| Terahertz(THz)wave falls in between infrared radiation and microwave radiation in the electromagnetic spectrum.It is usually defined as the electromagnetic wave between 0.1 THz and 10 THz.It has the properties of high security,strong penetration and rich spectral information,so it has a broad application prospect in various fields,including telecommunication,medical imaging,explosive detection and so on.However,the lack of efficient and compact THz emitter is one of the main reasons for the limitation of THz techniques and relative applications.THz photoconductive antenna,as a kind of THz radiation source,is widely used in THz time-domain spectroscopy system and THz imaging system because of its advantages of smaller size,running at room temperature and wide bandwidth of radiation spectrum.But the relatively low radiation power and efficiency limit the further expansion of photoconductive antenna applications.Based on this,by combining with the current surge model,this thesis systematically studied the enhancement effect of the subwavelength metal structures,including the plasmonic grating structures and the surface anti-reflection array nanostructures,on the enhancement of the radiant power of the photoconductive antenna,thus improving the efficiency of the photoconductive antenna.The specific research contents are summarized as follows:(1)The important factors that affect the efficiency of photoconductive antenna have been analyzed and summarized from the aspects of laser source,photoconductive antenna and other aspects.Then,based on the current surge model of small aperture photoconductive antenna,the influence of the femtosecond laser pulse power intensity,the laser pulse width and the carrier lifetime of the substrate material posed on the THz radiation of the photoconductive antenna was analyzed,which can provide a solid foundation for the systematic design of the efficient photoconductive antenna.(2)According to the characteristics of subwavelength metal structure,the necessary correction of the current surge model was studied after the subwavelength metal structure was added into the THz photoconductive antenna.The effects of metal materials,substrate materials and the gap width of gratings posed on the THz emission of the photoconductive antenna were discussed under the excitation of femtosecond laser pulse with 800 nm wavelength.The main parameters and optical transmission of the plasmonic gratings with the gap width of 20 nm,40 nm,60 nm,80 nm and 100 nm were simulated and analyzed by the Finite Element Method.(3)The electric field distribution in the substrate of plasmonic grating was simulated.The different influence of Cr adhesion layer and Ti adhesion layer posed on the THz emission of the photoconductive antenna was analyzed and discussed,and the mechanism and structure design of the mask layer was also interpreted.Besides,the overall design of the photoconductive antenna based on 800 nm photoexcitation and the GaAs substrate was proposed.(4)In order to achieve the enhancement of THz radiation from photoconductive antenna,the anti-reflection effect of different plasmonic nanostructure arrays textured on substrate surface has been studied.The different anti-reflection effects of square,disk and hexagonal Au nanostructure arrays were compared,the anti-reflection effects of square,disk and hexagonal GaAs nanostructures and the localized enhancement of electric field of the metal array added in the substrate was also compared.These sub-wavelength nanostructures are all theoretically beneficial to reduce the reflectance of the incident light or to enhance the localized intensity of the electric field in the substrate and improve the efficiency of THz photoconductive antenna at the end. |
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