Propagation of monochromatic light is analyzed by numerical calculations for an optical cavity enclosing a thin absorbent film. The computational study is performed by the CIP (constrained interpolation profile) method, which is shown to be able to solve temporal evolution of Maxwell's equations even in a strongly absorbing medium. Simulations on build-up processes reveal that such a cavity exhibits transmittance even higher than the intrinsic transmissivity of the absorbent, i.e., the absorbent is virtually transparent, when the following requirements are satisfied: the film is much thinner than the wavelength, it is located at a node of a standing wave, and the cavity consists of a pair of mirrors with optimal reflectivity depending on the film thickness. Optimal conditions are discussed as well for maximizing absorption in the film.
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