Band structure, strain, and polarization engineering of nitride heterostructures open unparalleled opportunities for quantum sensing in the infrared. Intersubband absorption and photoluminescence are employed to correlate structure with optical properties of nonpolar strain-balanced InGaN/AlGaN nanostructures grown by molecular-beam epitaxy. Mid-infrared intersubband transitions in m-plane (In)AlxGa1-xN/In0.16Ga0.84N (0.19≤x≤0.3) multi-quantum wells were observed for the first time in the range of 3.4-5.1 μm (244-360 meV). Direct and attenuated total-reflection infrared absorption measurements are interpreted using structural information revealed by high-resolution x-ray diffraction and transmission electron microanalysis. The experimental intersubband energies are better reproduced by calculations using the local-density approximation than the Hartree-Fock approximation for the exchange-correlation correction. The effect of charge density, quantum well width, and barrier alloy composition on the intersubband transition energy was examined to evaluate the potential of this material for practical infrared applications.Temperature-dependent continuous-wave and time-resolved photoluminescence (TRPL) measurements are also investigated to probe carrier localization and recombination in m-plane InGaN/AlGaN quantum wells. Average localization depths of 21 meV and 40 meV were estimated for the undoped and doped structures, respectively. Using TRPL, dual localization centers were identified in undoped structures, while a single type of localization centers was found in doped structures. At 2 K, a fast decay time of approximately 0.3ns was measured for both undoped and doped structures, while a longer decay time of 2.2 ns was found only for the undoped sample. TRPL in magnetic field was explored to examine the effect of doping sheets on carrier dynamics. Keywords: nitride semiconductors, intersubband absorption, photoluminescence
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