Investigation of the Non-equilibrium State of Strongly Correlated Materials by Complementary Ultrafast Spectroscopy Techniques

Photoinduced non-thermal phase transitions are new paradigms of exotic non-equilibrium physicsof strongly correlated materials. An ultrashort optical pulse can drive the system to a new orderthrough complex microscopic interactions that do not occur in the equilibrium state. Ultrafastspectroscopies are unique tools to reveal the underlying mechanisms of such transitions which leadto transient phases of matter. Yet, their individual specificities often do not provide an exhaustivepicture of the physical problem. One effective solution to enhance their performance is the integra-tion of different ultrafast techniques. This provides an opportunity to simultaneously probe physicalphenomena from different perspectives whilst maintaining the same experimental conditions. In thiscontext, we performed complementary experiments by combining time-resolved reflectivity and timeand angle-resolved photoemission spectroscopy. We demonstrated the advantage of this combinedapproach by investigating the complex charge density wave (CDW) phase in 1T-TiSe2. Specifically,we show the key role of lattice degrees of freedom to establish and stabilize the CDW in this material.