Complex rhombus lattice photonic crystals for broadband all-angle self-collimation

In this paper, we discuss two-dimensional photonic crystals of complex rhombus lattice structures to enable broadband, 'all-angle' light collimation. The proposed structures are made of complex elliptical dielectric rods (epsilon = 12) embedded in an air background. Based on the equi-frequency contours (EFCs) calculated by the plane wave expansion method (PWEM), these structures show a wide range of open, nearly flat EFCs, with the maximum angular deviation from the primary collimating direction less than 2 degrees. The broadest relative range of these open, nearly flat EFCs found is 17% of a chosen central wavelength. For a sample element centered at 1550 nm, this range covers wavelengths from 1429 to 1695 nm; the devices can be scaled to cover alternative wavelength ranges. These structures could benefit applications that require a broad operating wavelength range. Potential applications include collimating beam combiners, multiplexers and solar light collectors.