Tunable Properties Of One-dimensional Graphene Photonic Crystals In The Terahertz Band

As an emerging research band of the electromagnetic wave,terahertz waves have shown many excellent characteristics which have good application prospects in biomedicine,environmental monitoring,security checking,military,and wireless communications.Currently,China is putting forward the 5G application technologies and the 6G vision researches.It is very important to work hard to develop the terahertz technology if China wants to hold the power of speech in the future 6G.The most prominent features of photonic crystals are band gaps and energy localization,which can be used to steer terahertz waves well.In particular,one dimensional photonic crystal has been widely applied in the study of terahertz devices in the past decade due to its advantages of easy fabrication,low cost,and high compatibility.At the same time,terahertz devices are now developing toward the tunable direction in order to meet the application requirements of integrated and multiple-functional devices.Graphene,which shows excellent physical performance has been recently used as an optical characteristic material in the design of tunable terahertz photonic crystals.Under this background,in this thesis,we study the tunability of one-dimensional graphene photonic crystal to lay a scientific foundation for the development of various tunable terahertz devices in the future,such as absorbers,filters,and sensors.We have come up with three one-dimensional anisotropic photonic crystal(1D APC)models which have no defect layer,a VO₂ defect layer in the center,and a defect layer on the surface,respectively.Based on the generalized transfer matrix method,we have investigated the tunable band gaps and transmission characteristics of each model under different incident conditions and got the main results as follows:(1)For the 1D APC without a defect layer,the position and size of the band gaps can be modulated by changing the chemical potential of graphene.Under the condition of|k_x|<k₀,the terahertz waves outside the band gaps exhibit transmission characteristics with slight dispersion and polarization insensitivity.For the TM wave under the condition of6)₀<|6)|<?_n6)₀ and TE wave under the condition of6)₀<|6)|<?_t6)₀,the terahertz waves outside the band gap exhibit extraordinary optical transmission(EOT)with comb-shaped EOT peaks.The tunable sensitivity of the graphene chemical potential to the EOT peaks is about 0.18 THz/e V for the TM wave and 0.28 THz/e V for the TE wave,respectively.Under the condition of|k_x|>?_nk₀,the TM wave reflection exhibit a resonance with Fano-like lineshape.The tunable sensitivity of the graphene chemical potential to the Fano-like resonance peak is about 0.13THz/e V.(2)For the 1D APC with a VO₂ defect layer in the center,under the condition of|k_x|<k₀(take k_x=0 as an example),we can realize a switchable function between band-pass filter and perfect absorber by changing the phase state of VO₂,while the operating frequency of either band-pass filter or perfect absorber can be modulated by the chemical potential of graphene.Under the condition of|k_x|>k₀(taking the TM wave of k_x=2k₀ as an example),when VO₂ is in the insulating phase,the performance of band-pass filter is strongly affected by the thickness of VO₂.For both two phase states of VO₂,the terahertz waves outside the band gap exhibit EOT phenomenon.(3)For the 1D APC with a defect layer on the surface,the defect layer also serves as a phase matching layer.Under the condition of normal incidence,according to the standing wave theory,the thickness and refractive index of defect layer play a critical role on the absorption performance of the 1D APC.In particular,when they are equal to the thickness or refractive index corresponding to the standing wave series,a coherent perfect absorption peak with narrow spectral line,high absorption and large Q value can be obtained.At the same time,the coherent perfect absorption peak can be modulated by changing the chemical potential of graphene.The designed terahertz devices based on one-dimensional graphene photonic crystals in this paper have compact structure,excellent tunable performance,diverse tunable functions,and relatively convenient preparation process.They have potential applications in the fields of terahertz communication,sensing,and imaging in the future.