Nonachromaticity and reversals of topological phase as a function of wavelength

Abstract

In an earlier work (R. Bhandari, Phys. Lett. A 204 (1995) 188), it was shown that, contrary to the property of achromaticity (independence of wavelength) usually associated with topological phases, topological phases encountered in polarization optics can, under certain conditions, show sharp changes and reversals as a function of wavelength, a phenomenon originating in the occurrence of phase singularities, earlier observed in interference experiments. It was shown that a proposed flat lens capable of focussing light with pure geometric phase shifts can switch from being convex to a concave lens or vice versa under these conditions. In this paper conditions under which such reversals can take place for small changes in wavelength are described. Some general implications of phase singularities in the physics of interference are pointed out.