In this paper, a critical point model with three Lorentzian terms for interband transition was proposed for dielectric permittivity of metal films. After validated, it was incorporated into a two-temperature model (TTM) to study transient optical and thermal response for a copper film irradiated by an ultrashort laser pulse. The dynamic changes of reflectivity (R) and absorptivity coefficient (α) during laser irradiation, electron and lattice temperature, and phase change were investigated. It was shown that for an ultrashort laser pulse with relatively high laser fluence, both R and α could drastically decrease, leading to significantly different thermal response than that described by using constant R and α at room temperature (RT).

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