  In materials that do not allow birefringent phase-matching or periodic poling
we propose to use waveguides to exploit the tensor structure of the second
order nonlinearity for quasi-phase matching of nonlinear interactions. In
particular, we concentrate on curved waveguides in which the interplay between
the propagation direction, electric field polarizations and the nonlinearity
can change the strength and sign of the nonlinear interaction periodically to
achieve quasi-phase matching.
