Intersubband resonant enhancement of second-harmonic generation in GaN/AlN quantum wells

We report on the observation of resonant enhancement by intersubband transitions of the second-harmonic generation of 1 mum radiation in GaN/AlN quantum wells grown on AlN/c-sapphire templates. The quantum wells with a nominal well thickness of 10 monolayers have been investigated in terms of intersubband linear and nonlinear optical properties. A strong increase of the second harmonic conversion is observed at a pump wavelength of 2 mum, which is attributed to double-resonance enhancement of the nonlinear susceptibility by intersubband transitions. The second-order susceptibility at resonance is of the order of 114 pm/V in good agreement with calculations and corresponds to a factor of 4 enhancement with respect to bulk GaN.