Abstract
We present a general experimental concept for jitter-free pump and probe experiments at free electron lasers. By generating pump and probe pulse from one and the same X-ray pulse using an optical split-and-delay unit, we obtain a temporal resolution that is limited only by the X-ray pulse lengths. In a two-color X-ray pump and X-ray probe experiment with sub 70 fs temporal resolution, we selectively probe the response of orbital and charge degree of freedom in the prototypical functional oxide magnetite after photoexcitation. We find electronic order to be quenched on a time scale of (30 ± 30) fs and hence most likely faster than what is to be expected for any lattice dynamics. Our experimental result hints to the formation of a short lived transient state with decoupled electronic and lattice degree of freedom in magnetite. The excitation and relaxation mechanism for X-ray pumping is discussed within a simple model leading to the conclusion that within the first 10 fs the original photoexcitation decays into low-energy electronic excitations comparable to what is achieved by optical pump pulse excitation. Our findings show on which time scales dynamical decoupling of degrees of freedom in functional oxides can be expected and how to probe this selectively with soft X-ray pulses. Results can be expected to provide crucial information for theories for ultrafast behavior of materials and help to develop concepts for novel switching devices.
Highlights
The energy scales of the coupling between spin, orbital, and structural degrees of freedom (DOF) in functional solids imply that correlated dynamics in such materials occurs on femtosecond time scales
We present a general experimental concept for jitter-free pump and probe experiments at free electron lasers
We find electronic order to be quenched on a time scale of (30 6 30) fs and hence most likely faster than what is to be expected for any lattice dynamics
Summary
The energy scales of the coupling between spin, orbital, and structural degrees of freedom (DOF) in functional solids imply that correlated dynamics in such materials occurs on femtosecond time scales. Even when the jitter is corrected for by elaborate parallel single-shot cross-correlation measurements,[11,12] the time resolution is often limited to around 100 fs due to the combination of pulse lengths limitations in the FEL and available optical laser system and residual uncorrected transport jitter between those light sources often placed hundreds of meters apart In this contribution, we report on an experimental scheme that we successfully used at the FEL FLASH (Free-Electron Laser in Hamburg) and that allows for an a priori jitter-free pump and probe X-ray experiment. In a first attempt to disentangle electronic and structural dynamics in magnetite, we used infrared laser pulses to drive the electronic sector out of the low-temperature phase and X-ray pulses from the free-electron-lasers FLASH24 and LCLS (Linac Coherent Light Source)[23] to probe the response In both cases, the temporal resolution was limited by jitter between externally synchronized optical pump lasers and probe (FEL) sources. These time scales define the temporal resolution needed to observe dynamical decoupling
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