Revealing the Transformation Invariance of Full-Parameter Omnidirectional Invisibility Cloaks

Searching for an optimal solution among many nonunique answers provided by transformation optics is critical for many branches of research, such as the burgeoning research on invisibility cloaks. The past decades have witnessed rapid development of transformation optics, and different kinds of invisibility cloaks have been designed and implemented. However, the available cloaks realized thus far have been mostly demonstrated with reduced parameters, which greatly impact the predefined cloaking performance. Here, we report a general design strategy to realize full-parameter omnidirectional cloaks that can hide arbitrarily shaped objects in free space. Our approach combines a singular transformation with transformation-in-variant metamaterials. The cloaking device with extreme parameters is implemented using a metallic array structure. In the experiment, two cloak samples are designed and fabricated, one with nondiscrete cloaking regions and the other with separated hidden regions. Near-unit transmission of electromagnetic waves with arbitrary incident angles is experimentally demonstrated along with significantly suppressed scattering. Our work challenges the prevailing paradigms of invisibility cloaks and provides deep insight into how transformation optics could be harnessed to obtain easily-accessible metadevices.