Spatial walk-off limits second-harmonic generation in birefringent crystals by separating the fundamental and second-harmonic beams, particularly for tightly focused fields. We show that wavelength-scale periodic modulation of the refractive index can suppress this limitation by reshaping the angular dispersion near an existing birefringent phase-matching point. The modulation couples two second-harmonic propagation directions so that their transverse energy flows cancel, eliminating the leading walk-off contribution while phase matching is recovered on the modified dispersion branch. We derive simple design conditions for this regime and confirm the mechanism with spatially resolved simulations that include pump depletion and grating-induced modification of both frequencies. For representative type-I second-harmonic generation in beta barium borate, the modulated structure can maintain spatial overlap and enhance the interaction at a millimeter scale. The approach is therefore most relevant to compact, walk-off-limited frequency converters and tightly focused beams.

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• Beam optics
• Beam processes
• Nonlinear optics
• Second order nonlinear optical processes
• Nonlinear optical materials
• Nonlocal nonlinear media
• Optical second-harmonic generation