Spectral positions of plasmon resonances related to boundaries between a thin aluminum layer and dielectrics (air, glass, VDF-TrFE 65/35 ferroelectric copolymer, and indium tin oxide (ITO)) have been determined in the transmission spectra of aluminum gratings of three types with 30 × 30 μm dimensions and 350-, 400-, and 450-nm line periods. Experimental results agree well with spectral positions of plasmon resonances calculated for the normal incidence of TM-polarized light. In addition, maximum values of transmission coefficients in the region of λ ≈ 900-950 nm have been determined for glass-Al-copolymer and glass-ITO-Al-copolymer structures. These values are close to 100%, which shows that the effective optical aperture is two times greater than the geometric areas of slits. [ABSTRACT FROM AUTHOR]