Study In Oxide Superlattices And Actively Tunable Devices In The Terahertz Regime

Terahertz technology has attracted great attention since the 1960s.Moreover,science and technology associated with terahertz have made significant development in the last two decades.In the fundamental research area,various kinds of terahertz emitters and detectors have been proposed.The interactions between different kinds of materials and terahertz waves have been demonstrated widely.Many passively tunable metadevices and actively tunable metadevices have been designed to manipulate terahertz waves.What's more,integrating terahertz time-domain spectroscopy with other technologies,such as X-ray diffraction and scanning tunnel microscope,has also made great progress.In the practical application area,terahertz technology has already been used to detect and analyze materials in biology and medicine.Furthermore,terahertz imaging is also used in airports and railway stations to defend against terrorism.Particularly,terahertz waves have dramatical advantages in wireless communications which has great influences on future information technology.Although terahertz technology has enormous values in practical applications,there are some areas need to be studied deeply and improved.On the one hand,the interactions between many materials and terahertz waves are still unknown.Only electromagnetic responses of different kinds of materials have been demonstrated clearly,the tunability of terahertz waves can be efficient.On the other hand,to realize various functionalities based on terahertz technology,it is exceedingly necessary to achieve efficient manipulation of terahertz waves.Owing to natural materials are hard to control terahertz waves,the lack of functional devices in terahertz regime still limit the development of terahertz technology.Therefore,the main work of this thesis is about studying in dielectric responses of materials and designing actively tunable metadevices in the terahertz regime,which are listed below:1.We constructed a terahertz time-domain spectroscopy system based on photoconductive antennas which is high-stable and has extremely high signal-to-noise ratio.Then we integrated a continuous wave laser at a center wavelength of 532 nm and a liquid nitrogen cryostat into this terahertz system.Besides,we also constructed an optical-pump terahertz-probe spectroscopy system based on nonlinear crystals(zinc telluride).2.We measured some materials by the two home-made terahertz systems,especially investigated the interactions between the quantum functional materials and terahertz waves.We first measured the dielectric responses of two(La0.7Sr0.3MnO3)n/SrTiO3)m superlattices with different superlattice period but the same total thickness by means of terahertz time-domain spectroscopy under external continuous wave green laser and optical-pump terahertz-probe spectroscopy at room temperature.Experimental results show that significant tunability of the dielectric constants of the two samples under green laser excitation,which was explained well by the Drude-Lorentz model.Moreover,we also investigated the dynamics of free carriers of GaAs crystal,graphene,InSb nanowires by optical-pump terahertz-probe technology.3.Metasurfaces have been widely used to manipulate terahertz waves due to their great potential for achieving unique electromagnetic responses.However,to realize fast and efficient control of the light in active metasurfaces is still a critical challenge.We designed and fabricated an ultrafast tunable metasurface consists of silicon disk array.The silicon film was ion-implanted with boron ion(B+)to reduce the carrier lifetime of silicon,subsequently annealed to improve the crystalline quality of silicon.The silicon disk array was fabricated by standard UV-photolithography and subsequent reactive ion etching.By utilizing the optical-pump terahertz probe spectroscopy,the transmission modulation of terahertz waves is realized in the picosecond scale.Furthermore,a theoretical analysis that quantitatively models the time-dependence of the electron-hole plasma density and the permittivity of the silicon disks was proposed,which agreed well with the experimental data.4.Actively tuning optical transmission through hybrid metasurfaces incorporated with multifunctional active media holds great promise for the next generation optical devices.In the terahertz range,they remain rare due to the lack of dynamic and multifunctional designs and materials.Here,a vanadium dioxide(VO2)-based hybrid metasurface is proposed to present multifunctional control of terahertz waves via electrically triggering and ultrafast optical excitation.By minimizing the thermal mass of VO2 and optimizing the VO2 patterns within two side gaps of the asymmetric split-ring resonators,a hybrid metasurface which can tune the terahertz wave with an absolute modulation depth up to 54%and a figure of merit as high as 138%is hereby presented.The hybrid metasurface achieves a switching time of 2.2s under the electrically triggering and offers an ultrafast modulation within 30 ps under the femtosecond pulse excitation.More interestingly,owing to the intrinsic hysteresis behavior of VO2,the hybrid metasurface exhibits distinguishing multi-state transmission amplitudes with a single electrical input.