Band structure and optical absorption of GaN nanowires grown along the c axis

We study the optical properties of wurtzite GaN nanowires in which the geometrical axis coincides with the c axis of the crystal. The band mixing is typically very strong for the valence subbands and this results in rather complicated behaviors of the dispersion curves. However, in a very thin nanowire, the top valence subband is found to contain mostly z components near the zone center; this is in contrast to bulk GaN where the top state has x and y components only. A particularly interesting feature of bandmixing is that the nearest-to-band-edge valence state can be either optically dark or bright, depending on the radius of the nanowire. We show that near the band edge, the optical absorption is greater when the optical field is polarized along the wire and this polarization anisotropy becomes stronger as the radius decreases; above the band edge, the transverse component of absorption may dominate. This absorption dependence is again in contrast to that in bulk GaN where the band edge absorption is much greater for the optical polarization perpendicular to the c axis. The absorption properties are also directly relevant for the polarization of luminescence and laser emission in GaN nanowires.