Research And Applications Of Cloaks Based On Metamaterials

Metamaterials have experienced very fast development due to its extraordinary electromagnetic properties and enormous potential applications in the last few decades.Metamaterials are composed of periodic or gradient subwavelength structures.By properly choosing the structure parameters and materials we can obtain some extraordinary electromagnetic properties which don’t exist in natural materials.These unusual properties lead to many fantastic phenomena,such as negative refraction,sub-wavelength focusing and so on.Among those metamaterials-based research,invisibility cloak is still one of the major and promising outcomes.In this thesis,we will focus on the cloak theory and applications using metamaterials,including cloak for magnetic field and radar detection.In the first part,we propose three kinds of bilayer structures to realize cloak effect for magnetic field.Due to the complex material properties demanded by the traditional cloak based on transformation optics,the methods similar to scattering-cancellation technique employed in the thesis are more operational.Firstly,we demonstrated a perfect three-dimensional broadband magnetic cloak under the quasi-static approximation,and the working frequency band is from zero to 250 kHz.We also experimentally verified the practical cloak performance of the device by using a common commercial handhold metal probe.Furthermore,to overcome the requirement of the low-temperature environment due to employment of superconductors as diamagnetic materials.We used common metals(here we used copper)to replace superconductor and demonstrated a three-dimensional magnetic cloak at room-temperature.The device is also made up of a bilayer structure(magnetic material/metal).Experimentally,a good cloaking effect is achieved in a wide frequency band ranging from 5 to 250 kHz.The maximum field disturbance ratio is less than 0.5%.We also examined the work performance of the device through a commercial metal detector.Finally,to realize the cloak effect for static magnetic field at room temperature,we used the bilayer structure composed of an active current boundary and a magnetic inner shell with a high permeability.In the design,the magnetic shell shields and attracts the external magnetic field,and the active current boundary with specific current distribution can perform as a diamagnetic material like superconductor material outside.These two parts can balance out the disturbances to the external magnetic field caused by each other.And experimentally,we verified the good cloaking and shielding effect of the active bilayer design.Moreover,we theoretically showed that the device can overcome the permeability limitation and work for low frequency magnetic field if the current distribution of the active current boundary is optimized accordingly.In the second part,we employed an omnidirectional broadband absorber to realize the cloak effect for radar detection regime.The absorber proposed here is composed of multiple metamaterial resonators,and the effective dielectric constant profile satisfies Kramers-Kronig(KK)relationship spatially.The spatial dispersion of KK relationship and the time dispersion caused by metamaterials together contribute to the omnidirectional and broadband absorption property.What’s more,the absorption properties almost remain unaffected when the device is compressed or stretched uniformly at certain level(when the effective medium theory is satisfied).We proved that the sample remains good absorption property from 8 GHz to 12 GHz at three different thicknesses by numerical and experimental verifications.We numerically examined the cloak performance for arbitrary objects covered by the KK absorber finally.In the end,we summarize the whole work in this thesis,and put forward some prospects for future works in the relevant research.