Waveguiding based upon geometric phase

Abstract

Control of light propagation is usually obtained by appropriately designing the optical path, that is, the refractive index of the material. Common examples are standard lenses and dielectric waveguides. Recently, a new way to tailor the wave front emerged, based on the Pancharatnam-Berry phase (PBP). The PBP is a geometric phase related to changes in the polarization state of light along its propagation [1]. Such geometric phase delay is proportional to the solid angle defined by the polarization state along its path on the Poincare sphere [1]. In an anisotropic material, a transverse gradient of PBP can be induced by a local rotation of the crystal axes. When the thickness of an anisotropic plate corresponds to a half-wave plate (phase difference of π between the ordinary and the extraordinary polarization), the PBP between two points across the beam profile equals twice the relative rotation of the optic axis. Thus, the beam wave-front is polarization-dependent and can be molded at will by a space-dependent rotation of the optic axis in the transverse plane, using very short propagation distances [1]. The term planar photonics has been coined to describe this revolutionary optical technology.