamics in Tailored Intense Fields

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Research Projects

AFundamental problems in atomic physics

Laser-assisted ionization and spin-polarization of electrons in strong twisted and phase-controlled light fields

Project leader: Prof. Stephan Fritzsche

While the electron dynamics in femto- or atto-second pulses has attracted much recent interest, most of these pulses can be tailored by their shape, subcycle phase and polarization, and can thus be described in terms of their plane wave decompositions. Less attention, in contrast, has been paid so far to shaped "twisted" fields which offer a novel route of tailoring light pulses. Photon beams with such properties are now generated routinely over a large range of energies and with quite high intensities. In this research project, we shall explore the "twistedness" of the photons fields and light beams as an additional degree of freedom in order to control the motion and spin-polarization of electrons, apart from the duration, envelope and phase control of the pulses. In particular, we will investigate the interaction of atoms (and later also molecules) with more or less strong twisted light fields, and where a phase-controlled, near-infrared field is applied to time-resolve the details in the electron emission. For this, the strong-field approximation will be extended and applied to describe the ionization of atoms and the spin-polarization of electrons in such short-pulse helical light fields.

Momentum distributions from bichromatic ionization of atoms and molecules

Project leader: Prof. Manfred Lein

The response of atoms and molecules to intense laser pulses composed of two colours is investigated theoretically. In one part of the project, both colours are linearly polarized in orthogonal directions in order to probe the ionization dynamics induced by the fundamental field with a weak second harmonic field. In the other part, we choose both colours circularly polarized and counter-rotating in order to exploit the unusual mapping from ionization times to final electron momenta in such bicircular fields. Bircircular fields have recently found remarkable attention in the area of high-harmonic generation, but the photoelectron momentum distributions remained mostly unexplored. The momentum distributions are calculated by numerical solution of the time-dependent Schrödinger equation in order to capture both the direct and the rescattering electron trajectories. Comparisons are made with the strong-field approximation. In atoms, application of bicircular fields for the study of sub-cycle ionization dynamics is envisaged analogous to the attoclock method. In molecules, we analyse the orientation dependence of quantities relevant for strong-field processes such as ionization times and the lateral width of the momentum distributions. In the rescattering electron signal from molecules in bicircular fields, we search for the signature of electron diffraction to perform ultrafast imaging of molecular structure.

Electronic structure and dynamics in strong fields: precision spectra for multi-electron systems

Project leader: Prof. Armin Scrinzi

Strong and precisely controlled electric fields are available in the form of extremely short laser pulses. These fields are used to literally observe how electrons move in atoms and molecules and to control the transformation of electronic structure by such pulses. Electrons and photons reach us as the messengers of these processes by their distribution in energy and emission angles. The precise interpretation and theoretical and computational verification of such distributions is the overall purpose of this project. Our principal guideline is to establish unambiguous theoretical data, and to do so for a set of atomic and molecular systems that are widely used in experiments with strong pulses. The systems include (in increasing complexity) the Helium atom and the Hydrogen molecule, heavier noble gas atoms Ne, Ar, etc., molecules consisting of only two atoms (diatomics), and finally larger molecules with no or strongly reduced symmetry. In our computations, only the electronic dynamics will be investigated, which dominates the processes up to time-scales of about 1 femtosecond. The initial stage of the project is entirely based on mathematical and computational developments by our group that were established during recent years an that, for the first time, allow the precise calculation of such phenomena with manageable computer resources. To progress further in this direction, method development constitutes about 25% of the research time in this project. Our data will be used to answer questions like: what causes the observed time-delays in the emission of electrons? (Not fully explained even for the simplest noble gas atom of Helium). Can we imagine electron detachment as a process involving only a single electron, or does the collective motion of electrons determine emission spectra? The answer to this question is expected to depend on the molecular species. What does the emitted light tell us about the internal structure of the molecule? Do we only see the electronic skin (valence electrons), do we see the core, do we see motion of electrons?

Generation and Characterization of Chiral Attosecond Pulses

Project leaders: Prof. Mikhail Ivanov, Dr. Nickolai Zhavoronkov

Development of robust, flexible, and practical approaches to the generation of circularly polarized femtosecond and attosecond pulses in the extreme ultraviolet (XUV) spectral range is a frontier of research in intense field physics. Such pulses have numerous important applications in chiral-sensitive light-matter interactions: chiral recognition via photoelectron circular dichroism, study of ultrafast chiral-specific dynamics in molecules, X-ray Magnetic Circular Dichroism spectroscopy, including time-resolved imaging of magnetic structures. So far, most chiral studies have been performed at large-scale facilities (e.g. synchrotrons, XFELs), with time resolution above 100 fsec. However, recent breakthrough experiments show that high harmonic generation, which up-converts intense infrared light into XUV spectral range, can be a viable table top source of circular and elliptically polarized XUV radiation. Such source would enable laboratory-scale studies at ultrafast time-scales. Currently, the duration of circularly polarized XUV pulses available from table top sources is several tens of femtoseconds. The first goal of our joint experimental and theoretical project is to improve this state of the art by two orders of magnitude, achieving efficient generation of attosecond pulses, both isolated and pulse trains. These pulses will have controlled polarization, tunable from nearly circular to linear. We intend to achieve this goal by generating high harmonics of tailored intense light fields in a gas of atoms and/or molecules. The second goal of our project is to follow the nonlinear response of atoms and molecules during frequency up-conversion to study the effects of spin-orbit interaction and multi-electron correlation on strong-field electron dynamics, with attosecond resolution. Last but not least, we will also theoretically study the possibility to characterize both the duration and the polarization state of the generated attosecond pulses, in a single measurement.

Imaging Electrons and Ions produced by two colour counter rotating laser fields of variable ellipticity

Project leader: Prof. Reinhard Dörner

Femtosecond lasers today reach routinely intensities in the 1014 W/cm2 range. Atoms and molecules exposed to such laser pulses are efficiently ionized. At the maxima of the oscillating laser field short electron wave packets are set free which are then driven by the laser field. Shaping the electric field thus allows steering theses wave packets, which is at the heart of Priority Program 1840. The present experimental project will use a very flexible waveform (a superposition of two counter rotating elliptical fields of 400nm and 800nm). In the simplest configuration such a field has threefold symmetry, the electric field vector resembles a three leave shamrock. Tuning the relative field strength, the ellipticity and relative phase between the two colours breaks this symmetry and gives rise to a multitude of different field shapes. The complex field shapes will give rise to a complex three dimensional momentum distribution of the electrons which carries all the information on the field driven quantum dynamics. We will measure these three dimensional electron momentum distributions using the COLTRIMS technique. For each electron we will measure in coincidence the ion charge state and momentum and for small molecules the direction and energy of the charged molecular fragments. The flexibility of field shapes will allow us to learn about the interplay of coulomb potential and laser field in the ionization process. It will secondly allow us to steer the electron wave packets emitted at different times during one cycle to bring them to interference (intra cycle interference) or steer them to separate regions of momentum space to avoid these interferences. These interferogramms of the electron wave packets carry information on the angular dependent phase of atomic or molecular orbital from which the electrons emerged. We will use this to explore molecular orbitals. The flexible field shapes will thirdly allow us to control the energy distribution and direction of the electron wave packet recolliding with its parent ion. Our simulations suggest that even almost mono energetic electron beams can be made by tuning the field. A particular advantage of the suggested field forms is that the recolliding electrons can be directed to a region of phase space where they are not swamped by the direct electrons. This in turn allows to study and control otherwise inaccessible features and energy ranges (low energies) in the recollision. We will exploit this to explore shape resonances in small molecules in a laser field. We will furthermore use these electron for excitation, multiple ionization and to drive molecules to dissociation. In summary, we will combine the most advanced 3dimensional electron and ion imaging together with the highly flexible shape of counter rotating elliptical two colour fields to study and control electron wave packets emitted from and recolliding with atoms and small molecules in unpresented detail and completeness.

Quantum control with intense X-ray pulses: A theoretical approach

Project leaders: Prof. Michael Rost, Prof. Ulf Saalmann

The goal of this project is to extend optimal control to the X-ray regime, i.e., beyond the well established coherent control schemes based on near infrared laser pulses. Pulses in this new regime have typically bandwidths, available for pulse shaping, which are much smaller than the carrier frequency. It will be explored if and how optimal control can be made possible under these conditions. The theoretical investigations will be based on the so-called envelope Hamiltonian. This Hamiltonian separates the effect of the oscillations of the electric field with the carrier frequency from the effect of the envelope of the laser pulse. Thereby, it directly takes advantage of the aforementioned characteristic difference of bandwidth and carrier frequency in the high-frequency domain. After formulating the envelope Hamiltonian for high frequencies the most suitable control scheme in connection with the envelope Hamiltonian have to be identified and implemented numerically. In order to gain insight into the optimal control mechanisms the dynamics will be described in terms of a perturbation theory. It remains to be studied whether the perturbative approach can be directly used for optimal control. Whereas most of these studies will be done for test cases, ultimately the performance of the X-ray optimal control has to be applied to realistic systems, to be identified.

Phase-dependent ionization and CE-phase measurement at long wavelengths

Project leader: Prof. Gerhard G. Paulus

The characteristic signature of laser pulses tailored on the sub-cycle time scale is a spatially asymmetric temporal evolution of the electromagnetic field. Linearly polarized fields, e.g., exhibit different field in opposing directions, i.e. there is no inversion symmetry. For few-cycle pulses, this property can be described by specifying the ("absolute") phase of the carrier wave with respect to the pulse envelope, for two-color fields by their mutual phase. Tailoring waveforms on the sub-cycle time scale and using them requires measurement and control of this phase, which is extremely sensitive to laser and environmental instabilities. For the visible spectral regime, our group has invented a phase-meter that exploits the asymmetry of photoelectron emission caused by asymmetric laser fields. The instrument has several features that make it particularly attractive for experiments that measure ionization and dissociation processes of atoms and molecules in a highly differential way. One of these is that the asymmetry of photoelectron emission is an obvious and very robust indicator of the pulse duration, i.e. such a phase-meter can be used to monitor laser performance and thus the integrity of the recorded data during extended data acquisition periods alongside recording the absolute phase.However, the current mode of operation of this phase-meter will not work for infrared wavelengths. Considering that long wavelengths are considered as one of the most promising future research directions in strong-field and attosecond laser physics, there is a need for respective research and development.We therefore propose to investigate strong-field photoionization and, particularly, its dependence on the absolute phase in the short-wave- and mid-IR in detail in order to identify effects that are suitable for phase measurement in these spectral regimes. Based on the results, a new phase-meter will be designed, built and tested. The device should provide high-precision, single-shot performance for phase measurement and, at the same time, provide reliable information on laser performance. We also propose to realize an interim solution for the problem of phase measurement at long wavelength. A proof-of-principle for this sub-project is already available. However, this interim solution is fully applicable only for pulse sequences that are phase-stable at least on short time scales.

Theoretical and experimental investigation of Kramers-Henneberger states in alkaline and noble gas atoms

Project leaders: Prof. Mikhail Ivanov, Prof. Milutin Kovacev

The main objective of this project is to provide direct spectroscopic evidence for the bound states of a free electron an unusual quantum state created by the concerted actionof the attractive core potential and a strong laser field. Electrons in such states respond to the laser field almost like free electrons, yet on average they remain bound to the ionic core. The states are often referred to as the Kramers-Henneberger (KH) states, after first theoretical predictions made about 50 years ago. For many decades, the KH states looked like a purely theoretical concept of academic interest. However, today there is mounting indirect experimental evidence suggesting that these states are ubiquitous and can emerge almost any time an atom or a molecule is exposed to sufficiently intense infrared laser fields. The combined action of the attractive core potential and the laser electric field creates a potential barrier through which a bound electron can escape. As the laser intensity increases, the top of the potential barrier can descend below the energy of a bound electronic state. Yet, the state does not necessarily become free. While the electron oscillation amplitude in such a state reaches some ten angstroms, it remains stable against ionization. Crucially, this situation is typical for all excited atomic states exposed to infrared laser fields with intensities in mid-1013 W/cm2 and higher. Such restructuring of the atomic spectrum has important implications for nonlinear light-matter interaction, including such processes as laser filamentation. The emergence of Kramers-Henneberger states inside a laser filament is extremely likely, and their role in the filamentation process can be substantial. One of the key goals of our project is to investigate the emergence of these states, their role in the filamentation process, and provide conclusive, direct spectroscopic evidence of their existence. Our goal is to provide two complementary spectroscopic observations, based on angle-resolved photo-electron spectroscopy of isolated atoms and transient absorption spectroscopy in laser filaments, to resolve this intriguing situation.

Quantum dynamics of electrons on the boundary to continuum and its control in strong tailored fields.

Project leaders: Dr. Ihar Babushkin, Prof. Uwe Morgner

Electrons, ionized under the action of strong tailored optical fields and then moving in vicinity of atoms (without recollision), emit so called Brunel radiation. The part of the Brunel radiation in the terahertz (THz) range contains fingerprints of purely quantum dynamics, including soft re-scattering (without re-absorption) with the atomic core, high Rydberg level interaction with the continuum, and others. This information was up to now mostly inaccessible experimentally because of insufficient power of Brunel radiation, so that quantum features were hidden on the strong semi-classical background. In this project we are going to access the genuine quantum dynamics aspects of Brunel radiation using intracavity- -enhanced driving fields at MHz repetition rate. Three-colour strong tailored fields will be used to generate significant Brunel radiation from even a small spot. Moreover, we are going to cavity-enhance the Brunel radiation as well, thus making it self-amplified. In this way, the electron trajectories will be 'self-influenced' via return of Brunel radiation over the round trip time, thus also opening new possibilities to their control. This will allow us to enter the strong-field regime allowing to induce nonperturbative quantum effects for noble gases in THz range. In the theoretical part of the work we support our experiments with full quantum mechanical simulations, develop the theory of a Brunel radiation laser. This opens a wide range of possibilities for the next financial period such as study of Brunel radiation in molecular gases which gives the picture of interaction of rotational levels with ionized electron, joint detection of both intracavity Brunel and high-harmonic radiation as well as study the question, to which extend the electronic wave function can be reconstructed by detection of Brunel radiation only (instead of detection of electrons).

BMolecular physics in strong controlled fields

Coherent Control of a Binary Reaction: Mg + H2

Project leaders: Prof. Christiane Koch, Zohar Amitay, Ph.D.

About thirty years ago, coherent control was conceived as a method to determine the fate of chemical reactions using laser fields. It has been extremely successful for destructive unimolecular processessuch as fragmention or dissociation but controlling the reverse, bimolecular process of bond formation still remains an unresolved puzzle. Within this project, we seek to demonstrate for the first time the coherent control of a complete gas-phase binary reaction. This includes the controlled formation (or photoassociation) of a new chemical bond, the controlled dynamics of the intermediate complex through a conical intersection, the controlled cleavage of another chemical bond as well as the stabilization of the reaction product. We consider a triatomic reaction which provides the simplest non-trivial model in which all these steps can be examined. The experimental part of the work will be carried out in a heat pipe, using strong, shaped near-infrared femtosecond laser pulses. Theoretically, the reaction will be studied from first principles, combining ab initio electronic structure and molecular quantum dynamics calculations. The project builds on our earlier work demonstrating femtosecond multi-photonphotoassociation of magnesium dimers and its coherent control, generalizing it to a triatomic system with stable reaction products.

The HeH+ isotopologues in intense asymmetric waveforms

Project leaders: Prof. Stefanie Gräfe, Prof. Manfred Lein, Prof. Gerhard G. Paulus

We investigate the sub-cycle dynamics of dissociation and ionization of the HeH+ isotopologues experimentally and theoretically. The HeH+ ions are produced in an ion beam apparatus and exposed to intense few-cycle pulses. The momenta and charge state of all fragments, including neutrals, are measured in coincidence and as a function of the ("absolute") carrier-envelope (CE) phase. In parallel, quantum-mechanical non-Born-Oppenheimer simulations including all possible fragmentation pathways, i.e. dissociation, single and double ionization, are carried out. HeH+ is a particular attractive molecule for investigating the sub-cycle dynamics of elementary photochemical processes because of its lack of symmetry, a property shared with few-cycle pulses. As compared to the symmetric homonuclear case, the strongly localized initial charge distribution simplifies dynamic electron localization in strong-field molecule interaction. A particularly attractive feature of HeH+ is the availability of the four isotopologues 4HeH+, 4HeD+, 3HeH+, and 3HeD+, which offer the possibility to vary the mass ratio of the two nuclei over a broad range from 4:1 to 3:2, thus affording systematic investigations of the entanglement of electronic and nuclear dynamics on the sub-cycle time scale.

Laser-tailored, ultrafast spin-dependent dynamics in chiral/helical molecules

Project leaders: Dr. Ingo Barth, Prof. Jamal Berakdar

Laser-controlled, spin-dependent charge dynamics in molecules is studied with a particular focus on the influence of the topology on the coupled spin-charge time evolution. Methods of ab-initio quantum chemistry, quantum dynamics in intense fields, classical and quantum spin dynamics, and a reductive symmetry analysis render possible a reliable description and a clear picture of the driven spin dynamics in chiral/helical and other molecules in tailored intense fields. Laser fields with engineered helical waves may generate internal orbital currents that we will calculate and utilize to steer in a spatio-temporal way the spin degree of freedom. Furthermore, we will develop and implement a theory for the molecular nonadiabatic tunnel ionization in strong circularly polarized laser fields and explore thereby the spin polarization effects and the role played of the underlying geometric molecular structure.

Diatomic Molecules in Intense Laser Fields

Project leader: Prof. Alejandro Saenz

Already in the past diatomic molecules have played a prominent role in understanding molecular processes. Therefore, an existing theoretical approach for the description of diatomic molecules with two electrons (like molecular hydrogen) exposed to ultrashort intense laser fields should be extended to diatomic molecules with an arbitrary number of electrons. A number of different approaches beyong single-electron models should be implemented. With the aid of these codes simulations shall be performed that study the behaviour of diatomic molecules in shaped laser fields or sequences of laser pulses. This should allow for the verification of control schemes.

Laser induced electron diffraction off strongly aligned and oriented molecules by tailored electric and laser fields

Project leaders: Prof. Jochen Küpper, Dr. Arnaud Rouzée

Laser-induced electron diffraction (LIED) is an approach that allows for the atomic resolution imaging of structures and structural dynamics of molecules. Re-scattered electrons, emitted from strong-field ionization of molecules by intense mid-infrared pulses, are known to contain information on the exact (time-dependent) structures of simple molecules that can be extracted to record a "molecular movie". However, the application of this technique to complex molecules and molecular dynamics requires strong control over the molecular sample, which needs to be spatially separated according to size, structural isomer, and quantum-state and to be strongly aligned and oriented.Here, we set out to advance methods to strongly control molecular samples of complex molecules and to use them to record precise structures of molecules and their dynamics using LIED. We will create beams of cold molecules using supersonic expansions and disperse these beams according to quantum-state, in order to create pure samples of individual states, conformers, or cluster sizes.Subsequently, the molecules will be strongly three-dimensionally aligned and oriented using moderately strong, tailored laser and dc electric fields. These samples, with all molecules looking identically in the laboratory frame, will be irradiated by an intense, mid-infrared, femtosecond pulse. The very strong electric field will ionize the molecules and accelerate the produced electrons.Eventually, the electron will re-scatter at the molecular ion, which was left behind. We will measure the momentum distribution of these electrons, in the molecular frame, and extract the electron diffraction pattern that will be inverted to yield a precise structure of the molecule. Adding an ultrashort UV pulse to start chemical dynamics will allow us to perform pump-probe experiments and to record snapshot of photo-initiated dynamics.We will implement these experiments for complex polyatomic asymmetric-top molecules and molecular clusters to investigate structural rearrangement reactions and so-called half collisions, in order to create clear pictures of these complex chemical-dynamics processes with high resolution. The investigated systems range from the dissociation dynamics of the OCS molecule to the solvent-solute interaction in indole-water clusters. Our results will provide new insight into the molecular basis of chemistry and chemical reactions.

CApplications in chemistry

Phase locked bichromatic polarization tailored femtosecond laser fields to study and control electron dynamics in chiral molecules

Project leaders: Prof. Thomas Baumert, Prof. Arne Senftleben

Photoelectron Circular Dichroism (PECD) is a CD effect based on an electric dipole transition being large in comparison to ordinary CD effects. Recently we have measured a PECD up to the ten percent regime in a 2+1 resonance enhanced multi photon ionization (REMPI) scheme with femtosecond laser pulses on randomly oriented molecules of Camphor, Fenchone and Norcamphor in the gas phase at an excitation wavelength of 400 nm. We observed contributions from higher order Legendre polynomials up to two times the number of photons absorbed. Different modulations and amplitudes of the contributing Legendre polynomials are observed despite the similarity in chemical structure and absorption spectrum. Our intensity studies revealed dissociative ionization as the origin of the PECD effect and ionization of the intermediate resonance is dominating the signal. So far there is a lack of a consistent theoretical description of the PECD in the multiphoton case and the role of the intermediate is theoretically unclear. Progress is expected as several theoretical groups are currently working on this topic. Here we propose to put PECD measurements to a new level and use phase locked bichromatic (400 nm / 800 nm) polarization tailored femtosecond laser fields to study and control electron dynamics in chiral molecules, where the above mentioned bicyclic ketones serve as prototypes. The experiments are directed to study the influence of the photons angular momentum on resonances and continuum states and to study PECD in unusual polarization fields. A systematic study using bichromatic polarization tailored laser fields on the ionization dynamics in general has not been performed so far. This is partly due to the lack of a proper optical set-up to create these tailored light fields. One of the objectives of this proposal is therefore the implementation and characterization of a phase locked bichromatic bipolarization set-up with independent selectable polarization states and intensities for the two radiation fields as well as allowing for phase stable tuning from the optical interference regime (temporal overlap of the two radiation fields) to the pure quantum interference regime (temporally separated pulses) and a pump-probe regime with attenuated pulses. Besides a test of the set-up on achiral potassium atoms, we will for the first time apply such fields to ionize randomly oriented chiral molecules. The three dimensional momentum distribution can be inferred from tomographic reconstruction techniques developed recently in our laboratories. Amongst a whole variety of different physical topics, we expect promising results from two specific approaches: We will investigate to what extent the PECD effect can be increased using laser prepared nonisotropic distributions. We will study PECD in unusual polarization fields like for example 'cloverleaf' or 'butterfly' shapes, where the latter might open a route to create a PECD effect within one polarization field.

Controlling ultrafast charge migration by short laser pulses

Project leader: Dr. Alexander Kuleff

Exposing molecules to ultrashort laser pulses can trigger pure electron dynamics in the excited or ionized system. In the case of ionization, these dynamics may manifest as an ultrafast migration of the initially created hole-charge throughout the system and were termed charge migration. Due to the coupling between the electronic and the nuclear motion, the control over the pure electron dynamics offers the extremely interesting possibility to steer the succeeding chemical reactivity by predetermining the reaction outcome at a very early stage. That is why, the charge-migration phenomenon increasingly attracts the attention of the scientific community and at present several groups are preparing experiments to study the process using novel attosecond pulse techniques. For being able to design successful protocols, one needs to acquire a detailed knowledge on the influence of control pulses on the pure electronic step. The ambitious goals of the present proposal are to further develop and employ our methodologies to study the influence of laser pulses on the pure electronic charge-migration dynamics, and to develop experimentally accessible strategies to control the charge migration process by appropriately tailored ultrashort laser pulses. Such calculations will, on one hand, provide the experimentalists the needed conceptual understanding and help in interpreting the experimental results, and, on the other hand, will guide future experimental efforts by proposing interesting systems and effects to be measured.

Probing molecular chirality and chiral dynamics using High Harmonic generation

Project leader: Dr. Olga Smirnova

The main objective of our project is to develop new methods sensitive to molecular chirality, both its structure and dynamics. Our goals include resolving and controlling multielectron chiral dynamics in molecules at its natural, sub-femtosecond time-scale. So far, this ultrafast time-scale in chiral response has remained hidden in experiments. The proposed method is based on high harmonic generation from a randomly oriented gas of molecules (chiral HHG, cHHG). Its key component is the application of tailored intense pulses to induce, enhance and manipulate the chiral response.Chirality is a basic property of living matter, crucial for its function. Left and right enantiomers of molecules have identical chemical and physical properties unless they interact with another chiral object, such as chiral light. Traditional methods of chiroptical discrimination in rotationally isotropic media are linear spectroscopic techniques, which measure the difference of the optical response of left and right enantiomers to chiral (e.g. circularly polarized) light. In these techniques, the chiroptical effects arise from the interplay between the light-induced electric- and magnetic-dipole transitions. The magnetic effects in light-matter interaction are generally very weak. This results in a weak chiroptical response, often four to six orders of magnitude smaller than e.g. light absorption. The weakness of the chiral response poses challenges for time-resolved measurements.Constant efforts towards the enhancement of the chiral response have led to a remarkable recent progress in very challenging gas-phase studies. One of the most sensitive techniques is photoelectron circular dichroism spectroscopy, PECD. The PECD detection requires resolving the direction of the electron final momenta: photoelectron angular distributions from randomly oriented chiral molecules show an asymmetry with respect to the direction of the light propagation. This asymmetry originates from the electron interaction with the chiral potential of the core. Crucially, it does not rely on the weak effects of the magnetic field. The chiral dichroism signal achievable with PECD, up to about 10 %, sets the ''golden standard'' for the new cHHG methods proposed here.We aim to match these high values of chiral dichroism with the new cHHG method, while also being able to simultanelously image the underlying chiral dynamics with the time resolution that is two orders of magnitude better then the available state-of-the art techniques.

Simulation of the electron dynamics of chiral systems in CEP-stabilized, intense laser fields

Project leader: Prof. Stefanie Gräfe

This project aims at developing a numerical model system describing a chiral potential environment for an electron. Interaction of this system with intense, circularly polarized laser fields leads to ionization, i.e. emission of the electron. The asymmetric angular distribution of the electron (multiphoton-photoelectron circular dichroism, PECD) will be numerically calculated. This simple model does not provide information on multi-electron effects, however, the role of intermediate, excited states and the continuum, which are not properly described in existing approaches, can be well examined. The results of the quantum dynamical calculations of this model system will be compared to existing approaches calculating PECD. The focus lies on the examination of the interaction of chiral systems with intense, ultrashort laser pulses with only a few optical cycles. Such pulses inherently possess a strong asymmetry. We intend to investigate if and how this additional asymmetry of the laser pulse may lead to a modification of the (asymmetric) angular distribution of photoelectrons.We plan to closely collaborate with the group of Prof. Dr. Matthias Wollenhaupt (Universität Oldenburg) who experimentally aim at investigating PECD for intense, few-cycle laser pulses.

Control of symmetry breaking in multiphoton ionization of chiral molecules

Project leader: Prof. Matthias Wollenhaupt

Symmetry breaking is a recurring theme in the natural sciences with subject areas ranging from fundamental problems in particle physics to applications of chiral molecules in chemistry, biology and medicine. A particularly intriguing example of symmetry breaking in the interaction of light and matter is the Photoelectron Circular Dichroism (PECD). PECD describes a forward / backward (axial) asymmetry in the photoelectron angular distribution along the light propagation direction arising from photoionization of randomly oriented chiral molecules in the gas phase with circularly polarized light. The PECD is ideally suited to study symmetry breaking of light matter interactions associated with chirality because the observed asymmetries are many orders of magnitude more pronounced compared to the conventional circular dichroism. Recently, we have demonstrated the use of femtosecond laser sources for PECD measurements via resonance enhanced multiphoton ionization of small organic chiral molecules. With the advent of novel ultrafast light sources capable of producing few-cycle laser pulses, an additional symmetry breaking perpendicular to the light propagation direction (lateral) has become feasible by Carrier Envelope Phase (CEP) stabilization of the pulse. The main objective of this project is a demonstration of lateral symmetry breaking in the interaction of CEP-stabilized tailored intense light fields with chiral molecules. We plan to investigate the implications of CEP stabilization on the PECD and to carry out complementing parameter studies to reveal the underlying quantum dynamics. In the experiment, we will employ CEP-stabilized ultrashort pulses with tailored polarization state and a broad range of excitation wavelengths as an advanced light source for ionization and tomographic reconstruction of three-dimensional photoelectron angular distributions by velocity map imaging for detection. By combining CEP-stabilization with pulse tailoring techniques, an unprecedented degree of control on symmetry breaking in multiphoton ionization of chiral molecules will be attained.

DExtending strong-field quantum dynamics to new media

Attosecond spectroscopy in the liquid phase

Project leaders: Prof. Reinhard Kienberger, Prof. Hans Jakob Wörner

The goal of the present proposal is to extend attosecond time-resolved spectroscopy to the liquid phase. Whereas attosecond spectroscopy in the gas phase is well established by now and first promising attosecond experiments have been performed on solids, the liquid phase has remained entirely unexplored on the attosecond time scale. However, the liquid phase is the natural environment of most chemical and biological processes. Therefore, the extension of the tools of attosecond spectroscopy to the liquid phase is an essential step in understanding the role of electronic dynamics in chemical and biological processes. Solvation of molecules is known to modify their electronic structure. Hence electronic dynamics of solvated molecules will almost certainly differ from that of gas-phase molecules. The liquid phase also offers unique dynamics compared to the gas phase such as the dynamics of the solvated electrons including its formation, relaxation and tunneling dynamics. This proposal will focus on demonstrating the attosecond streak camera on a liquid target with the goal of measuring attosecond photoionization delays from water and solvated species and to compare them with the corresponding gas-phase delays. These studies will be followed by measurements of the dynamics of formation and relaxation of the solvated electron using infrared (IR), ultraviolet (UV) and extreme-ultraviolet (XUV) pulses, all with few- to single-cycle durations.

Spatio-Temporal Tailoring of Light Fields for Sub-Cycle Resolved Measurements of Strong-Field

Project leader: Prof. Adrian Pfeiffer

The goal of the project is to investigate processes that evolve during one optical cycle of a laser field. The laser fields used are short pulses comprising only a few optical cycles, with an optical spectrum spanning from 600 nm to 900 nm. The processes under scrutiny are strong-field effects, which means that they scale highly-nonlinearly with the instantaneous laser intensity and exhibit dynamics rapid enough to evolve during one optical cycle of about 2.6 fs. Specifically, the effects addressed by the project are strong-field ionization, transiently induced polarization in atoms and molecules, and transient conduction band population in dielectrics. The basic concept to obtain sub-cycle resolution is the use of spatio-temporal tailoring of the few-cycle pulses, especially wavefront rotation, together with pump-probe measurements. Wavefront rotation means that the wavefront, or the plane of constant phase, rotates in space as the pulse passes through the focal plane. In this project, a tailored pump-probe geometry will be used, where the pump-field is a plane wave that induces strong-field effects, and the probe-field is a weak pulse (so it does not induce strong-field effects without the pump) that samples the medium with wavefront rotation. The strong-field effects are encoded in the probe field through dispersion and diffraction, for example the probe field experiences a retardation as it propagates through the strong-field dressed medium. Due to the wavefront rotation of the probe, the sampling times of the pump-probe interaction are sent to angularly separated directions. It can be regarded as photonic streaking, where sampling time is mapped to the propagation direction of the probe field. The subsequent observation of the angularly dispersed probe-field retardation is accomplished by fluorescent imaging: the probe field meets a counter-propagating reference field in a Fluorescein-filled cuvette and is imaged with a microscope. Despite the broad applicability to targets in gas phase, liquid phase and also condensed phase, the capability to investigate bulk media should be highlighted, because most other methods in strong-field science are not applicable to these targets. Coherence properties of transient conduction band populations will be studied, and dynamical Bloch oscillations in bulk dielectrics will be observed. A concomitant effect of transiently populated conduction bands in strongly driven dielectrics is the generation of UV and VUV light. By transient spectroscopy of UV and VUV light in a tailored pump-probe field, the role of higher-order nonlinearities and the role of transient conduction band population in the generation process will be studied. Wavefront rotation simultaneously in both the pump and the probe field will be used to send the generated light bursts into time-varying directions, thereby creating an attosecond-lighthouse. This is a development for an alternative route to sub-femtosecond pulse generation.

Strong-field electron dynamics at nanostructures controlled by spatiotemporal near-fields synthesis

Project leaders: Prof. Thomas Fennel, Prof. Peter Hommelhoff, Prof. Matthias Kling

This proposal focuses on coherent electron dynamics at nanostructured solids - nanospheres and sharp metal tips. Collective electron motion in laser driven nanostructures generates near-fields that are localized on the sub-wavelength scale and can be enhanced substantially with respect to the incident field. These features are ideal to drive, probe, and control coherent strong-field dynamics at nanostructures in the intermediate regime between atoms and macroscopic solid-state matter. Here we propose to study how electron dynamics on sub-cycle time and nanometer length scale can be controlled with and are affected by spatial, temporal, and vectorial near-field sculpting, collisional dephasing and intertwining of single and multielectron effects, andmulti-spectral optical field control.

Femto- and attosecond dynamics of helium nanodroplets and nanoplasmas

Project leaders: Prof. Marcel Mudrich, Prof. Thomas Pfeifer

The goal is to study the dynamics of electronically excited and ionized helium nanodroplets using ultrashort NIR, UV and XUV laser pulses. The time-evolution of a dopant-induced nanoplasma driven by intense NIR pulses will be probed by few-cycle NIR-NIR, NIR-UV and NIR-XUV pump-probe ion and electron imaging spectroscopy. Photoelectron spectra will directly reveal the evolution of the cluster potential as well as electron-ion recombination. Anisotropy effects in the nanoplasma evolution will be explored by controlling the pulse polarizations and the carrier-envelope phase of the driving NIR pulse. Using attosecond XUV pulses, the ultrafast charging dynamics of a dopant-induced nanoplasma will be directly followed.The relaxation dynamics of singly excited He droplets will be probed by XUV-UV pump-probe ionization in combination with electron and ion imaging detection. In particular the dynamics of ionization of dopant atoms attached to the He droplets by energy exchange akin to inter-atomic Coulombic decay (ICD) will be characterized. Upon double and multiple excitation, He droplets form an unusual nanoplasma by collective autoionization, which we will follow in real time. Comparative measurements using neon clusters will be instrumental for working out the peculiarities of quantum liquid He nanodroplets with respect to XUV excitation. The project will be carried out in collaboration with the groups of T. Pfeifer and R. Moshammer at MPI-K in Heidelberg.

Attosecond time-resolved streaking spectroscopy as a probe of strong field effects at the solid-vacuum interface of layered materials

Project leader: Prof. Walter Pfeiffer

Strong electromagnetic fields in the vicinity of the solid-vacuum interface can dynamically change surface properties and might serve to control interfacial charge transfer processes on a sub-cycle time-scale. The implementation of such strong field control requires both, the complete knowledge of the actual field distribution on an atomic length scale and the understanding of the complex processes occurring in this massively perturbed many-body system. Presently the atomic-scale local dynamic field distribution in the vicinity of the interface is unknown. In addition, above threshold photoemission (ATP) drowns other more specific emission channels and systems with low ATP yield are needed to extend the range of accessible field amplitudes. These two issues are addressed in this project that aims to improve the understanding of the fundamental processes governing the action of strong IR fields at interfaces and addresses open questions concerning the dynamics of the photoemission process. In addition it opens a pathway to increase the accessible field amplitudes which might then enable the application of strong field control schemes at the solid-vacuum interface. To achieve these goals attosecond streaking spectroscopy at solid surfaces is employed. Photoelectrons excited by a single attosecond XUV pulse propagate through the substrate-vacuum interface and interact with the simultaneously present intense IR streaking field. The fundamental mechanisms and processes determining the observed delays in photoemission are still under debate and no complete theoretical model is available yet. Transition metal dichalcogenides forming layered van der Waals crystals and other layered materials are employed to localize the origin of the emitted core level photoelectrons with atomic precision. By the investigation of single or few layers of these materials the problem of above threshold photoemission at solid surfaces is addressed. A single layer of this material supported on a substrate that exhibits a small ATP yield will allow increasing the IR field strength. Based on this improved understanding of intense fields at interfaces we will then target the actual modification of electronic phenomena. In first approximation electronic coupling phenomena, such as spin-orbit interaction, are determined by an effective potential. Intense fields at the surface can alter this potential and thus we anticipate that related phenomena such as the spin-polarization of emitted photoelectrons can also be affected. Therefore, in a feasibility study we will implement a spin-polarization sensitive photoelectron detector to demonstrate that spin-resolved and attosecond time-resolved photoemission spectroscopy is possible.

Identification and control of ultrafast spin dynamics in ferromagnetic solids by tailored fields

Project leaders: Dr. Andrea Eschenlohr, Dr. Sangeeta Sharma, Prof. Uwe Bovensiepen, Prof. Eberhard K. U. Gross

We will analyze spin dynamics in the transition metal ferromagnets Co and Fe driven by intense electromagnetic fields and aim at controlling the dynamics by optimizing and tailoring the driving intense fields in the visible and infrared spectral region. In the framework of our collaborative theoretical and experimental project we perform a fully ab initio investigation of femtosecond laser induced demagnetization, a phenomenon which we exploit in order to investigate the electronic charge and spin dynamics with particular emphasis on spin-orbit interaction and coupling to nuclear motion. Simultaneously, we will carry out an experimental investigation employing the time-resolved complex magneto-optical Kerr effect. This will allow to analyze dynamic spin-dependent effects due to spin-orbit coupling, electronic scattering with lattice and magnetic excitations, and transport effects induced by the externally applied electromagnetic fields, which drives the ferromagnet far out of equilibrium. At first we will distinguish the microscopic processes contributing to the magnetization dynamics, where we will strongly profit from our coordinated experimental-theoretical effort. On this basis, we will design laser pulses to optimally control the spin-dynamics. These optimal pulses will then be used in our experimental effort for further validation and development. To achieve these goals we will need to write two codes: a nuclear dynamics code which is fully coupled to the already existing spin-charge-dynamics code (http://elk.sourceforge.net) and a quantum optimal control code interfaced to this code such that laser pulses can be tailored. This tailoring will be done by employing experimental constraints on the target functionals. Within the experimental part we will improve the time resolution using sub 15 fs pulses generated by non-collinear optical parametric amplification and increase the intensity of driving laser fields up to 10^15 W/cm^2 employing optical parametric chirped pulse amplification. Thereby, we will access the predicted time and intensity scales for spin-orbit mediated spin dynamics. Identification of experimentally observed signatures with microscopic processes and the theoretical optimal pulse design will enable pulse shaping to manipulate and control the spin dynamics towards a desired response driven by tailored intense electromagnetic fields.

SOLSTICE - SOLids in Strong Terahertz and Infrared CE-phase-stable waveforms

Project leaders: Prof. Mackillo Kira, Prof. Franz Xaver Kärtner, Prof. Stephan W. Koch, Dr. Oliver D. Mücke

Rapid progress in the synthesis of sub-cycle optical waveforms has opened the door to the previouslyinaccessible realm of Waveform Nonlinear Optics. This new research field aims to study and control the nonlinear interactions of matter with rapidly changing optical waveforms custom-tailored within a single cycle of light. The capability to cycle-sculpt the electric field brings unprecedented opportunities for attosecond strong-field physics and extreme nonlinear optics, in which nonlinear interactions sensitively depend on the time evolution of the electric field. Some exciting new opportunities have already been explored and demonstrated in several pioneering works on atoms and molecules: For example, attempts to synthesize the so-called perfect waveform for high-harmonic generation (HHG), generation of intense isolated attosecond eXtreme UltraViolet (XUV)/soft-X-ray pulses for attosecond-pump/attosecond-probe spectroscopy, cycle-engineered steering of photoelectrons, and a ground-breaking experiment, in which a sub-cycle pulse was used to field-ionize krypton atoms within a single half-cycle and launch an electron wavepacket into a valence-shell orbit with a well-defined initial phase. In contrast to the research area of AMO physics, the great potential of cycle-sculpted waveforms for attosecond and strong-field physics in solids has been largely unexplored so far.In this project, we investigate attosecond strong-field physics in solids, using high-energy sub-cycle optical waveforms with cycle-sculpted electric fields, tightly synchronized to high-energy 0.3-THz transients of 1-10 GV/m field strength, for controlling extremely nonlinear interactions in solids. With these THz pulses, free electrons can readily reach ponderomotive energies in the keV to MeV, i.e. relativistic regime, in the solid. As we have previously shown, extreme nonlinear excitations can lead to the generation of electrons in multiple bands throughout the entire range of the Brillouin zone. In addition to the created charge-carrier densities and polarizations, the electric field also generates many-body correlations via Coulomb and phonon interactions. Experiments will be guided by a rigorous theoretical modeling. In our theoretical approach, we treat dephasing, relaxation, and energy renormalizations for the single-particle dynamics originating from electron-electron and electron-phonon scattering completely systematically by applying a cluster-expansion scheme. On the theoretical side of this project, we are particularly interested to study how these effects quantitatively modify optically driven solid-state HHG and XUV sources.These studies will permit new insights into complex electronic dynamics in solids under extreme conditions and will elucidate the physical mechanism of damage in high-gradient electron accelerators, important for tabletop hard-X-ray sources, and next-generation high-speed electronics operating at THz frequencies.

Coherent interactions of strong optical near-fields with free and weakly bound elecrons

Project leaders: Dr. Petra Groß, Prof. Christoph Lienau, Prof. Claus Ropers, Dr. Sascha Schäfer

The project proposed here addresses quantum coherent interactions of free and weakly bound electrons with strong optical near-fields. Specifically, we target the controlled manipulation of electronic populations in two quantum systems belonging to widely separated ranges of the energy spectrum: (i) Free electrons traversing optical near-fields at high kinetic energies, allowing for a quantum control of free electron momentum superpositions and the first generation of attosecond electron pulse trains (Göttingen group). (ii) Weakly bound electrons emitted into and out of localized image-potential states, demonstrating coherent control over Rydberg wavepackets in a nanoscale geometry (Oldenburg group). These two systems exhibit pronounced quantum phenomena by effectively decoupling the electronic dynamics from the rapid dephasing mechanisms typically encountered in the high electron densities of solids. Thus, the project combines concepts from coherent interactions in atomic and molecular systems with collective optical excitations in solids. This approach yields access to quantum coherent control of electronic populations while maintaining the spatial, temporal and vectorial field control facilitated by nanostructures.

Design based on bluewebtemplates