  Multiphoton absorption of wide band-gap semiconductors has shown great
prospects in many fundamental researches and practical applications. With
intensity-modulated femtosecond lasers by acousto-optic frequency shifters,
photocurrents and yellow luminescence induced by two-photon absorption of InGaN
and GaN photodetectors are investigated experimentally. Photocurrent from InGaN
detector shows nearly perfect quadratic dependence on excitation intensity,
while that in GaN detector shows cubic and higher order dependence. Yellow
luminescence from both detectors show sub-quadratic dependence on excitation
intensity. Highly nonlinear photocurrent from GaN is ascribed to absorption of
additional photons by long-lived electrons in traps and impurity bands. Our
investigation indicates that InGaN can serve as a superior detector for
multiphoton absorption, absent of linear and higher order process, while GaN,
which suffers from absorption by trapped electrons and impurity bands, must be
used with caution.
