3 Ultrafast and Nonlinear Phenomena:
   Condensed Phase
Group coordinator:

Research focus 3 is dedicated to the investigation of ultrafast and nonlinear phenomena in condensed matter and at solid surfaces. Ultrafast electron and structural dynamics, their interplay and the study of natural and artificially grown nanosystems represent long-term research topics in this focus area.

In a broad, interdisciplinary approach a number of complementary systems are studied to unravel the elementary non equilibrium dynamics and relaxation of elementary excitations as well as structure changing processes, such as phase transitions. Different types of nanosystems, correlated materials, molecular layers at surfaces, and optoelectronic devices are investigated. For such research, novel experimental techniques of ultrafast spectroscopy are applied. In the centre of interest are presently time-resolved photoelectron spectroscopy with short pulse lasers, in part also in combination with synchrotron radiation, combination of optical near-field techniques with ultra high time resolution, nonlinear terahertz spectroscopy, and femtosecond X-ray diffraction. Applications, resulting from these studies, are actively pursued in collaborations with external partners from research and industry.

The present activities are grouped into four projects concentrating on the following subjects:

Electron dynamics and laser-induced reactions at clean and adsorbate covered surfaces

Time-resolved photoelectron spectroscopy and X-ray absorption experiments aim at a microscopic understanding of fundamental electronic excitations and atomic motion at surfaces by unravelling the ultrafast response of the surface geometric and electronic structure. Comprehensive information of momentum, energy, spin and life-time of excited two-dimensional electronic states is obtained by means of angle-, energy-, spin- and time- resolved photoemission. While laser-based studies with a time-resolution of below 10 femtoseconds allow to access coherent electronic phenomena at metal and semiconductor surfaces, the MBI-BESSY experiment with synchronized laser and synchrotron radiation will focus on processes where element specific spectroscopy at elevated photon energies is essential to address diffusion of adsorbates and collective conformation dynamics in ensembles of molecular switches. In the low-alpha mode at the Berlin synchrotron an enhanced time-resolution below 5 picoseconds is envisaged and opens up the possibility to further study collective excitations at semiconductor surfaces. Access to shorter time-scales will be achieved by building up a new multi-kHz high-harmonics (HHG) facility for two-photon photo-electron emission experiments with time-resolution below 100 femtoseconds. Furthermore, the potential of the newly developed incoherent X-ray source will be explored for time-resolved ESCA experiments at surfaces. The basic research in project 3-01 is strengthened by fundamental studies comprising material modification with ultra-short laser pulses. The research focusses on dielectric and semiconductor targets and is intended to outline the potential of femtosecond laser technology and adaptive pulse shaping for high quality material processing. Main activities in 3-01, close collaboration with 3-02 and 3-04.

Ultrafast dynamics of individual nanosystems

Single semiconductor nanostructures and individual molecular switches at surfaces are studied by combining optical near-field techniques including apertureless near-field microscopy with nonlinear ultrafast spectroscopy. Such techniques which have in part been pioneered by MBI researchers allow highly specific studies of electronic and/or vibrational excitations in individual nanosystems, thus avoiding complexities introduced by ensemble averaging. Combined semiconductor-metal nanosystems as well as (macro)molecular switches at surfaces will be major objects of future research. Recent experiments using apertureless near-field probes promise an extension of the spatial resolution down to the 10 nm length scale which is to be combined with a sub-100 fs time resolution. Main activities in 3-02, close collaboration with 3-01.

Low-energy excitations in bulk and nanostructured solids

Few-cycle mid-infrared and THz pulses with field strengths up to several MV/cm and phase-resolving detecting schemes like electrooptic sampling allow for the generation and detection of highly nonlinear quantum-coherent excitations which are relevant for the basic optical and transport properties of solids. Experiments on low-energy excitations in semiconductor nanostructures and correlated materials provide insight into basic dynamics and couplings of carrier systems in a wide range of carrier density and will shed new light on correlation effects. Field-induced phenomena rather than resonantly driven dynamics will be an important aspect of future research, including work on optoelectronic device structures. Main activities in 3-02, close collaboration with 3-04.

Time-resolved experiments on highly correlated condensed-matter systems

An important trend of all projects within research area 3 is a shift of focus from the well established physics in the single particle picture to the physics of highly correlated condensed-matter systems. The latter is a thriving field of research because of a broad range of unusual phenomena which are of interest from the point of view of both fundamental research and practical applications. An essential focus will be antiferromagnetic systems since many high-correlation phenomena like high-temperature superconductivity, colossal magnetoresistance, or exchange bias are closely related to the antiferromagnetic state. In nonlinear magneto-optics we highlight the spin dynamics of antiferromagnetic systems. This includes the dynamics of sublattice correlations in compounds with multiple magnetic or electric ordering and should lead to methods for magnetic or magnetoelectric phase control. Main activities in 3-02.

Transient structures in condensed matter investigated by ultra fast X-ray techniques

Based on the leading role of the MBI in high-repetition rate femtosecond X-ray plasma sources and first successful application on coherent lattice dynamics in solids, studies of reversible phase transitions in correlated condensed-matter systems and - to some extent - in molecular crystals will be a key area of MBI research in the next 5 years. Such research aims at a distinction of electronically vs. thermally driven structural changes by separating the different phenomena in the time domain. In molecular crystals, the photoinduced change of molecular geometries and effects of sterical hindrance are investigated. Main activities in 3-04, close collaboration with 3-01 and 3-02.

Applied research on optoelectronic devices

The MBI has developed a number of spectroscopic techniques to characterize key parameters of optoelectronic devices, e.g., semiconductor lasers, and to assess their aging behaviour. In addition, new materials for optoelectronics are being studied by time-resolved spectroscopies. This work which is performed in close collaboration with external partners from research and industry and gives access to new materials and device structures, will remain an important part of the MBI research program. Main activities in 3-03 and 3-02.