By monitoring the quasiparticle relaxation rates following a pulse of light, we have established a new window into the intrinsic rates of recombination that occur as Cooper pairs become re-established. We have demonstrated the ability to directly perturb and monitor the superconducting gap in cuprates, using near-infrared pump pulses to suppress superconductivity in the low-fluence regime and to fully induce a metastable pseudogap state in the moderate- to high-fluence regime. These results serve both as a vivid demonstration of an ultrafast phase transition involving a topological change in the nature of the Fermi surface of a material, and as a novel probe into the factors that influence the superconducting order parameter. These new results prelude planned trARPES studies of charge-density wave dynamics.  Ultrafast time-resolved ARPES studies of high Tc superconductor reveal a softening of the electron-boson interaction in concomitance with a softening/closure of the superconducting gap. The effect is negligible in the normal state and for a metallic compound, suggesting that the electron-boson interaction might be a booster for pairing in cuprates.

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