In a study concerned with second-harmonic generation at grating couple
rs, Kull et al. [Opt. Lett. 16, 1930 (1991)] observed a curious phenom
enon in which the second-harmonic efficiency exhibits dips, instead of
peaks, when the angle of incidence of the pump beam is scanned throug
h its resonant value. This surprising result has not been explained. I
t is our aim to show that this effect is closely related to the existe
nce of nonlinear zeros.