  We present design guidelines for realizing side-coupled photonic crystal
nanobeam (PCN) cavities with efficient coupling to low order resonances through
adjusting the overlap of the PCN cavity mode and a feeding bus waveguide in
both physical and k-space. We show that optimal side-coupled configurations
function at a non-zero k-vector offset between the bus waveguide and PCN cavity
modes. The straightforward design of side-coupled PCNs with high contrast
resonances opens the door to their practical implementation in multiplexed,
on-chip photonic devices.
