Despite the identification of various possible applications, the terahertz region (1-10 THz) has remained one of the least developed spectral regions. Photonic approaches to direct terahertz generation are limited due to the lack of appropriate materials with sufficiently flexible small bandgaps. In this thesis, we investigate the applications of semiconductor nanostructures as MIR/THz sources and detectors. The optical properties of graphene-based materials as well as quantum cascade structures are studied.
We theoretically studied the optical properties of AGNRs/BN, employing tight-binding (TB) calculations. We demonstrate that in AGNRs/BN only optical transitions from subbands with odd (even) indices to subbands with odd (even) indices are allowed. This transition rule is more restricted for AGNRs and completely different from that of ZGNRs. Our TB results are in agreement with first principle calculations which verify the accuracy of our model. The applicability of AGNR/BN as photodetectors is investigated. Our results indicate that due to more allowed transitions compared to conventional GNRs a larger photo current in AGNR/BN structures can be achieved. Two analytical approximations of the discrete energies in ZGNRs are presented. Relations for the wave functions and the energy dispersion show good agreement with those obtained from numerical calculations. Our simple approximation is applicable for a wide range of ZGNR indices from N=6 to N=500. While the results show good agreement for narrow N-ZGNRs, the accuracy increases for wider ZGNRs. The analytical model developed is used to derive optical transition rules. Our model shows that transitions from odd to odd and even to even subband numbers are allowed and that other transitions are forbidden in ZGNRs. The model is applicable for the evaluation of optical properties of ZGNRs, such as dielectric response, absorption coefficient, and energy loss spectrum.
Furthermore, the optical properties of graphene nanoribbon superlattices embedded in boron nitride sheets and the possibility of using such structures as photodetectors are studied. We propose a set of TB parameters for the investigated structures, which yields an excellent agreement with first-principles results. The results indicate that the optical spectrum of a BN-confined AGNR superlattice contains more absorption peaks and allow more optical intersubband transitions compared with a hydrogen passivated superlattice of the same geometry. Employing the NEGF method, the photocurrents and quantum efficiencies are evaluated and compared for both devices at various incident photon energies. Using statistical approach, the role of line-edge roughness on the optical properties of GNR-based superlattices is investigated. The results indicate that the quantum efficiency and photoresponsivity decrease in the presence of line-edge roughness. For hydrogen-passivated superlattices (HSL), induced states appear and increase with the roughness amplitude, which result in the appearance of an additional peak in photocurrent spectrum. In comparison with HSLs, BN superlattice (BNSL) photodetectors exhibit more robust optical properties in the presence of line-edge roughness due to the stable edge atom configuration.
As intersubband transition structures provide small energy gaps suitable for THz, we comprehensively investigated the bandgap-engineering in quantum cascade lasers (QCLs) and detectors (QCDs). A framework has been developed to optimize and engineer the band structure by modifying the structural parameters, including the barrier and well thicknesses, from the perspective of predefined figure of merits. For QCLs, we maximize the optical gain with regard to the instability threshold for MIR/THz short pulse applications. This instability, caused by saturable absorber (SA), leads to passive mode-locking and short pulses in ring cavity QCLs. Optimized structures exhibit a larger optical gain and operate below the instability threshold in comparison with the reference design. The results indicate the instability threshold decreases faster with SA coefficient for mid infrared QCL sample while the terahertz QCL sample still operates below the instability threshold.
Two different active-region designs are investigated. The first one is the original design based on a three quantum well (3QW) active-region separated from the injection/relaxation region by a tunneling barrier, whereas the second one consists of a superlattice (SL) active-region. The SL active-region QCL indicates more stable operation and higher instability threshold. However, the matrix element and lifetimes of the lasing transition, which are the key parameters in linear stability analysis, are proportional to optical gain. The 3QW QCL exhibits larger optical gain at nearly the same wavelength. The optical gain of 3QW structure is maximized by delocalizing the lasing states, while in the SL active-region QCL, there is no significant lifetime variation because of the bound states. However, due to the larger matrix element, better instability condition is achieved for the SL active-region QCL. The dynamics of all the optimized designs above the threshold instability is numerically analyzed by solving the Maxwell-Bloch equations. The results indicate that the lasing instability, which appears as side modes in the optical spectrum, occurs with the increase of the SA coefficient even at low pumping strength. The increase of the SA coefficient and pumping strength reduce the instability threshold, whereas the instability characteristics is broadened by the pumping strength. We applied the developed optimizer to QCDs with responsivity and the bandwidth as figure of merits. Larger responsivities are achieved for optimized QCD structures. However, for larger responsivities the bandwidths of optimized QCDs are increased. The results indicate the excellent performance of our developed optimizer for QCDs.