Out-of-plane losses are the major issue in the integration of two-dimensional photonic crystal devices in photonic integrated circuits. In this paper, we show that the out-of-plane losses of pillar-based photonic crystal waveguides can be vastly reduced, even for pillars with a lowvertical index contrast, such as in InP/In-GaAsP/InP technology. These low losses are obtained by creating confinement between the pillars with a polymer layer stack. We show that the spatial frequency component of the Bloch mode in the first Brillouin zone (i.e., the component inside the light cone), is significantly suppressed by the optimized polymer layer stack. © 2009 IEEE.