  Significantly enhanced second order nonlinearity resulting from local
symmetry breaking in domain walls is reported. By this new giant nonlinearity,
novel hexagonally, conically and trigonally distributed second harmonic beams
closely related to domain wall patterns are generated without contribution of
reciprocal vectors. Experimental results show that such nonlinearities have
made important impact on quasi-phase-matching technique and electro-optic Solc
filter, which needs careful reconsiderations.
