Design,fabrication And Characterization Of Broadband Terahertz Invisibility Cloak

It is well known that an object may reflect or scatter(a fraction of)light that is incident upon it,and the reflected or scattered light can be detected by eyes or other sensors.Developing cloaking techniques to render an object completely invisible has attracted a great deal of interest over the past few decades.Several methods have been explored for realizing invisibility cloaking,such as transformation-based cloaks,carpet cloaks,plasmonic cloaks,mantle cloaks,transmission-line cloaks,complementary media cloaks.Most of these cloaking schemes work only for a single specific frequency or within a very narrow bandwidth.Carpet cloaks by using of non-resonant material elements can realize broad-bandwidth cloaking with low loss,polarization-insensitive cloaks made of glass or polymer can hide large objects in incoherent light at visible-light region.However,these broadband cloaks require large volume of such materials.Therefore,invisibility cloaks made of small volumes of materials,which can cover objects to make them undetectable to electromagnetic radiation over a wide range of frequencies,are highly desirable for various practical applications.In this paper,using the broadband transmission of electromagnetic waves in the subwavelength oblique metallic gratings,we propose a novel approach to realize broadband invisibility cloak in terahertz waves,and the main contents are as follows:Firstly,we utilize the broadband transmission property of subwavelength oblique metallic gratings to construct broadband terahertz invisibility cloaks.We numerically simulate transmission spectra,dispersion,and electric field of dielectric and metallic gratings by finite-difference time-domain method.This study shows that when electromagnetic waves illuminate on the surface of the metallic gratings,it will excite the spoof surface plasmons(SSPs),which transmit through the slit to reach the bottom of the gratings,oscillate and emit wavelets,resulting in high-broadband transmission.Since the SSPs are only excited on the surface of the metal film,the transmission is independent of the material beneath the film.Therefore,it can be used in constructing broadband invisibility cloak.Secondly,we extend the broadband cloaks from periodic structure to disordered structures,We first theoretically derivate the structure of the discorded gratings based on Maxwell equations,and then we numerically simulate the transmission spectra using finite-difference time-domain method.We find that disordered grating achieve broadband transmission as well.Disordered structures can both suppress Wood's anomaly and Fabry-Perot resonance,further broadened the working bandwidths of invisibility cloak.Thirdly,we experimentally demonstrate that the disordered structure can be a broadband invisibility cloak.We fabricated a broadband invisibility cloak in the terahertz region,which is composed of a dielectric gratings and metal films covered on its surface.The sample is measured by real-time terahertz spectrometer and then compared to the transmission spectra measured using dielectric sturcture.The transmission of the cloaked structure is obviously higher than that of the dielectric structure,and the phase shift is relatively flat.In order to further suppress the phase shift,we designed and constructed a curved structure,achieving high transmission with relatively small and flat phase shift.In conclusion,we analyze the broadband transmission of subwavelength periodic and disorder gratings.The electromagnetic waves can excite SSPs and transmit through the slits,robustness against disorder,and its high and broadband transmission is not affected by the objects inside the strips.Therefore,any objects can be hidden in the strips,to realize broadband invisibility cloaks.Besides,we experimentally demonstrate that oblique grating with disordered structures can be used in invisibility cloaks in the terahertz region.In order to suppress the phase shift of the cloak,we construct a curved structure and achieve better results in cloaking.These cloaks are easy to fabricate with low cost.By tuning the parameters of the structure it can be achieved in other frequency regions.This type of invisibility cloak provides a new approach to develop broadband optical cloaking materials and devices.