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The optical laser system in the laser hutch
Pump-and-Probe Experiments
The extremely bright pulses of extreme-ultraviolet radiation and soft X-rays from FLASH can be as short as 10 femtoseconds, corresponding to only 3 μm of optical path length. These ultrashort pulses can be used to explore the temporal evolution of various processes such as atomic motion, phase transitions, expansion of hot plasmas, and chemical reactions.
To measure events happening in femtoseconds or picoseconds, researchers mainly utilize the pump-and-probe technique. Two short pulses are required: The pump pulse starts the reaction, while the probe pulse investigates the state of the system after a defined time delay. A sequence of probe pulses, at different time intervals after a reaction has been initiated by the pump pulse, allows recording a “film” of a molecular reaction.
Both the pump and the probe pulse could originate from the FEL itself by splitting the pulse in two and delaying one of them by sending it on a longer path. However, an alternative and much more flexible approach is to use an ultrashort pulse from an optical laser as the pump source. With an optical laser, it is much easier to change the wavelength, the polarization, and the angle of incidence, or to expand the pump delay from the femtosecond range up to several nanoseconds.
Currently an optical laser system is routinely provided for pump probe experiments at FLASH. A THz beamline, that is hoped to deliver naturally synchronized THz and IR pulses for pump probe experiments in the future, is currently under comissioning.
