Efficient(∼1&percent;)second harmonic generation, resonantly enhanced near&lgr;=8.6 &mgr;m,has been observed in asymmetric double multi-quantum well structures. We used (i) edge-emitting waveguide geometry where the phase matching was achieved by incorporating a separate multiple quantum well region which modifies (via the Kramers–Kronig relation) the dispersion of light and (ii)45°wedge multi-bounce geometry where the phases of second harmonic waves generated at sequential bounces were synchronized by changing the angle of incidence. ©1998 American Institute of Physics.