Design, fabrication, and characterization of quantum cascade terahertz emitters

In an effort to develop a compact coherent source in the terahertz (THz) frequency range, a phonon mediated intersubband scheme using GaAs/Al _xGa_1−xAs quantum wells is proposed and THz emission is experimentally demonstrated. The emitters were designed and optimized after computing all the relevant scattering mechanisms and carrier kinetics. The concept of wavefunction engineering when implemented provides a paradigm for the efficient design of such structures. In this context, interface phonon modes in quantum well systems were tailored to improve device performance and this dissertation reports the first ever demonstration of interface phonon mediated quantum cascade THz emission.; The THz emitters were fabricated using Molecular Beam Epitaxy (MBE) and the emission spectrum was characterized using a Fourier Transform Infrared Spectrometer in the step scan mode. Emission at 4.2 THz (17.5 meV) and 2.9 THz (12 meV) with full width at half maximum (FWHM) of 2.01 meV and 0.75 meV was observed from samples, S00-035 and S00-045 respectively at T = 10 K. These emission peaks agree very well with the designed values of 16 meV and 11.8 meV for the two structures. Dual frequency emission has also been observed from S00-045 at 2.9 THz and 1.45 THz. Intersubband absorption measurements were performed to estimate the interface phonon-mediated lifetimes of subbands in quantum wells. The results obtained experimentally (0.83 ps) compare very well with our theoretically predicted value (0.75ps).