Marcel Krenz

Transient Electronic Structure and Melting of a Charge Density Wave in TbTe3

Obtaining insight into microscopic cooperative effects is a fascinating topic in condensed matter research because, through self-coordination and collectivity, they can lead to instabilities with macroscopic impacts like phase transitions. We used femtosecond time- and angle-resolved photoelectron spectroscopy (trARPES) to optically pump and probe TbTe3, an excellent model system with which to study these effects. We drove a transient charge density wave melting, excited collective vibrations in TbTe3, and observed them through their time-, frequency-, and momentum-dependent influence on the electronic structure. We were able to identify the role of the observed collective vibration in the transition and to document the transition in real time. The information that we demonstrate as being accessible with trARPES will greatly enhance the understanding of all materials exhibiting collective phenomena.

Analysis and control of laser induced fragmentation processes in CpMn(CO)3

Abstract In this paper we present experimental and theoretical findings about the dynamics of ultrafast fragmentation processes which occur during resonant multiphoton ionization of CpMn(CO) 3 with femtosecond laser pulses. Employing a two-color pump and probe scheme, it was possible to retrieve lifetimes of the electronically excited parent molecule and its first fragment CpMn(CO) 2 . The observed time of 66 fs for the loss of the first CO-ligand is in good agreement with the results of one-dimensional quantum dynamical model simulations based on three relevant adiabatic ab initio potentials and the related components of the transition dipole matrix elements, in C s symmetry. Subsequently, smaller fragments appear somewhat later during approximately 100 fs. Based on these findings we performed feedback control experiments on the system in order to optimize individual fragmentation/ionization paths. With the routine a considerable increase of – for example – the CpMn(CO) + /CpMn(CO) + 3 intensity ratio was achieved. The obtained optimized laser pulses correlate well with the fast dynamics of the photoinduced preparation of CpMn(CO) + 3 versus CpMn(CO) + product ions, respectively.

500 kHz OPCPA delivering tunable sub-20 fs pulses with 15 W average power based on an all-ytterbium laser

An optical parametric chirped pulse amplifier fully based on Yb lasers at 500 kHz is described. Passive optical-synchronization is achieved between a fiber laser-pumped white-light and a 515 nm pump produced with a 200 W picosecond Yb:YAG InnoSlab amplifier. An output power up to 19.7 W with long-term stability of 0.3% is demonstrated for wavelength tunable pulses between 680 nm and 900 nm and spectral stability of 0.2%; 16.5 W can be achieved with a bandwidth supporting 5.4 fs pulses. We demonstrate compression of 30 microjoule pulses to sub-20 fs duration with a prism compressor, suitable for high harmonic generation.An optical parametric chirped pulse amplifier fully based on Yb lasers at 500 kHz is described. Passive optical-synchronization is achieved between a fiber laser-pumped white-light and a 515 nm pump produced with a 200 W picosecond Yb:YAG InnoSlab amplifier. An output power up to 19.7 W with long-term stability of 0.3% is demonstrated for wavelength tunable pulses between 680 nm and 900 nm and spectral stability of 0.2%; 16.5 W can be achieved with a bandwidth supporting 5.4 fs pulses. We demonstrate compression of 30 µJ pulses to sub-20 fs duration with a prism compressor, suitable for high harmonic generation.

Poor man’s source for sub 7 fs: a simple route to ultrashort laser pulses and their full characterization

A practicable and economic method for the generation and full characterization of laser pulses ranging down to sub 7 fs duration with energies spanning the full microJ domain is presented. The method utilizes a self-induced and self-guiding filamentation of titanium-sapphire based, amplified pulses in air for spectral broadening, a standard chirp mirror compression scheme and transient grating frequency resolved optical gating for determining the spectral phase over the full visible to near infrared range. In this manner, few-cycle laser pulses with a high quality in the spatial beam profile have been generated in an robust arrangement with a minimal amount of standard optical components for their full characterization. The optical scheme demonstrates an uncomplicated, versatile access to this regime of pulsed laser radiation accompanied by a comprehensive analysis.

Coupling of spin and vibrational degrees of freedom of adsorbates at metal surfaces probed by vibrational sum-frequency generation.

Vibrational spectroscopy using sum-frequency generation has been used to investigate the coupling between a ferromagnetic thin film and adsorbed molecules, here CO on Ni/Cu(100). The CO stretching vibration exhibits a strong magnetic contrast with a pronounced temperature dependence, underlining the high sensitivity of this adsorbate-specific spectroscopy method. Our results indicate that the strong temperature dependence is caused by dynamical changes in the surface chemical bond when the CO stretch vibration is coupled to thermally excited external vibrational modes.

Ultrafast dynamics of coherent optical phonons in α-quartz

Femtosecond laser excitation of α-quartz launches coherent optical phonons modulating the refractive index of the sample. The observed oscillations in the transmission and ellipticity of probe light decays due to phonon-phonon scattering. With decreasing temperature, the vibrations shift towards higher energies and are accompanied by a rise of the phonon lifetime caused by lattice stiffening and freezing of phonon modes, respectively.

Water droplet formation in a cloud chamber induced by ultra short laser pulses

Summary form only given. The interaction between ultra short pulses with peak powers exceeding the critical power of self-focusing in gases and the atmosphere have investigated by several groups today. Highly nonlinear effects like self-phase modulation, self-focusing or four-wave mixing together with ionization processes could lead to the formation of filaments and the generation of white light. Within the Teramobile project, we have observed propagation distances of these filament ensembles, which also called a channel, of 20 m or more. We have designed a diffusion cloud chamber to study the ionization process of femtosecond laser pulses. The laser light ionizes the supersaturated atmosphere along its path inside the chamber. Using different techniques the traces of droplets due to condensation processes could be observed. Furthermore, dependencies of several parameters for the droplet generation process like humidity, temperature, chirp setting and laser power could be changed.

500 kHz OPCPA-Based UV-XUV Light Source For Time-Resolved Photoemission Spectroscopy

A 15 W, sub-20 fs OPCPA based on an Yb laser is used to demonstrate 6.3 eV fourth harmonic-based UV photoelectron spectroscopy with sub-60 fs time resolution and XUV high harmonic generation at 500 kHz.