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Photonic crystal slabs combine a slab waveguide with an in-plane photonic crystal. Light is then confined in-plane by the photonic crystal and out-of-plane by the slab waveguide. The etched structures will cause light to scatter out of the waveguide plane. We studied the out-of-plane scattering losses using a two-dimensional approximation of this three-dimensional structure, with etched slots instead of holes. Our simulation techniques include mode expansion with perfectly matched layer absorbing boundary conditions. We show that the losses increase with higher index contrast, but that with very high-index contrasts light can be coupled into lossless Bloch modes.