| Terahertz waves are electromagnetic waves with frequencies from 0.1 to 10 THz and play an important role in many fields such as non-destructive inspection,quality control,medical diagnosis and defense & security because of their unique properties such as low photon energy,strong penetration and rich spectral information.In recent years,as the applications of imaging technology continue to expand,the demand for high-resolution images has become more urgent,and terahertz super-resolution imaging technology has therefore attracted much attention from researchers.The emergence of metamaterials has provided an effective way for the modulation and super-resolution imaging of terahertz waves.Among them,hyperbolic metamaterials,as an important branch of metamaterials,have unique electromagnetic waves modulation properties,which can effectively transmit evanescent waves carrying sub-diffraction information to reach the image plane and participate in imaging,and thus are expected to realize super-resolution imaging that breaks the diffraction limit.However,the current terahertz super-resolution imaging methods based on hyperbolic metamaterials suffer from complex imaging structures,high transmission loss and single performance.In order to improve the resolution and flexibility of imaging and further expand the application scenarios of super-resolution devices,this paper constructs multilayer hyperbolic metamaterials with different geometries based on the flexible tunability of graphene materials and analyzes their super-resolution imaging performance in terahertz band through simulations,which mainly includes the following three aspects:(1)The transmission and dispersion properties of graphene hyperbolic metamaterials are investigated,including the transmission loss of SPP on the graphene surface,the transmissivity of the graphene-dielectric multilayer structure,and the dispersion relations of graphene hyperbolic metamaterials.The analyses of the transmission and dispersion properties of graphene hyperbolic metamaterials can provide a reference for the tuning range of chemical potential in the hyperlens structures.The theoretically better super-resolution imaging performance can be obtained when the chemical potential of graphene is set to a smaller value that satisfies the condition of hyperbolic dispersion.(2)Graphene-based cylindrical and planar multilayer hyperlens structures are proposed,both of which are composed of 10 pairs of alternating graphene and dielectric.With the incident transverse magnetic waves at a wavelength of 100 μm,both structures can clearly resolve the two-line object spaced by 10 μm,corresponding to a resolution of λ/10,and the cylindrical structure has a mechanism to magnify the sub-diffraction features.Moreover,an outstanding advantage of these two hyperlenses is that by adjusting the chemical potential of graphene by applying an electric field,etc.without changing other parameters,they can achieve the super-resolution effect for the two-line object in the broadband range of 3～5 THz,corresponding to resolutions of λ/10～λ/6.On this basis,the factors affecting the super-resolution performance of graphene hyperlenses,including the relaxation time,the thickness of dielectric and the pairs of graphene/dielectric,are also investigated.In order to combine the respective advantages of cylindrical and planar structures,a hybrid graphene hyperlens is also constructed,which allows easy localization of the imaging target as well as the magnification of sub-diffraction features,and therefore has greater potential for application.(3)Terahertz imaging applications based on graphene hyperbolic metamaterials are investigated,including super-resolution imaging of the grating structure and micron-scale particles,and the implementation of a tunable concave/convex lens.For the grating structure,a relatively uniform grating interference resolution of ～λ/12 is obtained by adjusting the chemical potential of graphene.For randomly distributed micron-sized particles,a resolution improvement of about 5-fold is achieved,and the planar hyperlens can also clearly resolve two particles at a very close distance that cannot be distinguished in diffraction-limited method.In addition,by exploiting the tunability of chemical potential,the graphene hyperbolic metamaterial structure can also realize the effect of a diverging or focusing lens.Due to many outstanding advantages,including super-resolution imaging capability in a wide frequency range,flexible tunability,stability and robustness,and customizability of imaging magnification,it is believed that this work can provide a reference for the design and development of terahertz super-resolution devices,and also play a positive role in the development of terahertz modulation devices based on hyperbolic metamaterials. |
