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A rigorous approach is proposed to excite plane waves in time/envelope domain finite-element methods with perfectly matched layer (PML) termination. The essential idea is to apply the equivalence principle on an inner boundary. The radiation of the equivalent electrical and magnetic current sources forms plane wave incidence inside the boundary and zero field outside the boundary. It provides a concise, computationally efficient, and physically clear way to implement plane wave incidence for electromagnetic scattering problems.