We have designed, fabricated and measured the first plasmon-assisted normal incidence GaN/AlN quantum cascade detector (QCD) making use of the surface plasmon resonance of a two-dimensional nanohole Au array integrated on top of the detector absorption region. The spectral response of the detector at room temperature is peaked at the plasmon resonance of 1.82 mu m. We show that the presence of the nanohole array induces an absolute enhancement of the responsivity by a factor of similar to 30 over that of the bare device at normal incidence and by a factor of 3 with respect to illumination by the 45 degrees polished side facet. We show that this significant improvement arises from two phenomena, namely, the polarization rotation of the impinging light from tangential to normal induced by the plasmonic structure and from the enhancement of the absorption cross-section per quantum well due to the near-field optical intensity of the plasmonic wave. (C)2014 Optical Society of America