  We present a full formalism for the calculation of the linear and
second-order optical response for semiconductors and insulators. The
expressions for the optical susceptibilities are derived within perturbation
theory. As a starting point a brief background of the single and many particle
Hamiltonians and operators is provided. As an example we report calculations of
the linear and nonlinear optical properties of the mono-layer InP/GaP (110)
superlattice. The features in the linear optical spectra are identified to be
coming from various band combinations. The main features in the second-order
optical spectra are analyzed in terms of resonances of peaks in linear optical
spectra. With the help of the strain corrected effective-medium-model the
interface selectivity of the second-order optical properties is highlighted.
