  Quasinormal frequencies of electromagnetic and gravitational perturbations in
asymptotically AdS spacetime can be identified with poles of the corresponding
real-time Green's functions in a holographically dual finite temperature field
theory. The quasinormal modes are defined for gauge-invariant quantities which
obey incoming-wave boundary condition at the horizon and Dirichlet condition at
the boundary. As an application, we explicitly find poles of retarded
correlation functions of R-symmetry currents and the energy-momentum tensor in
strongly coupled finite temperature N=4 supersymmetric SU(Nc) Yang-Mills theory
in the limit of large Nc.
