Terahertz Multifunctional Modulation Study Based On EIT Metasurfaces

Due to the strong coherence,low photon energy,deep penetration into non-polar substances and large operating bandwidth,terahertz wave has shown significant applications in both military and civil fields such as spectral detection,high-resolution imaging,non-damage detection and wireless communication.Terahertz has become a hot topic of researchall round the world in recent years.Among them,the study of terahertz metasurfaces modulation have promoted the development of terahertz science and technology.The electromagnetic induction transparency(EIT)effect of metasurface,with its slow-light effect and high quality factor,has shown great applications in terahertz metasurface modulation devices.At present,the modulation capability of terahertz EIT metasurface is limited to,and realizing the multifunctional modulation of one device is a goal that many researchers are striving to pursue.Besides,,forward design of terahertz metasurface modulation devices often involves multiple parameters which requires highly complicated skills.In contrary,the inverse design method can automatically optimize the design of metasurface structures based on the targeted results and can dramatically improve the design efficiency.At present,the application of inverse design into terahertz metasurface still needs further study.This thesis focuses on the studying of terahertz EIT multifunctional modulator and the inverse design of terahertz metasurfaces,by conducting the following work.(i)Literatures investigation laying the foundation for theoretical and experimental studies..Firstly,the basic characteristics and applications of terahertz waves are described in general,and the current status of research on active terahertz modulation and inverse design in photonic devices are investigated and analyzed.Then the basic of optimization algorithms and processes of simulation and optimal design are described.Finally,the details of optical setup and experimental operation of terahertz time-domain spectroscopy system is introduced.The experimental data is collected by the LABVIEW then processed by Python codes.(ii)Forward design and implementation of a nanosecond-scale dual-function terahertz EIT metasurface modulator.Different EIT modulation functions are realized under different optical pump fluences by using Si-bridge coupled metal metasurfaces.The modulation of the EIT resonance amplitude is realized on the metasurface under low-pump optical excitation.Under high-pump optical excitation,the molecularization process occurs on the metasurface and the modulation of EIT resonance Q-factor is realized.The modulation speed of both functionatilies is on the order of nanoseconds.(iii)A terahertz EIT metasurface modulator with picosecond-scale time response was inversely designed and implemented.The inverse design method combining particle swarm optimization algorithm with finite-difference time-domain method is used to design a terahertz metasurface with a specific EIT spectral profile.The time-domain finite-difference method is used for the electromagnetic simulation of the metasurface to obtain the terahertz transmission spectra,andthe particle swarm optimization algorithm is used for the tuning of the metasurface structure and the optimization of the spectral profile so that the transmission spectra gradually converges to the designed EIT profile.The metasurface structure was frabricated by photolithography and an amorphous germanium film is vaporized on its surface as the photoactive layer.The EIT modulation of the composite metasurface on the picosecond scale was experimentally measured.In addition,the dual EIT metasurface design work well verifies the universality of this inverse design scheme.