Generation of cross-polarized light and the effect of magnetic moments on plasmonic metasurfaces

There have been many reports on the generation of cross-polarized light in the infrared-to-radio-wave regions, but few reports on the generation of cross-polarized light in the visible region because this would require fabricating metasurfaces that consist of structures smaller than the wavelength in the visible region. In this study, we aimed to realize a circular polarization conversion element using a reflective plasmonic metasurface that generates cross-polarized light across a wide band of short visible wavelengths. To achieve this, we designed an aluminum (Al)-based reflective plasmonic metasurface and investigated a cross-polarization conversion mechanism by analyzing the role of the magnetic moment in the electromagnetic field distribution. The metasurface, which is composed of Al rectangular structures, demonstrated a reflectance of 62% for circularly cross-polarized light within the range of 450–600 nm on the visible spectrum, and co-polarized light was notably absent in this wavelength region. Our findings highlight the pivotal role of the magnetic moment in the generation of cross-polarized light. Specifically, a magnetic moment is induced on the surface of the Al rectangular structures, where the electric field is concentrated at the ends of their longer sides. This concentration extends the distance over which the current flowing through the rectangle is propagated, thereby enhancing the magnetic moment generated along the shorter sides. As a result, a significant phase delay is introduced, which enables an efficient cross-polarization conversion. In conclusion, we clarified the method and mechanism for generating cross-polarized light over a wide band in the short visible wavelengths of light.