Understanding charge density wave dynamics in 2D materials with femtosecond spectroscopy

2D materials based on transition metal dichalcogenides exhibit a unique combination of electrical and optical properties. In this talk I will review our experimental efforts in understanding collective excitations such as charge density waves in TaSe2 ad NbSe2. Our work is primarily focused on the star-like charge density wave with hexagram symmetry, which has gathered much interest for optoelectronic memory devices. Using femtosecond spectroscopy, I will explain the details of electron phonon coupling by describing the coherent phonon dynamics. This coherent dynamics can be also observed in time- and angle-resolved photoemission spectroscopy, providing a direct link between the structural and electronic degrees of freedom. Finally, I will illustrate our work on NbSe2 and how the structure of the material can be tuned by metal doping and its implications to phonon dynamics.