Experimental setups for FEL-based four-wave mixing experiments at FERMI

The recent advent of free‐electron laser (FEL) sources is driving the scientific community to extend table‐top laser research to shorter wavelengths adding elemental selectivity and chemical state specificity. Both a compact setup (mini‐TIMER) and a separate instrument (EIS‐TIMER) dedicated to four‐wave‐mixing (FWM) experiments has been designed and constructed, to be operated as a branch of the Elastic and Inelastic Scattering beamline: EIS. The FWM experiments that are planned at EIS‐TIMER are based on the transient grating approach, where two crossed FEL pulses create a controlled modulation of the sample excitations while a third time‐delayed pulse is used to monitor the dynamics of the excited state. This manuscript describes such experimental facilities, showing the preliminary results of the commissioning of the EIS‐TIMER beamline, and discusses original experimental strategies being developed to study the dynamics of matter at the fs–nm time–length scales. In the near future such experimental tools will allow more sophisticated FEL‐based FWM applications, that also include the use of multiple and multi‐color FEL pulses.