  Two-photon polymerization has been demonstrated as an effective technique to
define embedded defects in three-dimensional photonic crystals. In this work we
demonstrate the ability to precisely position embedded defects with respect to
the lattice of three-dimensional photonic crystals by imaging the structure
concurrently with two-photon writing. Defects are written with near-perfect
lattice registration and at specifically defined depths within the crystal. The
importance of precise defect position is demonstrated by investigating the
optical properties of embedded planar cavities written in a photonic crystal.
The experimental data is compared to spectra calculated using the Scalar Wave
Approximation (SWA) which further demonstrates the importance of defect
placement.
