Abstract: Materials typically undergo phase changes as a function of external parameters such as temperature, pressure or magnetic field. Light can also be used to both switch between equilibrium phases and to create new photo-induced states that may have no equilibrium counterparts. Even though there are fascinating examples of photoinduced phase transitions, the detailed microscopic mechanisms and overarching principles that govern these are still not known.
In this talk, I will describe how we used ultrashort laser pulses to capture light induced melting and recovery of a charge density wave phase with femtosecond time resolution. During this process, a new state that does not exist in equilibrium is also transiently created. In a different material, I will show how circularly polarized light can induce and detect a chiral phase of electrons. Understanding light induced phase transitions could pave the way for optical engineering of new quantum states of matter.