Studying of high-temperature superconductivity and other correlated systems is one of the core areas of condensed matter physics. The formation of superconductivity is often accompanied with collective charge density wave, spin density wave, quantum spin liquid, Mott physics, etc. They arise from the complex interactions, long-range order, quantum fluctuations, etc. They form complex relations with superconductivity such as phase competition, evolution, coexistence, etc. There have been a lot of seminal research accomplishments in both theory and experiment. However, most of the studies are based on the ground state. Equally important is the study of excited states, which has been less investigated.
         Our group mainly uses ultrafast optical spectroscopy to study the ultrafast dynamics in correlated quantum materials. For example, ultrafast optical spectroscopy can be used to measure the electron-phonon coupling constant, which is important for understanding whether phonon is the superconductivity pairing glue and the role of phonons in high-temperature superconductivity. We can detect the decrease of superconductivity gap by the phonon bottleneck effect and obtain superconductivity gap, superconductivity transition temperature, etc.

         Complex quantum ground states are formed as a result of the coupling between lattice, charge, spin, and orbital. It is vital to understanding the coupling between multiple degrees of freedom. Compared to the equilibrium measurements, ultrafast non-equilibrium studies offer a unique perspective and advantage by decoupling the interactions and decay processes in different timescales.
         Our group utilizes multiple techniques to track the evolution of different degrees of freedom, such as ultrafast optical spectroscopy, ultrafast second harmonic generation, ultrafast electron and x-ray diffraction. The study of the coupling process provides insights for control of quantum states.

         There usually exists phase competition or coexistence in superconducting and other strongly correlated materials. Laser excitation modifies the potential energy surface and the minimum energy pathways, breaking the equilibrium of phase competition or coexistence. Therefore, the system might be driven to new states that do not exist in nature. In recent years, ultrafast laser control of quantum states have become one of the forefronts of condensed matter physics.
         Our group utilizes spatial self-phase modulation method and realized laser-induced electron/hole coherence in multiple materials. Team member Faran Zhou uses ultrafast laser pulse to tune the phase competition between to charge density wave states and drive the system to a hidden state that does not exist in equilibrium.