Transmission properties of light waves through a horn-opened subwavelength-size single slit in finite-width homogeneous or inhomogeneous metallic films are studied by using the boundary integral method. We calculate transmission spectra in the visible wavelength regime, display the optical field intensity distributions, and demonstrate the directions of the energy flow. The results show that the transmission spectrum crucially depends on the physical and geometrical parameters of samples, for instance, the split angle in the horn-opened region of the slit, the width and depth of the slit, and the metallic materials surrounding the slit, etc. Two peaks in the transmission spectrum are observed for the nanoslit with a horn opening in the homogeneous metallic film, which originate from the metallic film and air slit, respectively. However, there are three peaks for the nanoslit in the hybrid metallic films consisting of two metallic films on either side of the slit, stitched together. Two peaks correspond to two different metallic materials, and the other one corresponds to the air slit, for the slit surrounded by two metallic materials. An equivalent structure is proposed to give an approximate description of the transmitted behavior.
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