Ultrafast recovery of charge density wave order controlled in a pump-pump-probe free electron laser experiment*

Understanding and controlling charge density wave (CDW) order in metals can lead to better command of the materials’ superconducting properties and give dynamical access to hidden electronic states. In previous experiments at a free electron laser (FEL) LCLS, we demonstrated our capability to enhance charge density wave in thin chromium films by photoexcitation and measure its amplitude with an x-ray probe pulse with a precision of tens of femtoseconds. We now use two optical pump pulses, with x-ray FEL pulse providing feedback, to fine-tune the state of the material. Precise control over the state and high temporal resolution open the way to measurements of out-of-equilibrium states of the materials. We discover that recovery time of the charge density wave order decreases with the amplitude of the excited "frozen" acoustic phonon. We observe the effect of incoherent addition between two photoexcitations.