Photo-induced Phase Transitions in Charge Density Waves

The mechanism underlying photoinduced phase transitions has long been researched, but many details still remain to be clarified. Towards this end, we studied light induced phase transitions in two different charge density wave (CDW) systems. First, we investigated the photo-induced melting of a unidirectional CDW in LaTe3. Using a suite of time-resolved probes, we independently track the amplitude and phase dynamics of the CDW. We find that a fast (approximately 1 picosecond) recovery of the CDW amplitude is followed by a slower re-establishment of phase coherence dictated by the presence of topological defects in CDW. Furthermore, after the suppression of the original CDW by photoexcitation, a different, competing CDW along the perpendicular direction emerges. The timescales characterizing the relaxation of this new transient CDW and the reestablishment of the original CDW are nearly identical, which points towards a strong competition between the two orders. Secondly, I will also report the realization of optical chiral induction and the observation of a gyrotropically ordered CDW phase in 1T-TiSe2. These results may provide a framework for understanding other photoinduced phase transitions.