Light of wavelength 632.8 nm andp‐polarization is incident on a prism–air gap (varied from 0.7 to 7 &mgr;m)–Al‐GaAs arrangement. Both the photosignal generated by the Schottky diode and the reflectance are measured as a function of the internal angle of incidence in the prism. There is significant, well‐defined enhancement of the photosignal, up to a factor of ∼7.5, associated with two different types of enhanced absorption modes. For air gaps <1.5 &mgr;m there is photosignal enhancement due to an enhanced absorption feature (reflectance dip) that occurs at an angle of incidence just above critical angle in the prism; this feature corresponds to the excitation of a surface plasmon polariton at the Al‐air interface. For air gaps ≳1 &mgr;m there are between one and ten photoresponse peaks at input angles less than the critical angle. The corresponding enhanced absorption features are due to leaky guided wave modes set up in the air gap.