Optical characteristics and mode analysis of aluminum nanohole arrays fabricated by character projection electron-beam lithography

In this study, aluminum nanohole arrays with well-defined periods from 460 to 520 nm were fabricated by character projection electron beam lithography, and their optical transmission and reflection properties were systematically examined. The measured spectra show two sharp transmission dips in the visible region together with a broad transmission band at longer wavelengths. All spectral features shift to longer wavelengths with increasing array period, demonstrating that the optical response is primarily controlled by the geometrical periodicity. Finite-difference time-domain simulations reproduce the essential spectral characteristics observed experimentally. Analysis of the calculated electric field distributions reveals that the sharp transmission dips originate from LSPR-like modes localized at the nanohole rims, with distinct resonance energies associated with the top and bottom metal interfaces. In contrast, the long wavelength transmission band arises from coupled SPP-like modes extending across both sides of the aluminum film, enabling enhanced optical transmission through the nanoholes. These results establish a clear correspondence between optical spectra and hybridized plasmonic mode formation in aluminum nanohole arrays, providing general insight into wavelength selective transmission in periodic thin metal structures.