  This paper assesses intersubband transitions in the 1 to 10 THz frequency
range in nonpolar m-plane GaN/AlGaN multi-quantum-wells deposited on
free-standing semi-insulating GaN substrates. The quantum wells were designed
to contain two confined electronic levels, decoupled from the neighboring
wells. Structural analysis reveals flat and regular quantum wells in the two
perpendicular inplane directions, with high-resolution images showing
inhomogeneities of the Al composition in the barriers along the growth axis. We
do not observe extended structural defects introduced by the epitaxial process.
Low-temperature intersubband absorption from 1.5 to 9 THz is demonstrated,
covering part of the 7 to 10 THz band forbidden to GaAs-based technologies.
