Author Affiliations

A. J. Vickers,^a P. Aiyarak,^a A. B. Walker,^b J. R. Watling,^b and J. S. Roberts^c

^aPhysics Department, University of Essex, Wivenhoe Park, Colchester, CO4 3SQ, UK; Telephone : +44-1206-872876; Fax : +44-1206-873598 (Department of Physics);

^bSchool of Physics, University of East Anglia, Norwich, NR4 7TJ, UK

^cDepartment of Electronic & Electrical Engineering, University of Sheffield, Sheffield, S1 3JD, UK