This paper introduces a new boundary element formulation for transient electromagnetic scattering by homogeneous dielectric objects based on the time-domain PMCHWT equation. To address dense-mesh breakdown, a multiplicative Calderon preconditioner utilizing a modified static electric field integral operator is employed. Large-timestep breakdown and late-time instability are simultaneously resolved by rescaling the Helmholtz components leveraging the quasi-Helmholtz projectors and using temporal differentiation and integration as rescaling operators. This rescaling also balances the loop and star components at large timesteps, improving solution accuracy. The resulting discrete system is solved using a marching-on-in-time scheme and iterative solvers. Numerical experiments for simply- and multiply-connected dielectric scatterers, including highly non-smooth geometries, corroborate the accuracy, stability, and efficiency of the proposed approach.

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