Phase-preserved macroscopic visible-light carpet cloaking beyond two dimensions

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA.Transformation optics, a recent geometrical design strategy of light manipulation with both ray trajectories and optical phase controlled simultaneously, promises an invisibility cloaking device that can render a macroscopic object invisible even to a scientific instrument measuring optical phase. Recent "carpet" cloaks have extended their cloaking capability to broadband frequency ranges and macroscopic scales, but they only demonstrated the recovery of ray trajectories after passing through the cloaks, while whether the optical phase would reveal their existence still remains unverified. In this paper, a phase-preserved macroscopic visible-light carpet cloak is demonstrated in a geometrical construction beyond two dimensions. As an extension of previous two-dimensional (2D) macroscopic carpet cloaks, this almost-three-dimensional carpet cloak exhibits three-dimensional (3D) invisibility for illumination near its center (i.e. with a limited field of view), and its ideal wide-angle invisibility performance is preserved in multiple 2D planes intersecting in the 3D space. Optical path length is measured with a broadband pulsed-laser interferometer, which provides unique experimental evidence on the geometrical nature of transformation optics. Optical phase cloaking is demonstrated with an almost-three-dimensional carpet cloak designed from transformation optics. Both ray trajectories and optical path lengths have been measured at the macroscopic scale. The optical path lengths are determined with a broadband pulsed laser interferometer. This study provides, from the viewpoint of optical phase, unique experimental evidence on the geometrical nature of macroscopic cloaking.