Broadband measurements of the refractive indices of bulk Gallium Nitride

New measurements of the index of refraction for free-standing samples of gallium nitride are reported. A simple dispersive prism technique is used to obtain the birefringent indices from 500 to 5100 nm, covering most of the transparency range of this wide band gap semiconductor. Millimeter thick samples prepared by both ammonothermal growth and hydride vapor phase epitaxy are found to have nearly identical refractive indices. The observed dispersion fits well to a two-pole Sellmeier equation with an estimated overall accuracy of + or - 0.002. Our results are found to be in good agreement with previous visible and near-IR measurements on thin-film GaN samples, however we observed significantly less dispersion in the mid-IR. Moderate heating of the samples also provided a new determination of dn/dT.