
Bibliography 215
a susceptibility χ
(3)
δ(kz −jπ)/N. The medium susceptibility χ
(NL)
can thus
be represented by:
χ
(NL)
(z) =
N
j=1
χ
(3)
N
δ(kz − jπ) (3.191)
Insert this susceptibility in Eq. (3.159), and average (integrate) over the
thickness of the medium, to find an average third-order polarization.
Show that the coupling term in this polarization, which in an homoge-
neous medium would average to zero, has now a contribution of the same
order as the other terms.
It is interesting to contrast the result from the MQW with that of the
homogeneous medium. First, the nonlinear polarization is larger in the
case of the MQW sample: if the fields are equal, P
(3)
1
=