Integrated dispersion compensation for Terahertz photonics

Abstract

Terahertz quantum cascade lasers (THz QCLs) are compact sources of radiation in a frequency range that has been labelled the terahertz gap due to the lack of sources as well as optical components. A topic of special interest here is the generation of THz frequency combs for spectroscopy and metrology. Since THz radiation couples heavily to the crystalline lattice it is much more dispersive than the radiation in the near and mid infrared frequency range. For frequency comb spectroscopy applications it is imperative to operate the laser with near zero dispersion. Therefore the aim of this thesis is concepting new waveguide structures that counteract the dispersion of the waveguide and the material in a THz QCL. We use micro-patterns on the top metal contact of the QCL waveguide to build devices that cancel out the waveguide and material dispersion. These devices are based on conventional dispersion counteracting structures like the Gires-Tournois interferometer (GTI) and a chirped mirror. Patterning the top contact changes the refractive index in the active region underneath. We make use of this aspect to build multilayered mirrors by alternating patterned and unpatterned sections. We start by assessing the influence of the micro-patterns on the modal index of the active region material by means of FEM simulations. We present refractive index shifts to 3 % in micro-patterned waveguides. These remarkable deviation can be achieved by using only standard processing techniques. We make use of this significant refractive index difference for the design of mulitlayered mirror based dispersion compensating devices. We present the first designs for integrated GTI structures, working at frequencies of 2.5 and 2.8 THz. These GTIs compensate the QCL dispersion with bandwidths extending to 75 GHz. Furthermore, the design frequency can be shifted by simple adaptations of the mirror and interferometer geometries. We also demonstrate three novel chirped mirror structures for dispersion compensation at frequencies of 2.5 THz, 2.7 THz, and 2.9 THz with bandwidths as high as 125 GHz. These devices hold powerful potential for broadband compensation when adequately adapted.