Terahertz metamaterials for sensing and detection
The function of metamaterials targeted for operation at terahertz frequencies was investigated including split-ring resonators, perfect absorbers, and flexible absorbers. The devices were designed through simulation by finite element method commercial software and fabricated by photolithographic lift-off process using electron beam evaporation for the deposition of metal layers. The terahertz time domain spectroscopy measurement technique was used to characterize the devices in both transmission and reflection modes. Each device was composed of arrays of subwavelength resonant structures. The split-ring resonator study focused on the effects caused by the addition of a passivation layer, which served to effectively lower the resonant frequencies of the rings, as well as the addition of a second layer of rings that led to a broadening of the resonant response of the structures. The perfect absorber investigation was focused on the polarization insensitive nature of the frequency selective surface elements. A mechanism for this property was proposed along with an equivalent circuit model that can describe and predict the resonant behavior. The perfect absorber were based on a unit cell structure with four-fold symmetry that consisted of a metal ground, a polyimide dielectric spacer layer, and top frequency selective surface layer. The flexible absorber chapter reports the simulation and characterization of perfect absorber structures fabricated on a polyimide flexible substrate that consisted of a metal ground plane, a dielectric spacer, and a frequency selective surface composed of two layers of differently sized, nonconcentric, circular rings separated by another dielectric spacer layer. Specifically, the frequency selective surface was seen to exhibit a frequency response dependent upon the position of the individual resonant elements with respect to each other. The functionality of the flexible absorbers during deformation was also evaluated through the use of two custom designed vacuum holders. Characterization of the flexible absorbers before and after removal from the sacrificial substrate, as well as under both types of deformation, revealed that the structures had little change in functionality. The flexible absorbers were shown to be suitable for future incorporation in sensing applications and to have the potential to lead to future applications in cloaking.