Experimental observation of multi-functional plasma-metamaterial composite for manipulation of electromagnetic-wave propagation

Although microwaves are widely used in our daily life, their propagations are left free with disordered absorption and scattering in most scenes, without any sophisticated controls of their paths. Microwave metamaterials have increased our regulation options in which cloaking is extensively studied, but their exploratory aspects so far are insufficient for multiple requests for wireless-communication availability in our daily life space. Here, we show our plasma-metamaterial composite device that becomes an absorber, a scatterer, or a flattener (including cloaking effects) by changing a narrow-range microwave frequency (2.6–3.1 GHz, in the middle of the S band). A radio frequency high-power input ranging to 400 W is used to generate plasma in vacant space in a metamaterial array, and this metamaterial creates a space with an abnormal permeability ( μ) in which 0 < μ < 1, while the plasma works as a dynamic negative- or positive-permittivity ( e) medium with a spatial gradient. In this e– μ parameter region, a state with negative e is a significant source for absorbance, and scattering is dominant with a small positive value of e. Among them, propagation flattening of microwaves mainly due to the cloaking condition takes place with the adequate set of μ and e and suitable spatial gradient of refractive index. This microwave manipulation is experimentally verified by monitoring scattering energy of propagating microwaves.