Dr. Paul Beaud

Paul Beaud

Senior Scientist
Bernina Group

Paul Scherrer Institute
Forschungsstrasse 111
5232 Villigen PSI


Paul Beaud is a Senior Scientist in the Bernina (former FEMTO) group at the PSI Photon Science Division. Paul Beaud received his diploma degree in Physics in 1983. For his PhD studies he then moved to the University of Bern where he first had to built his first femtosecond laser before investigating with that laser the nonlinear propagation of ultrashort light pulses in single mode optical fibers near zero group velocity dispersion. Paul got his PhD in 1988 and stayed in Bern for another three years before he moved to Florida joining the Laser Plasma Laboratory at CREOL with a SNSF advanced researcher fellowship in his pocket. In 1994 he was hired at the PSI ENE Division to establish in the Molecular Dynamics Group a femtosecond laser laboratory to study ground and excited state dynamics in combustion relevant molecules. From 2001 Paul also engaged in the FEMTO project at the Swiss Light Source to build a source of tunable femtosecond hard X-rays designed for ultrafast diffraction and absorption experiments. His responsibility during the initial design and construction phase was to ensure optimum and stable interaction of the laser pulses with the storage ring electron bunches copropagating in a periodic magnetic device. Paul switched departments in 2006 at the same time as FEMTO started to operate and for a decade produced  significant scientific output. This success prompted the decision to dedicate one of the first scientific instruments at SwissFEL to studies of non-equilibrium dynamics in correlated electron systems. In 2017 the SwissFEL Bernina instrument started with first pilot experiments and the slicing source was discontinued. Since 2010 Paul is also Principal Investigator in the MUST network – a National Center of Competence in Research funded by SNSF.

Institutional Responsibilities

The Bernina team is responsible to operate the Bernina instrument at the SwissFEL that is designed for studying ultrafast cooperative phenomena in condensed matter science. Paul supports experiments performed at Bernina and is responsible for the former FEMTO laser laboratory offering PhD students and Postdocs the opportunity to perform optical pump-probe experiments in preparation of potential time resolved x-ray studies.

Scientific Research

Throughout my career I was attracted in understanding the dynamics of complex nonlinear systems. This started during my diploma thesis looking at the development and motion of large hailstorms and continued during my PhD with the dynamics of laser mode locking or nonlinear processes leading to soliton formation and continuum generation in optical fibers. In that period I also plunged into the femtosecond time scale which became a second constant in my professional life including the acquaintance with complex laser systems to generate ultrashort pulses of light. Early at PSI my personal research interests focused on collision-induced inter- and intramolecular energy transfer of combustion relevant molecules experimentally investigated with picosecond LIF and femtosecond CARS. Moving to hard x-rays the focus of my research moved from molecular to condensed matter physics primarily employing femtosecond (non-resonant and resonant) x-ray diffraction to disentangle the photo-excited non-equilibrium dynamics of the coupled atomic, electronic and magnetic structures in solid materials with an emphasis on photo-induced phase transitions. A primary goal is to advance our understanding of functional materials - another to find ways to control their properties on ultrafast time scales.

Selected Publications

For an extensive overview of recent PSI publications we kindly refer you to our publication repository DORA. You may find a more comprehensive list of my publications on ResearcherID or Google Scholar.

Towards X-ray transient grating spectroscopy, C. Svetina, R. Mankowsky, G. Knopp, F. Koch, G. Seniutinas, B. Rösner, A. Kubec, M. Lebugle, I. Mochi, M. Beck, C. Cirelli, J. Krempasky, C. Pradervand, J. Rouxel, G. Mancini, S. Zerdane, B. Pedrini, V. Esposito, G. Ingold, U. Wagner, U. Flechsig, R. Follath, M. Chergui, C. Milne, H. Lemke, C. David, P. Beaud, Optics Letters 44, 574 (2019).

The extension of transient grating spectroscopy to the x-ray regime will create numerous opportunities, ranging from the study of thermal transport in the ballistic regime to charge, spin, and energy transfer processes with atomic spatial and femtosecond temporal resolution. Studies involving complicated split-and-delay lines have not yet been successful in achieving this goal. Here we propose and validate a novel, simple method to extend transient grating spectroscopy to the x-ray regime.

The photoinduced transition in magnetoresistive manganites: a comprehensive view, V. Esposito,  L. Rettig, E. Abreu, E. Bothschafter,G. Ingold, M. Kawasaki, M. Kubli, G. Lantz, M. Nakamura, J. Rittman, M. Savoini, Y. Tokura, U. Staub, S. L. Johnson and P. Beaud, Physical Review B 97, 014312 (2018).

The dynamics of the structural and charge order response are qualitatively different when excited above and below a critical fluence fc. For excitations below fc the charge order and atomic superlattice is only partially suppressed and the ground state recovers within a few tens of nanosecond. Above fc the superlattice vanishes promptly followed by a change of the unit cell parameters on a 10 picoseconds timescale. At this point all memory from the symmetry breaking is lost and the recovery time increases by many order of magnitudes due to the first order character of the structural transition.

Nonlinear electron-phonon coupling in doped manganites, V. Esposito, R. Mankowsky, M. Fechner, H. Lemke, M. Chollet, J. M. Glownia, M. Nakamura, M. Kawasaki, Y. Tokura, U. Staub, P. Beaud, and M. Först, Physical Review Letters 118, 247601 (2017).

Resonantly exciting a high-frequency infrared-active lattice mode in Pr0.5Ca0.5MnO3 we find that the charge order reduces promptly and highly nonlinearly as function of excitation fluence. Density-functional theory calculations suggest that direct anharmonic coupling between the excited lattice mode and the electronic structure drives these dynamics.

Ultrafast formation of a charge density wave state in 1T-TaS2: observation at nanometer scales using time-resolved X-ray diffraction, C. Laulhé, T. Huber, G. Lantz, A. Ferrer, S. O. Mariager, S. Grübel, J. Rittmann, J. A. Johnson, V. Esposito, A. Lübcke, L. Huber, M. Kubli, M. Savoini, V. L. R. Jacques, L. Cario, B. Corraze, E. Janod, G. Ingold, P. Beaud, S. L. Johnson, and S. Ravy, Physical Review Letters 118, 247401 (2017).

A detailed femtosecond x-ray diffraction study of the photoinduced phase transition between the nearly commensurate (NC) and the incommensurate (I) charge density wave (CDW) states in 1T-TaS2. Structural modulations associated with the NC-CDW state disappear within 400 fs and the photo-induced I-CDW phase develops through a nucleation and growth process whithin ~100 ps. The newly formed I-CDW phase is fragmented into nanometric domains that grow following the universal Lifshitz-Allen-Cahn law, which describes the ordering kinetics in systems exhibiting a non-conservative order parameter.

A time-dependent order parameter for ultrafast photoinduced phase transitions, P. Beaud, A. Caviezel, S.O. Mariager, L. Rettig, G. Ingold, C. Dornes, S.-W. Huang, J.A. John­son, M. Radovic, T. Huber, T. Kubacka, A. Ferrer, H.T. Lemke, M. Chollet, D. Zhu, J. Glownia, M. Sikorski, A. Robert, H. Wadati, M. Nakamura, M. Kawasaki, Y. Tokura, S.L. Johnson, and U. Staub, Nature Materials 13, 923-927 (2014).

Time-resolved resonant X-ray diffraction is applied to disentangle the evolving electronic and atomic structure of the ultrafast melting of the charge and orbitally ordered phase in a perovskite manganite. Although the actual change in crystal symmetry associated with this transition occurs over different timescales characteristic of the many electronic and vibrational coordinates of the system, the dynamics of the phase transformation can be well described using a single time-dependent ‘order parameter’ that depends exclusively on the electronic excitation.