  Ultrafast time-resolved optical spectroscopy in high-temperature
superconductors enables the direct real-time measurement of non-equilibrium
quasiparticle recombination dynamics. In addition, it also gives detailed
information about the symmetry of the superconducting gap and the "pseudogap",
their doping dependence and their temperature dependence. Experimental data,
together with theoretical models on the photoinduced transmission amplitude and
relaxation time as a function of temperature and doping in YBa2Cu3O7-d gives a
consistent picture of the evolution of low-energy structure, where a
temperature-independent gap is shown to exist in the underdoped state which
evolves with doping into a two-component state near optimum doping, where the
dominant response is from a T-dependent BCS-like superconducting gap.
