Nanostructures made from graphene and graphene related materials or from traditional compound semiconductors are promising building blocks for light sources and detectors in a broad frequency range. By cutting graphene into a few nanometer-wide nanoribbon, the bandgap can be tuned to a certain extent by the confinement of the electronic wave function. One dimensional graphene nanoribbons and superlattices provide precisely tunable energy gaps for optical applications. The optical properties of such nanostructures are investigated. The nearest neighbor tight-binding model is employed to describe the electronic bandstructure. In addition, an analytical solution for the dispersion relation and the wave functions are introduced in this study. Based on developed models, selection rules for optical transitions of each structure are obtained. The results are verified against first principles calculations. Single-layer hexagonal boron nitride can be patterned into nanoribbons which exhibit large enough band gaps and qualitatively different properties from those of graphene related materials. Embedding graphene nanostructures in boron nitride lattices increases flexibility of bandgap engineering for optical transitions. The optical properties of such embedded graphene nanoribbons and superlattices are investigated. The optical spectrum, the quantum efficiency, and the photoresponsivity of those nanostructures are evaluated and their application in photodetector devices is investigated. The role of line-edge roughness on the optical properties of such devices is carefully studied.
Since the intersubband transition energy can be varied to cover a broad wavelength range, intersubband optoelectronic devices are very agile in frequency. For a quantum cascade laser, both the emission frequency and optical gain can be tailored by design of the heterostructure. Employing the flexibility offered by bandstructure engineering, we present an approach to use quantum cascade lasers for ultrashort pulse generation in the infrared and terahertz frequency range. Laser design parameters, including the barrier and well thicknesses and applied electric field are modified for maximizing the laser performance and desired dynamic operation. For this purpose, particle swarm optimization — a multi-variable multi-objective optimization algorithm — is employed. For short pulse generation, we study passive mode locking in a ring cavity quantum cascade laser where the instability condition is introduced by means of a saturable absorber. The effects of saturable absorber and pumping strength on the instability threshold are investigated. Various quantum cascade laser designs, including three-well vertical, superlattice, and terahertz designs, are employed in the optimization study. A large optical gain below the instability threshold is achieved for the optimized quantum cascade laser designs. To analyze the optimized structure above the instability threshold, numerical calculations based on the Maxwell-Bloch equations are performed. A finite-difference discretization scheme is employed to find the evolution of electric field, polarization, and population inversion in the spatial and time domain. The results indicate side-mode instabilities due to Risken-Nummedal-Graham-Haken-like instability.