Preparation And Terahertz Performance Study Of ReS₂/Si Heterojunction Based Thin Films

Terahertz waves are widely used in the field of detection and modulation spectroscopy as a high-frequency electromagnetic wave due to its quality of having different absorption efficiency in different materials.In terms of modulation,all-optical modulation has become one of the main research tools for terahertz modulation because it is not interfered by electromagnetic signals,acoustic waves,or thermal energy.Metamaterials are electromagnetic response patterns of a specific structure,which can accomplish specific functions depending on the pattern and show many special properties in terahertz modulation.In recent years,a large amount of literature has used the combination of metamaterials and semiconductors to accomplish various aspects of terahertz tuning such as amplitude and phase of a single device.The current highly regarded two-dimensional materials have led scientists to a new direction for the study of terahertz modulation devices due to their unique optical properties and material structures.Among them,transition metal sulfides（TMDCs）,as the most studied and widely available class of 2D materials,have become one of the ideal candidates for terahertz modulation devices due to their unique properties of tunable band gap.Firstly,a highly stable ReS₂nanodispersion is prepared by liquid phase exfoliation and a uniform,dense and highly crystalline ReS₂layered film is prepared on high resistance silicon by drop casting method.The morphological profile,optical and structural properties of the ReS₂/Si heterojunction films are characterized by optical microscopy,scanning electron microscopy,atomic force microscopy,X-ray diffraction,Raman spectroscopy and photoluminescence spectroscopy,and the existence of ReS₂/Si heterojunction is demonstrated by Raman enhancement effect.The effects of annealing and film thickness on the properties of ReS₂layered films are investigated.Secondly,the terahertz performance of the samples is tested using a terahertz time-domain spectroscopy system（THz-TDS）,and the effects of different laser wavelengths and powers on the modulation performance of the samples are analyzed.The modulation depth of the sample is calculated from the transmission curve of the experimental results,and the variation law of refractive index and conductivity of the sample with frequency at the corresponding laser power is calculated.Based on the above conclusions,it is demonstrated that the ReS₂/Si heterojunction terahertz modulator of this experiment is able to reach 30%broadband modulation stably in the range of 0.3-1.5THz at a laser power density of 1555 m W/cm²,and its conductivity can reach up to 3.8Ω^-1m^-1.This is of potential value for ReS₂in active terahertz modulation devices.Finally,an experimental study of terahertz metamaterials is carried out in this paper,and a double-open metal resonant ring is designed by the finite-difference in time domain（FDTD）method,and the resonance of the metamaterials under different parameters and excitations is analyzed.Using the lithography process,a layer of metallic metamaterial pattern is prepared on the silicon wafer surface,and the effects of different process parameters on the metamaterial pattern are analyzed.The results indicated that the longer exposure time of lithography corresponds to the finer graphics obtained,while the increase of the development time makes the graphics coarser.ReS₂is prepared onto the metamaterial sample by drop-casting method,and the modulation effect of ReS₂-based terahertz metamaterial is tested by TDS.The resonant frequency of the sample is 0.8THz under TE wave and 1.3THz under TM wave,which is consistent with the simulation results.And under the pumping of continuous laser,the samples show a decrease in amplitude in the wide frequency range of 0.2THz-1.6THz,which indicates that the ReS₂metamaterial has a good tuning effect in the wide frequency range of 0.2THz-1.6THz.