Unidirectional Light Transmission In Hybrid Metamaterials

The phenomenon of unidirectional light transmission has attracted a great deal of interest due to its potential applications in optical isolators,optical diodes,polarization conversion and optical interconnects.The unidirectional light transmission device plays a key role for the logic functions in optical systems.Conventional methods for achieving unidirectional light transmission often require external magnetic field,which greatly limits their development in device miniaturization and integration.In recent years,metamaterials are emerging as a new way for realizing unidirectional light transmission.Current studies have proposed various metamaterial structures for obtaining unidirectional light transmission with fixed frequency bandwidth based on static designs.The dynamic tuning of unidirectional light transmission remains a challenge,which deserves further investigation.In this thesis,we propose a hybrid graphene-metal metamaterial,which exhibits unidirectional light transmission with tunable wide bandwidth.The main research results are as below:(1)According to the physical conditions of unidirectional light transmission,we design and propose a hybrid graphene-metal metamaterial structure,which exhibits tunable unidirectional light transmission.The hybrid metamaterial consists of a complementary split-ring graphene and a metallic grating.Simulation results show that for linearly polarized light propagating in forward direction,the hybrid metamaterial has a total transmittance of more than 0.1 in the frequency range of 27.1 THz to 31.4 THz.The maximum transmission efficiency reaches 18.8% with a FWHM bandwidth of 4.3 THz.For backward light propagation,the light is completely blocked.As the graphene Fermi energy is varied from 0.6 eV to 1eV,the unidirectional light transmission can be tuned by 6.6 THz,exhibiting a great potential for developing tuning functionality.(2)Mechanisms underlying the polarization conversion and unidirectional light transmission are illustrated in our proposed graphene-metal hybrid metamaterial.Three relevant resonant modes are calculated using numerical model,and their field profiles and surface currents are calculated.For linearly polarized light in forward propagation,surface currents are excited on perforated graphene via resonant interactions.Directions of the surface currents are different from polarization of the incident light,which thus rotate the incident polarization by 90o via re-radiation.The rotated light polarization coincides with transmission direction of the metal grating,leading to transmission peaks in forward propagation.For backward propagation,the incident light polarization is blocked by the metal grating,leading to the phenomenon of unidirectional light transmission.(3)Physical factors affecting efficiency of the unidirectional light transmission are analyzed.Besides transmission,we also calculated reflection and absorption spectra of the graphene-metal hybrid metamaterial.Relevant factors affecting the transmission efficiency are examined including surface reflection,material absorption and electron scattering time of graphene.Results show that absorption of the metal is negligibly small and can be ignored.The absorption of graphene is a major factor limiting the transmission efficiency when electron scattering time of graphene is less than 0.5 ps.For graphene electron scattering time larger than 0.5 ps,however,the surface reflection plays a major role.Our analysis lays a foundation for further optimization of the structure.