We present our studies on nonequilibrium carrier dynamics in high temperature superconductors, using the optical pump-probe spectroscopy method. We have performed our time-resolved measurements of the optical reflectivity change ΔR/R in HgBa₂Ca₂Cu₃O_8-x (Hg-1223), Pb- and Gd-doped Bi₂Sr₂CaCu₂O_10-x (Bi-2212), and MgB₂ thin-film and single-crystal superconducting samples. The measured optical signals were strongly related to the presence of a superconducting energy gap Δ in the electronic energy spectrum. The superconducting transition at the critical temperature T_c was also clearly observed. In Hg-1223 thin films, the ΔR/R signal measured in the superconducting state was only related to the Cooper pair-breaking and pair-formation dynamics, with no evidence of the phonon bottleneck effect, characteristic for conventional BCS materials. The pair-breaking time in Hg-1223 was measured to be ~350 fs and was temperature independent up to T_c. The quasiparticle recombination time was in the 5-10 ps range with the 1\Δ divergence at temperatures close to T_c and the 1\T divergence at low temperatures. In Bi-2212 single crystals, the Cooper pair dynamics below T_c was very similar to that observed in Hg-1223 films. Above T_c, however, we observed the change in the sign and shape of the ΔR/R signal, which we related to the presence of the pseudo-gap state. Finally, in MgB₂ films and crystals, the temperature dependence of the amplitude of the superconducting component of the ΔR/R signal followed the Δ(T) anomalous temperature dependence, which was interpreted for our mixed clean\dirty MgB₂ films as the two Δ's, closing at two different values of T_c's. The same two T_c's were also visible in the double-divergence of the temperature dependence of the electron-phonon relaxation time. The quasiparticle recombination time was on the order of 400 ps and was limited by the phonon escape time.

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