Research On Terahertz Wave Devices Based On Photonic Crystals

In recent years,terahertz wave is undoubtedly a new disquisitive field,which is between millimeter wave and infrared light of the electromagnetic spectrum.The breakthrough of the terahertz radiation source and the terahertz electromagnetic wave detection technology promote vigorous expansion of the terahertz wave functional devices.This paper designs four terahertz wave function devices based on the photonic crystal waveguides structure.The theory of photonic crystal coupling,multimode interference and self-imaging phenomenon and also electromagnetic properties of LuBiIG single crystal are used to analyze and simulate the characteristic of the proposed devices.The main research contents are summarized as follows:1.Compact four-channel terahertz demultiplexer based on directional coupling photonic crystal waveguide,in the proposed scheme,the terahertz wavelength demultiplexer consists of three cascaded directional couplers.In the first photonic crystal directional coupling region,the length of the coupling region is optimally chosen so that the two frequencies?i.e.,f₁=0.524 THz,f₂=0.537THz?and the other two frequencies?i.e.,f₃=0.560 THz,f₄=0.585THz?are separated.Then,the two frequencies of f₁=0.524 THz and f₂=0.537 THz are output from the same photonic crystal line defect waveguide,which are separated later by the second directional coupling region.Likely,the other two frequencies f₃=0.560 THz and f₄=0.585 THz are output from the same photonic crystal line defect waveguide,which are separated later by the third directional coupling region.Both the plane wave expansion method and finite-difference time-domain method are used to analyze and simulate the characteristics of the proposed device.The transmittances are higher than 91% for each terahertz frequency and four outputs channels with the average crosstalk is about 14.45 dB.The insertion loss corresponding to f1=0.524 THz,f2=0.537 THz,f3=0.560 THz,f4=0.585 THz are 0.127 d B,0.285 dB,0.313 dB and 0.399 dB,respectively.It was shown that a good frequency selection performance,a higher transmission,and a low crosstalk are achieved.Moreover,the present device is very compact and the total size is 6.8×10.6mm2.it may also be easily extended to other terahertz frequencies through changing the structure parameters.2.Compact and tunable-multichannel terahertz wave filter,in the proposed scheme,a compact terahertz tunable multichannel filter based on two cascaded photonic crystal waveguide directional couplers,formed by two closely spaced line defect waveguides in a two-dimensional photonic crystal of silicon rods in the air,is numerically studied using plane wave expansion and finite difference time domain methods.The theory of electromagnetic properties of LuBiIG single crystal is used to analyze and simulate the characteristic of the proposed devices.The filtering properties of the present terahertz wave filter,including channel number and frequency,can be tuned by adjusting excitation of external applied magnetic field.The dynamical shift of channels with external applied magnetic field intensity is studied in detail.The transmittances of the present filter are higher than 95% for each terahertz wave frequency with a compact size of 3.8mm 10 mm.The designed device can possess both multichannel and tuning features,which could be of technical use in the terahertz wave communications system applications.3.Tunable multi-channel terahertz wave power splitter,in the proposed scheme,the power splitter by use of a photonic crystal waveguide and surface-mode photonic crystal T-shaped waveguides which based on photonic crystal surface-mode waveguides and conventional photonic crystal waveguides.The tunable multi-channel transmission characteristics and high efficiency transmission ratio are investigated by using finite-difference time-domain method.Our results reveal that the number of the terahertz wave transmission channel could be controlled by changing of the external applied magnetic field of the LuBiIG rod.The proposed device can split the input terahertz wave energy equally into six output ports at the frequency of 0.6THz.When changing the external magnetic field,the input terahertz wave can be equally divided into four output ports with the aid of a magnetic-sensitive material Lu BiIG rod.The total output power is equivalent to 97.2% of the input power for the six-channel splitter and 96.8% for the four-channel splitter.The dimension of the presented terahertz wave polarization beam splitter is of 4.4×6.0mm2.With the help of the surface-mode PhC,the total size of the power splitter is reduced dramatically when comparing with the traditional PhC power splitter.4.Terahertz polarization beam splitter based on photonic crystal waveguide and traditional waveguide,in the proposed scheme,the polarization beam splitter structure consists of multimode interference section and photonic crystal.Both plane wave expansion method and finite-difference time-domain method are used to calculate and analyze the characteristics of the proposed device.The designed polarization beam splitter can split TE-polarized and TM-polarized THz waves into different propagation directions.The simulation results show that the extinction ratios are larger than 18.36 dB for TE polarization and 13.35 dB for TM polarization in the frequency range from 1.86 THz to 1.91 THz,respectively.The designed polarization beam splitter has the advantages of small size and compact structure with a total size of 4.825mm×0.400 mm.By using the internal photonic crystal structure,the total size of the polarization beam splitter is reduced dramatically.Furthermore,the proposed THz polarization beam splitter made of single silicon material is suitable for the integration with other THz wave devices.