Near- and far-infrared intersubband transitions in polar and non-polar III-nitrides

III-nitrides are promising materials for intersubband devices operating in the near- and far-infrared due to the large conduction band offset (\u3e 1 eV), longitudinal-optical phonon energy (~ 90 meV), and fast intersubband relaxation time (~ 80 fs). However, several challenges impede progress towards the realization of high performance III-nitride optoelectronic devices. To address these challenges, this thesis investigates the optical properties of intersubband transitions in the III-nitrides. The studies were carried out across several III-nitride material combinations, including the relatively well developed c-axis AlGaN/GaN material system, the lattice-matched AlInN/GaN system, and the non-polar m-axis AlGaN/GaN system. The latter systems are relatively unexplored, and promise to alleviate specific material challenges, namely the lattice-mismatch and internal polarization fields associated with the well developed c-axis AlGaN/GaN material system. In the c-axis AlGaN/GaN material system, we study the impact of doping profile on the transition energy, linewidth and magnitude. An optimal doping profile is determined and the results are understood through band structure calculations that consider the influence of many-body effects, interface roughness, and the transition lifetime. We also demonstrate quantum well infrared photodetectors and electromodulators on low-defect GaN-on-sapphire and free-standing GaN substrates, proving the suitability of low-defect substrates for III-nitride devices. The optimization and theoretical understanding of the optical properties, along with the successful demonstration of intersubband devices, is an important contribution to the future design of III-nitride devices. For the lattice-matched AlInN/GaN system, we demonstrate direct intersubband absorption for the first time. However, we find the strength of the intersubband absorption to be much smaller than expected. The poor performance of AlInN/GaN superlattices is attributed to columnar alloy inhomogeneity observed in structural characterization data. The impact of these inhomogeneities on the absorption properties is understood using 3D band structure calculations, and results suggest further optimization of the growth conditions is required. Terahertz intersubband absorption in the non-polar m-axis system is also demonstrated for the first time. The results promise to reduce the design complexity and increase the performance of III-nitride intersubband devices operating in the terahertz region