Exciting electrons in solids with intense light pulses offers the possibility of generating new states of matter through nonthermal means and controlling their macroscopic properties on femto- to picosecond time scales. One way to manipulate a solid is by altering its lattice structure, which often underlies the electronic, magnetic, and other phases. In this talk, I will discuss how structures of quantum materials are affected by photoexcitation and how their ultrafast dynamics are captured with time-resolved electron diffraction. I will focus on two recurring themes in the study of nonequilibrium phase diagrams: (i) phase competition, and (ii) electronic correlations. More specifically, an ultrashort light pulse can (i) perturb the subtle energy balance between proximal ground states, and (ii) modify the Coulomb interaction by exciting free carriers. These points will be discussed using examples from charge-density-wave compounds, which serve as a model system for other correlated materials to illustrate various light-matter interaction pathways.

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