Design And Application Of Metamaterials In THz Regime

Metamaterials have exotic dynamic properties that are absent in nature,while terahertz(THz)is the most elusive part in the electromagnetic spectrum.They have been rapidly investigated in recent years.Metamaterials could realize THz absorption,field enhancement and polarization control by properly designing the micron-structures.In this thesis,we will give our works on design and applications of metamaterials in THz regime,including ultra-broadband terahertz metamaterials absorber,bio-sensing,robustness analysis of metamaterials under ultra-strong THz field illumination and magnetic cloaks.First,we studied the broadband THz metamaterials absorbers.Most of previous THz metamaterials absorbers were designed with narrow band response,we designed and fabricated two broadband THz metamaterial absorbers.Based on slow-wave theory,we designed a metallic film stack structure that formed waveguides in adjacent sizes.Each sub-layer behaving as an effective waveguide is gradually modified in their lateral width to realize a wideband response by effectively stitching together the resonance bands of different waveguide modes..The device based on this theory is capable of producing a large absorption above 80%from 0.7 to 2.3 THz with polarization-insensitive and 127%of the FWHM(full width at half maximum)index.Then we propose an ultra-broadband and polarization-insensitive terahertz metamaterial absorber based on a patterned lossy silicon substrate.Experimentally,a large absorption efficiency more than 95%in a frequency range of 0.9-2.5 THz was obtained up to a wave incident angle as large as 70°.Second,THz metamaterials consisting of square patch arrays were investigated as bio-sensors.These THz metamaterials have strong localized field enhancement,which could be tailored to substantially improve the sensitivity of bio-molecules.Here,a THz metamaterials absorber was used as a chlorpyrifos sensor.The results demonstrate that sensitivity is greatly improved using THz metamaterials,with the limit of detection of chlorpyrifos reaching 0.2 mg/L,which is lower than the World Health Organization's provisional guideline limit.Gold nanoparticles can adhere proteins,and have big influence of the properties of metamaterials.We used an AuNP(gold-nanoparticle)-based THz metamaterial sensing method to detect avidin.The limit of detection of conjugated avidin-AuNPs reached 7.8 fmol/L,presenting greater than a 1000-fold sensitivity improvement compared with that of avidin alone.Third,the robustness of THz metamaterials has been studied.High field enhancement is the advantage of metamaterials,however,the strong electric field could affect the performance and even do harm to the micron-structure itself.We investigate various metals for their robustness against damage caused by strong THz field.Even though the damage process is not of a thermal nature we observe a correlation between robustness and the melting temperature.Influence of the substrate material on the damage pattern is also studied.A high robustness THz high field metamaterials,which was made of titanium nitride and sapphire substrate,was fabricated to demonstrate our analysis.In addition,based on scattering cancelling theory,we used superconductor and ferromagnetic materials to design and fabricate a three-dimensional magnetic cloak with a two-layer structure.The magnetic cloak could work from DC to 250 kHz under liquid nitrogen environment,a metal ball could be hidden in the cloak from a commercial metal detector.Then superconductor is replaced by copper to make the magnetic cloak to work in room temperature,with good performance working from 6 to 250 kHz.In the end,we make a conclusion of this thesis,and give perspectives about the application of THz technology and metamaterials.