| In the sensing field,the development of photonics and electronics has entered a new era(T-Era).The corresponding term "Terahertz(THz)" is now used to describe a special electromagnetic spectrum with a frequency in the range of 0.3-10THz(the wavelength range of 1mm?30?m).In recent years,the intensive development of the THz band in academia and business has filled the gap between the microwave and infrared spectrum.Especially in the THz sensing,imaging,and communication fields,a large number of theoretical research and applications have been invested by researchers from various countries.As a kind of optical sensor,the refractive index sensor can be used in many fields,including THz technology and so on.The high-sensitivity,multi-functional,and easy-to-integrate THz refractive index sensor is of great significance to promote the practical application of THz systems.THz waveguide devices based on Photonic crystal fiber(PCF)can solve the transmission and manipulation of THz waves in free space.PCF has a flexible structure(e.g.the shape,size,and separation of air holes),the controllable transmission mechanism(refractive index and band gap refractive index),and functional materials filled or placed in the channels.These features make the THz refractive index sensor based on PCF has the advantages of large design freedom,high sensitivity,and compatibility with existing optical fiber systems.Graphene is a kind of single-layer two-dimensional material formed by the honeycomb arrangement of carbon atoms.It has a unique band structure and excellent electrical and optical properties(e.g.high carrier mobility,reduced Fermi level,saturated absorption,and excitable surface plasmons,etc.).Graphene's intra-band transition and surface plasmon resonance(SPR)frequency cover the THz regime.The THz electro-optical properties of graphene can be flexibly controlled by applying external electric/magnetic fields and chemical doping.In recent years,many studies have shown that graphene is expected to promote the development of new functional devices in the THz regime.In this paper,THz PCF and graphene are combined to design a THz composite waveguide,and the transmission characteristics and refractive index sensing characteristics of the composite waveguide are systematically studied.The main research work is briefly introduced as below:(1)We designed a composite THz waveguide of Graphene and solid-core PCF.In this study,two larger air holes are introduced into the cladding hole adjacent to the PCF core for improving the sensitivity and introducing birefringence,and graphene is coated on the inner surface for filling the sample to be tested.Due to the high permittivity of graphene,the guided mode of G-PCF is mainly distributed in larger holes.For the analyte with a refractive index of 1.00-1.50,the introduction of graphene can increase the relative sensitivity coefficient by more than 5 times.when the analyte RI is equal to 1.03,the relative sensitivity coefficient increased by 15 times.The graphene-solid-core PCF has a high relative sensitivity coefficient of about 90%for the test sample with a refractive index of 1.37.what's more,we confirm that the transmission characteristics and refractive index sensing characteristics of the G-PCF can be adjusted by applying an electric/magnetic field.(2)We designed a composite THz waveguide of graphene-D-type PCF by coating graphene material on the inner wall of a specific air hole in the D-type PCF.It excites the surface-like plasmon in the THz regime The composite THz waveguide realized average refractive index sensitivity of 214.28GHz/RIU(Refractive Index Unit)and a refractive index resolution of 5.0×10-7RIU for analyte refractive indices ranging from 1.0 to 1.5.The surface plasmons resonance peak positions of the two polarization fundamental modes are blue-shifted owing to the graphene chemical potential changing.By tuning the graphene chemical potential ?c,simulation results show a maximum frequency sensitivity of 750GHz/eV for the X-Polarization and 765GHz/eV for the Y-Polarization with graphene chemical potential ?c indices ranging from 0.4eV to leV.With the conclusion,this composite waveguide can achieve higher electrical tuning sensitivity by applying an external electric field to control the chemical potential ?c of graphene.(3)We designed a graphene bandgap of hollow-core PCF composite THz waveguide.The structural parameters,the substrate material,and the analytes are studied to illustrate the variation of the bandgap with the PCF.Two filling methods with a selective filling(filling only in the core air holes)of the analyte and complete filling of the analyte are discussed.For the analyte selectively filled with the core air hole,when the refractive index of analyte is greater than the fundamental space-filling mode effective refractive index(nFSM),the PCF sensor transmission mechanism is converted from the bandgap guided type to the index-guiding PCF,and the transmission characteristics are converted from bandpass to broadband transmission.Outside the bandgap frequency,the "on-off" device of the THz regime can be achieved.For the analyte filled with entire air holes,the filling of the sample will not change the original hollow-core waveguide bandgap-guided transmission mechanism.Because the refractive index of gases and common liquids is lower than the refractive index of the substrate(1.53).The proposed sensor can realize the refractive index sensing function by changing the bandgap position and bandwidth.After the graphene-coated in the inner wall of the PCF central air core,the fundamental mode field distribution and the confinement loss of the G-PCF change significantly owing to the graphene chemical potential ?c changed.When graphene chemical potential ?c is equal to 0.6eV,the maximum confinement loss change is obtained.The confinement loss of the PCF is increased from 70dB/m to 1565dB/m.Therefore,an "on-off" device in a specific THz frequency regime can be achieved by introducing graphene and adjusting its chemical potential ?c.The research conclusions provide theoretical guidance for the design of functional devices such as PCF based THz sensors,filters,and switches.In conclusion,the research direction of this article belongs to the field of THz photonics.In this paper,the research on the transmission and sensing performance of G-PCF composite THz waveguide is systematically carried out.It provides the important theoretical basis and experimental guidance for the research and development and experimental preparation of multifunctional and tunable THz sensors. |
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