Achromatic polarization splitting effect of metallic gratings with sub-50 nm wide slits

We report on a broadband polarization splitting phenomenon in an aluminum grating with its period much smaller than the wavelength of incident light. Under oblique incidence, grating is highly transparent for one polarization, but it is opaque and reflects the orthogonal polarization. By using the Fourier modal method and planar waveguide theory, we show that, comparing with the traditional metallic wire grid polarizers, the improved confinement of TM0 mode within the sub-50 nm wide grating slits is responsible for this polarization splitting effect.