By applying a scattering-wave theory, the electromagnetic response of an arbitrary array of multiple slits perforated on a metallic film and filled with different slit dielectric materials can be studied in an analytical way. Here, the wavelength-dependent splitting of a light beam into two by asymmetrically filled slits in a metal film using intraslit and interslit dual-wave interferences is fully explored. We consider a triple-slit structure perforated on a gold film, where the middle slit is used for the surface-plasmon (SP) excitation by a narrow Gaussian beam while the two side slits are used for the detection of a transmitted SP wave propagated from the middle opaque slit either at a particular wavelength or at double that wavelength, respectively. For this proposed simple structure, we show that only one of the two side observation slits can be in a passing state for a particular wavelength, but the other blocked slit will change to a passing state at double that wavelength with a specific design for the slit depth, slit dielectric, and interslit distance in the deep subwavelength regime. In this sense, SP mediated light transmission becomes wavelength sensitive in our model, and a single light beam can be separated into two according to its wavelength in the transverse direction parallel to the array. This provides us with a unique way for direct optical reading in the near-field region using a nonspectroscopic approach.
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