1. Department of Engineering Science, University of Oxford, Oxford OX1 3PH, U.K.
2. Department of Materials, University of Oxford, Oxford OX1 3PJ, U.K.
3. Institute of Photonics, Department of Physics, University of Strathclyde, Glasgow G1 1RD, U.K.
4. Cambridge Centre for Gallium Nitride, University of Cambridge, Cambridge CB3 0FS, U.K.
5. Department of Physics, University of Warwick, Coventry CV4 7AL, U.K.