Nonlinear Optical Studies of Condensed Phases: From Equilibrium Structure to Non-Equilibrium Dynamics

The central questions in condensed matter physics concern the electronic structure and symmetries of systems at equilibrium, fluctuations of equilibrium systems, and relaxation of non-equilibrium states. I will describe our investigations of these questions in low-dimensional and interfacial systems via ultrafast and nonlinear optical techniques. Examples will include the change of electronic structure in atomically thin layered GaSe[1] and the electronic structure of biexcitons in graphene quantum dots[2]. More attention will be given to the dynamics of the hydrogen-bond network of water at an interface with a hydrophobic surface[3,4]. What makes water special is its strong, highly dynamics hydrogen-bond network, but this network is terminated at an interface. This leads to different properties of the interface compared to the bulk. We can probe these differences by using sum-frequency generation as a surface-sensitive probe after infrared excitation of the dangling OH stretch mode of interfacial water. The evolution of this dangling mode is dominated by reorientation to a hydrogen-bonded configuration, which we monitor by time- and polarization-resolved sum-frequency generation. I will conclude with a discussion of nonlinear optical approaches to studying exciton interactions in layered systems and novel orders in correlated and topological systems.