Femtosecondlasers for ultrafast X-ray diffraction and ultrafast spectroscopy (part I)

• Subject Area: Condensed Matter Physics
• Term: Funded in 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 556781414

Two reliable femtosecond laser systems with a high repetition rate and high average power are required, on the one hand, to operate a unique setup for X-ray diffraction with atomic length and time resolution (Ultrafast X-ray Diffraction - UXRD) and, on the other hand, to operate various methods of ultrafast spectroscopy under identical excitation conditions. This enables quantitative experiments on the coupling of the electron system of hybrid nanosystems with phonons, molecular vibrations and spin excitations. Overall, the new laser infrastructure is intended to maintain the existing portfolio of UXRD, transient absorption from the far infrared to the ultraviolet spectral range, transient magneto-optical Kerr effect measurements (tr-MOKE) and time-resolved Brillouin scattering. Moreover it should enable further development towards Raman scattering and highest time resolution. The various experiments were developed over many years on two laser systems that are approaching the natural end of their service life after more than 15 years of intensive use. The two laser systems applied for here are intended to further develop the essential parameters (pulse energy, pulse duration, stability) using modern and reliable laser technology in such a way that the time resolution and the signal-to-noise ratio are significantly improved for a series of novel experiments. In particular, in all pump-probe experiments (including UXRD), excitation pulses from the far infrared through the mid-infrared to the visible and ultraviolet spectral range should selectively carry out certain excitations: vibrations, magnons, phonons and plasmons. The UXRD femtosecond laser system will be integrated into the existing highly complex combined UXRD / trMOKE laboratory. It must provide sub 40 fs pulses at a central wavelength of approximately 800 nm and a repetition rate of 1 kHz with a total average power of >14 W. The power must be >9 and >5 W in two outputs simultaneously and synchronized. Pump pulses >5 mJ at 800 nm are required to generate tunable infrared and visible pump pulses with high intensity through nonlinear frequency conversion. The second femtosecond laser system will significantly further develop the diverse existing setups in the ultrafast spectroscopy laboratory. Because of comparability with the UXRD measurements, pulse parameters should be nearly identical: Sub 40 fs, 800 nm, >5 kHz, >6 W with at least 1 mJ per pulse. Here, the 5 times higher repetition rate is preferred because of the improved signal-to-noise ratio, while sufficient energy is still available for frequency conversion of pump pulses. The system is ideal for generating sub-10 fs pulses in optical hollow fibers in the future, which are particularly relevant for Raman scattering at high-frequency oscillations or for the short lifetime of plasmons.

• DFG Programme: Major Research Instrumentation
• Major Instrumentation: Femtosekundenlaser für die ultraschnelle Röntgenbeugung und Ultrakurzzeitspektroskopie (Teil I)
• Instrumentation Group: 5700 Festkörper-Laser
• Applicant Institution: Universität Potsdam
• Leader: Professor Dr. Matias Bargheer