Undercut etching is a necessary technique for a variety of device applications, including microdisk lasers. We have explored bandgap-selective photoelectrochemical etching of nonpolar m-plane GaN for undercut etching applications. including microdisks. These nonpolar optical devices are not limited by the quantum-confined Stark effect that hampers the performance of polar c-plane GaN devices. We discuss the dependence Of undercut quality Oil etchant concentration, illumination intensity, masking material, and epitaxial structure and use this technique to fabricate in-plane microdisks. In these nonpolar microdisks, the in-plane polarization fields have a dramatic effect on the symmetry of the etching in both the undercut etching and in the unwanted etching of the GaN disk layer. With a careful balance of etchant concentration and illumination intensity and a well-designed epitaxial structure, we have achieved smoother optical cavities than were possible in c-plane GaN. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3184156] All rights reserved.