Dr. Torsten Siebert

Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy

Max-Born-Str. 2a, 12489 Berlin

++49 (0)30 6392 1414



Freie Universität Berlin, Departement of Physics

Arnimallee 14, 14195 Berlin







































































2D-IR Photon-Echo Spectroscopy

The hydration pattern and counterion constellation in the vicinity of the DNA surface play a decisive role in determining the structure of the double-helix. The underlying dynamics in the aggregation, reorientation and exchange times of water as well as fluctuations in the geometry of counterion binding sites have remained largely unexplored in the ultrafast regime. An access to the time window relevant for the interactions of the polar and charged elements of the DNA surface with the water dipoles and counterion atmosphere can be obtained via the vibrational modes localized in the phosphate and desoxyribose substructures of the DNA backbone (Panel A). Ultrafast photon-echos from the vibrational transitions of these modes offer a view into the surrounding bath via the distortions that the fluctuating interfacial electric forces impose on the vibrational potentials. These fluctuations manifest themselves in the line shapes of two-dimensional infrared spectra take at sub-picosecond intervals after excitation (Panel B). Initial benchmarks have been acquired in this manor, showing fast structural fluctuations in the displacement of the water dipoles and counterions relative to the helix with a global time scale on the order of 300 fs. In contrast, the inhomogeneity of the interfacial constellation at individual sites along the helix persists beyond 10 ps. Further looking from the bath into the helix structure, signatures of vibrational couplings in the backbone modes reveal the pathway and kinetics of intra-helical energy dissipation. These processes are completed within several picoseconds in the desoxyribose-phosphodiester moiety and represent a sensitive probe of the influence that the interfacial constellation has on the couplings between the structural elements of the DNA backbone.

Few-Cycle Supercontinuum Pulse Synthesis

The extended bandwidth provided by a supercontinuum laser pulse and the precision of current phase modulation techniques constitute the two basic ingredients for realizing a highly versatile optical mixing console capable of synthesizing optical pulse forms and custom light-matter interactions on the ultrafast time-scale. The compression of a supercontinuum pulse spanning over an octave in the visible to the near infrared spectral range to few-cycle durations of sub 5 femtoseconds presents the starting point for inscribing analytical or arbitrary phase functions over the broadband spectrum in the construction of a desired pulse form with a liquid crystal array modulator (Panel A). In this manor, pulses can be generated that are composed of few-cycle phase-locked sub-pulse structures with variable spectral composition and relative delay (Panel B and C). The two essential components for ultrashort pulse form synthesis are accessible with established techniques. Optical filamentation of standard amplified femtosecond pulses in gaseous media allows for a cascade of nonlinear frequency conversion to be achieved in a self-initiating, self-confining and self-terminating mode of pulse propagation for distances far beyond the Rayleigh range of a focused laser beam. This extended nonlinear propagation drives spectral broadening up to an octave and beyond with an uncomplicated phase correction for generating highly structured temporal envelopes and broadband frequency sweeps. Complementary to this, liquid crystal arrays further offer a precision in the relative phase retardation of selected spectral components at a thousands of an optical cycle of the electric field in this spectral range. Furthermore, the sampling limit of an extended liquid crystal array translates to a maximal temporal shift ranging two orders of magnitude beyond the time-scale of a field cycle. Within this wide range of parameters, the approach delivers a virtually free design of an optical pulse form and the capability to achieve a substantial information content encoded in the phase of the broadband pulse for the control of diverse ultrafast light-matter interactions.

Control of Ultrafast Molecular Processes

The charge and spin state together with the associated geometry and symmetry of a particular ectronic configuration are central for the physical and chemical properties of a molecular system. The interaction with a sequence of phase-locked ultrashort laser pulses constitutes a possible route for controlling these fundamental electronic attributes. By guiding the molecular system of interest through the electronic manifold of its charge states in a photoinduced ladder-climbing process, its electronic state can be switched in a desired sequence of events. Initial experiments have been carried out on small metal cluster ions and natural chromophors isolated in the environment of an ion trap using phase-modulated supercontinuum pulses to drive a tandem photoelectron detachment and resonance enhanced ionization process. Control is achieved by the spectral composition and delay of ultrashort sub-pulse sequences within a single supercontinuum pulse that is tailored to the dynamic response of the system. The composition of these pulse sequences defines the order and residence times in the electronic configurations of the charge states accessed in a continuous phase-locked optical driving of the system (see figure below). Complex metal-oxide cluster systems aggregate with small hydrocarbons that model catalytic processes on a specific surface morphology and electronic structure have also been approached. Photoelectron detachment in these species allows for switching from the virtually inert anionic into a reactive neutral charge state that drives an oxidation reaction of the hydrocarbon on the cluster. The degree of selectivity in the chemical reaction is probed by laser-induced fragmentation and ionization of the reaction complex with mass spectrometry of the products. This type of scenario opens the possibility of testing different pulse sequences in their aptitude to modulate the electronic state of a reaction complex on the time-scale of the chemical transformation while competing against the fast electronic relaxation processes that act against the control of the system.