Minimum size for an illusion device

The illusion effect requires the establishment of the same scattering properties. Conventionally, the illusion effect is realized by a device that is larger than the target object, but the minimum size of the illusion device has not been discussed in the literature. Here, we first realize an illusion effect with a metasurface device that is smaller than the target object. This is achieved by obtaining the desired electromagnetic fields inside the target object and constructing a device with a surface-impedance metasurface. Then, more importantly, we discuss the minimum size of such illusion devices. Our results show that the minimum size is related to the size, material, and fine structures of the target objects. Furthermore, we discuss the realization of passive metasurfaces by introducing auxiliary evanescent TM modes. These results are verified with numerical simulations. Our work reveals the size limit of illusion devices, provides guidance for their miniaturization, and has the potential to generalize the application scenarios of illusion devices.