Transmissive invisibility cloak using phase gradient metasurfaces

Abstract

Abstract Different from the reflective approach of the carpet cloak designed to conceal surface irregularities on highly reflective surfaces, the transmission invisibility cloak, often capable of achieving perfect invisibility, operates within a transmission geometry. In this configuration, the cloaking mechanism ensures that an object neither reflects nor refracts incoming waves in free space, presenting opportunities for more versatile applications, though with the requirement for intricate designs. This paper introduces a novel methodology for designing a transmissive invisibility cloak, employing a simplified combination structure of two phase-gradient metasurfaces based on the generalized Snell’s law. Initially, we designed a highly transparent metasurface for the millimeter-wave band to yield diverse phase gradients. We confirmed the effectiveness of this metasurface through the observation of abnormal refraction. Then, through a deliberate arrangement of these phase gradients, we construct a transmissive invisibility cloak that guides electromagnetic waves around the cloaked region. Simulations and experimental measurements conducted under plane-wave conditions demonstrate the cloak’s effectiveness and practical applicability. The ensuing comparative investigations of results among free space, uncloaked objects, and cloaked objects validate the expected cloaking effect, offering valuable insights into the design and functionality of transmissive invisibility cloaks.