Study Of Resonances Of Terahertz Wave In Subwavelength Structures And Its Application In Imaging

Recently, along with the development of terahertz (THz) technology, the study of subwavelength structures has been extended to THz range. Through a large number of researches, people found that a kind of bound THz surface waves mimicking surface plasmons (SPs), namely spoof surface plasmons (SSPs), can be sustained by a structured metallic surface. For longer wavelengths, such as microwave and THz, SSPs can lead to resonance phenomenon in reflection and transmission spectra of metallic subwavelength structure arrays. Based on this property, a lot of new applications of wave manipulation could be expected. However, microscopic physical mechanisms of these extraordinary reflections and transmissions are still unknown. Furthermore, it is not clear that what kind of surface modes can be sustained by complex plasmonic structures, such as compound hole arrays and double-layer meta-surface. To resolve these problems, through experimental and theoretical approaches, the scattering and resonances of THz wave in subwavelength structures are studied systematically and thoroughly in this thesis. The main content is listed as follows:(1) We analyze the scattering and resonance properties of THz wave in one-dimensional metallic subwavelength grooves. By considering the additional scattering between grooves, an extended waveguide mode (EWGM) method is developed. Though this method, the coupling between scattering waves in groove doublet configurations is investigated. Moreover, the scattering properties of continuous and pulsed THz wave in subwavelength grooves are also analyzed. Our study offers a simple alternative to design compact THz directional antennas, couplers, and other devices.(2) The SSPs sustained by two-dimensional metallic compound holes array are investigated theoretically and experimentally in THz range. Here we propose a kind of compound structure which consists of larger and smaller periodic holes, and then discuss the coupling between SSPs in this structure and external field. By using our theoretical model, the resonant wavelengths of extraordinary transmissions through the compound holes array are deduced. Moreover, we find that the transmission peaks will show a redshift due to the existence of smaller hole arrays, and we also prove this phenomenon experimentally. These results may find applications in tunable filters in microwave and THz range.(3) By using theoretical and experimental approaches, we investigate hybrid spoof surface plasmons (HSSPs) in dielectric-metal subwavlength structure arrays. The existence of HSSPs in THz range is experimentally demonstrated based on THz time-domain spectroscopy. Moreover, we show that the resonances of HSSPs for low frequencies are redshifted when the filling fraction of the plasmonic structure is increased. To explain these experimental results, a dipole-scattering model is also developed. We believe that these results may find important applications in controlling THz wave in spectral and spatial domain, including tunable absorbers and filters.(4) From the point of view of practical application, the improvement of THz imaging by using of subwavelength structures is studied. We propose a kind of plasmonic sensor fabricated by dielectric-metal subwavlength structure arrays, and experimentally demonstrate that the detection sensitivity and the imaging contrast of THz surface imaging can be improved by use of this sensor. Through theoretical analysis, we show that HSSP modes can be sustained by this plasmonic structure, and the coupling between HSSPs and external field may compensate the detailed informations of the imaging. This study offers a simple and inexpensive alternative to improve the quality of THz imagine.