Abstract: Spontaneous symmetry breaking results in the formation of broken-symmetry states such as superconductivity, ferromagnetism, and charge-density wave (CDW). Such broken-symmetry states are usually the energetically preferred ground state of matter at low temperatures. With a femtosecond laser pulse excitation, a natural result is the excitation of electrons and subsequent lattice heating, which would suppress ordered broken-symmetry states. For example, it’s common to observe the ultrafast melting of a ferromagnetic order or CDW, but very rare to see the opposite: light-induced new symmetry broken states such as superconductivity.

In this talk, I will present a case where we observed light-induced CDW in CeTe₃. Such a new state is a hidden ground state that is not allowed thermodynamically, but is driven to form after an interaction quench by an intense infrared laser pulse. With ultrafast electron diffraction, we capture the entire course of the transformation from stripe CDW into a bidirectional checkerboard CDW state. This experiment demonstrates a new dimension of phase exploration in the nonequilibrium regime. I will also discuss the photo-doping and photo-thermal effect based on experiments on another CDW material, 1T-TaS₂.