Polarization-resolved second-harmonic spectra are obtained from the resonant modes of a two-dimensional planar
photonic crystal microcavity patterned in a free-standing InP slab. The photonic crystal microcavity is comprised
of a single missing-hole defect in a hexagonal photonic crystal host formed with elliptically-shaped holes. The
cavity supports two orthogonally-polarized resonant modes split by 60 cm<sup>-1</sup>. Sum-frequency data are reported
from the nonlinear interaction of the two coherently excited modes, and the polarization dependence is explained
in terms of the nonlinear susceptibility tensor of the host InP.