| Terahertz modulator is the key component of terahertz communication system.With the rapid development of terahertz communication technology,the development of miniaturized and integrated terahertz modulator has attracted more and more attention.Bismuth ferrite films have good optical and photoelectrical properties as the representative of multi-iron materials.The emergence of metamaterials has changed the human perception of materials in nature,and its response to electromagnetic waves can be customized by changing geometric parameters.In this thesis,terahertz wave modulators based on bismuth ferrite films and metamaterials are studied by terahertz time domain spectroscopy under laser pumping.Furthermore,the microscopic mechanism of terahertz wave modulation based on bismuth ferrite films under the external optical field is discussed.The main results are as follows:(1)The terahertz modulator based on bismuth ferrite/silicon heterostructure was studied under the external optical pump.The modulator showed the optical power dependent modulation effect on the incident terahertz wave,and the maximum modulation depth can reach 91.13%.This was the presence of a built-in electric field in the bismuth ferrite/silicon heterojunction that promoted the separation of photo-generated carriers(electron-hole pairs),thus changing the conductivity of the device and thus modulating the terahertz wave.The photoconductivity of bismuth ferrite films on silicon substrate was studied,and a large number of photocarriers were generated by photopumped excitation,which led to a significant increase in the conductivity of bismuth ferrite films.(2)The terahertz resonant switching modulator based on SRR/bismuth ferrite/silicon hybrid structure was studied under the external optical pump.The device had an absorption peak caused by LC resonance of the SRR at 0.5THz.With the increase of optical pump power,the resonance peak of the device disappeared,and the modulation effect of a resonant switch was realized.In order to analyze the internal physical mechanism,the terahertz conductivity of bismuth ferrite film was studied,and it was found that the change of photoconductivity of bismuth ferrite film directly affected the resonant switching modulation effect of the device.Through simulation,it was verified that the increase of photoconductivity of bismuth ferrate film causes shorted circuit at the gap of the SRR,thus inhibiting the LC resonance and realizing the resonant switching modulation effect.(3)The terahertz plasmon-induced transparency modulator based on metamaterial/bismuth ferrite/silicon hybrid structure was studied under the external optical pump.Due to the destructive interference between the two modes of metamaterial,the device realized the plasmon-induced transparency effect.With the increase of optically pumped power,the plasmon-induced transparent window gradually disappeared,and the modulation depth at the transparent window was 82%.The photoconductivity of bismuth ferrite film was investigated,and it was found that the photoexcitation caused it to produce a large number of photogenic carriers,which shorted the gap between the two structures of metamaterial and inhibited the plasma-induced transparency effect,thus realizing the modulation of terahertz wave.(4)Terahertz metamaterial devic based on annealed and unannealed silicon-based bismuth ferrite films was studied under the external optical pump.The device realized the plasmon-induced transparency effect,the internal mechanism was analyzed by simulating electric field distribution and surface current density,which was the result of direct coupling of the two modes.The annealing treatment made the bismuth ferrite film change from amorphous state to polycrystalline state.With the increase of optical pumping power,the modulating effect of the device changed significantly,and the modulation depth at the transparent window was up to 45%.This was attributed to the photoconductivity of annealed bismuth ferrite films changed significantly.. |
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