Design Of Terahertz Metamaterials For Refractive-index-based Sensors

Metamaterials is a new class of artificially engineered electromagnetic(EM) materials constructed by the sub-wavelength metallic building elements. Asignificant breakthrough of metamaterials is to engineer the desired EM properties,especially some exotic EM phenomena that cannot be obtained with natural materials.This includes extraordinary transmission, invisible cloaking, negative refraction, etc.As a result, this new class of materials has attracted considerable interest.In recent years, with the development of the THz generation and detectiontechnologies, considerable progress has been made in terahertz technologies. Moreand more metamaterials, as function devices, has been applied in terahertz region,such as absorbers, filters, modulators, sensors and polarizers. Especially terahertzmetamaterials has great potential applications in non-invasive biological andchemical sensing. Not only because many substances have a specific spectralfingerprint in THz interval, but also because THz metamaterial-based sensing hasadvantages of simplicity and higher sensitivity over standard terahertz time-domainspectroscopy (THz-TDS). Therefore, THz metamaterial-based sensing has been a hotspot of THz technologies.In this paper, we designed and fabricated two kinds of metamaterial-based THzsensors and a bilayer wire-grid polarizer. The performances of the metamaterialsdevices were numerical simulated and experimental measured by CST Microwavestudio and terahertz time-domain spectroscopy (THz-TDS). The experimental resultsand simulated data agree very well. The main works of the dissertation are listed asfollows:(1) The analogue of electromagnetically induced transparency (EIT) is achievedin a planar terahertz resonator consisting of a square closed loop (SCL) and a splitring resonator (SRR). Finite integration time domain (FITD) simulations are carriedout to analyze the EIT-like effect and its potential applications asrefractive-index-based sensor. Results show that the EIT-like resonance exhibits highrefractive-index sensitivity and a high FOM of4.06, higher than that for SCL (0.09) and SRR (2.48) resonators. The refractive index and dielectric loss of the substrate,which influences the sensing performance of the SCL/SRR resonator, has also beenstudied. Finally, a device of the SCL/SRR resonator is fabricated by usinglaser-induced and chemical non-electrolytic plating with copper on polyimidesubstrate, the transmission properties of which characterized by TDS is in goodagreement with numerical simulations.(2) Sharp Fano resonant characteristics is investigated in a planar terahertzresonator composed of symmetric U-shaped split ring resonators (SRRs) and metalliccut wire by CST Microwave Studio, which is caused by the coupling between abroad bright eigenmode and narrow dark modes. The Fano resonator features high Qresonance and a strong localization of electromagnetic fields in the resonatorstructure, enabling it a good candidate as high-sensitivity THz sensor. Simulationresults show that, Fano resonance in the designed metamaterial exhibits highrefractive-index sensing sensitivity and FOM, which have potential application inbiological and chemical sensing.(3) THz polarizer, as a quasi-optical device to manipulate the THz waves,plays an important role in the THz spectral, imaging and sensing system. Wefabricated a bilayer wire-grid polarizer (BWGP) on both sides of a low-losspolyimide substrate by simple laser-induced and chemical non-electrolytic platingwith copper technologies. The performance of the polarizer sample was characterizedusing a THz time-domain spectroscopy (THz-TDS). In the frequency range from0.2to1.5THz, the transmittance of TM wave through this polarizer was over67%. Theextinction ratio was better than33dB, and with an average extinction ratio of41dB,which is greatly higher than the conventional single layer wire-grid THz polarizer.