Metamaterials for terahertz frequencies

Abstract

Metamaterials are artificial structures designed to manipulate electromagnetic radiation in a way no other material can do. They are composed of subwavelength resonators, so-called meta-atoms. Although usually known for their negative index property, they can be used as an efficient technique for coupling free space radiation to different quantum systems. In this case they act as a cavity and the coupling is provided by the z component of the electric field. Their major advantage is the easy tunability of the resonance frequency which can be achieved by simply scaling their geometrical dimensions. The aim of this thesis is to investigate the properties of terahertz (THz) metamaterials based on double split-ring resonators for coupling free space radiation to intersubband transitions (ISBTs) in quantum cascade lasers (QCLs). In this work, finite element simulations as well as spectroscopic measurements are used to characterize the resonant behavior of THz metamaterials. Different tuning possibilities are explored and the impact of a structure used for QCL biasing is getting analysed. The simulations are supplemented by transmittance measurements using an FTIR spectrometer and a time domain spectroscopy setup. Two different QCL designs are used to probe the light-matter coupling additionally. For this purpose two structures are fabricated and experimentally characterized. The active region of the first sample is getting patterned in form of pillars and two different polymers are employed as planarization layers. In addition, the impact of the used dielectrics on the metamaterial resonance is getting investigated. The second structure uses a silicon nitride insulation layer and the resonators are used as an etch mask to get a laterally confined QC structure. The measured spectra have conclusively shown good agreement with the simulation results.