Ultrafast intersubband transitions in GaN/AlGaN heterostructures at ≈1.55-μm wavelength

We present a study of the electron relaxation times in different GaN/AlGaN multiple quantum well heterostructure samples. In particular, a comparative study of room temperature intersubband electron scattering lifetimes in a variety of single and coupled quantum well samples is presented for different excitation powers and wavelengths. All samples were grown by molecular beam epitaxy on sapphire substrate. Typical quantum well widths of similar to10 to 15 Angstrom were used together with AlN bulk and superlattice barriers with an AlN-mole fraction of similar to0.65 and similar to0.9, respectively. In absorption measurements, the single quantum well samples show intersubband absorption peaks ranging from similar to1.4 to similar to1.7 mum wavelength. Two absorption peaks at similar to1.75 and similar to2.45 mum were measured for an asymmetric coupled double quantum well structure. The intersubband electron scattering lifetimes were determined with time resolved pump-probe measurements using pump and probe beams with a pulse width of similar to130 fs and wavelengths of similar to1.55 and similar to1.7 mum, respectively. For the single quantum well samples, intersubband scattering lifetimes of similar to260 to similar to300 fs have been measured. For the upper excited state of the coupled double quantum well, a lifetime of similar to230 fs has been found.