Research On Propagation Properties Of Terahertz Waves In Subwavelength Metal Structures And Its Applications

Terahertz technology has important application values in the field of sensing and high-speed wireless communications. However, its development is seriously hampered by the shortage of basic beam control devices in the terahertz frequency. In this thesis, based on the theoretical studies of surface plasmon polariton (SPP) and spoof surface plasmon (SSP) effects in the terahertz domain supported by the subwavelength metal structures, we investigated the terahertz beam steering technologies by the subwavelength metal structures, and proposed a series of schemes. The research details are listed in the following:(1) We analyzed the transmission properties of subwavelength metal structures in the terahertz regime, then proposed an electrically controlled terahertz switch based on the metallic grating-liquid crystal-metallic grating (MG-LC-MG) structures. The switching mechanism is based on the shift of the resonance peaks, which is realized by modifying the effective refractive of the LC using different bias electric fields. The innovation of this design is that the subwavelength metal structures not only support the super transmittance at certain frequencies, but also act as the electrodes. Therefore, the proposed device has the potential advantage of avoiding high insertion loss due to the tin indium oxide (ITO) electrodes used in previously reported LC-based terahertz switch devices. Moreover, we proposed an electrically controlled broadband terahertz switch based on the LC-filled multilayer metallic grating (MMG) structures. The on and off state of the switch is realized by the changing of the absorption coefficient of the LC in the presence of different electric fields. The relations among the device design parameters and switching performances are studied by the rigorous coupled-wave analysis (RCWA) method.(2) We analyzed the dispersion relation of the SSP, which is supported by the subwavelength metal surface grating structures, then proposed a method to realized dynamic trapping of terahertz waves using a silicon-filled metallic grating structure. Using the finite element method, we demonstrated that if a graded refractive index distribution in the grooves is optical induced, the device has the ability to dynamic trap terahertz waves of different frequencies at different positions. Moreover, we demonstrate that the trapped position of a certain frequency of the terahertz waves can be moved continuously along the grooves in subwavelength scale by ingenious control of the distributions of the refractive indices of silicon filled in the grooves.(3) We proposed a subwavelength metal slit-groove structures to control the propagation characteristics of the terahertz waves passing through a subwavelength metal slit. The most significant feature of the design is that the surface gratings consist of two sets of grooves of different depths. The shallow grooves are designed to support the SSP, and the deep grooves are utilized to diffract the terahertz surface wave into the free space. Theoretical analysis and numerical simulations based on the finite element method confirm that various beaming effects including splitting, on-axis and off-axis beaming can be realized by controlling the distributions of the deep grooves.(4) We measured the transmission spectrum of a subwavelength metal slit grating fabricated on an epoxy glass substrate by the terahertz time-domain spectroscopy (THz-TDS) system, then presented a detailed analysis of the formation mechanism and characteristics of the transmission peaks and absorption minimums in the transmission spectra. The narrow transmission peaks are the result of the coupling between the SPP and the guided modes. Besides, Fabry-Perot-type transmission peaks can also be formed due to the multiple reflections effect in the substrate, which are relatively wide and will be red shifted if a SPP mode is simultaneously excited. The absorption minimum in the transmission spectra is emerged when a pure SPP mode is excited at the upper or lower surface of the metal grating, or a guide mode is solely formed in the substrate.