Second-harmonic generation (SHG) is investigated in a planar waveguide geom
etry under conditions for which the Use of a linear grating fabricated on t
op of the waveguide reproduces a photonic-bandgap structure. The fundamenta
l mode of the guide, which coincides with the fundamental pump frequency, i
s tuned at the photonic band-edge resonance in order to enhance field-local
ization effects. However, the linear grating alone is not able to produce b
oth field confinement and phase matching of the second-harmonic-generation
process. Phase matching is obtained with the additional modulation of the n
onlinear susceptibility chi ((2)), as in conventional quasi-phase-matching
schemes. The conversion efficiency achieved with both linear and nonlinear
gratings is orders of magnitude greater than that of a quasi-phase-matched
device of the same length. (C) 2001 Optical Society of America.