II-IV- N2 wide band gap semiconductors such as ZnSi N2, ZnGe N2, and ZnSiGe N2 have potential uses for nonlinear materials and as lattice matching compounds for the growth of SiC and GaN devices. In this study, the dispersion of the TE and TM indices of refraction has been measured systematically using the prism coupling technique for an orthorhombic ZnSi N2 epitaxial layer grown on r -plane sapphire. The resulting index dispersion is extracted from the measured optical modes using a layered biaxial waveguide analysis, which shows that although the ZnSi N2 crystal is orthorhombic, for practical purposes it can be treated as a uniaxial material. © 2005 American Institute of Physics.