Discrete quality-factor control in a side-coupled photonic crystal microcavity: evanescent Bloch tunnelling and the finite-cell correction

The resonance moves continuously through the transverse-magnetic band gap with the square of the radius of the defect rod, as first-order perturbation theory predicts for the dielectric area restored to the lattice site.

The quality factor is set instead by the separation between cavity and waveguide, counted in lattice rows: each added row multiplies it by 7.10 at the reference defect radius, whereas moving the defect rod by up to a tenth of a lattice period changes it by less than 7 per cent. The per-row factor follows, at two defect radii and with no adjustable parameter, from the decay of the slowest evanescent Bloch channel of the crystal at the wavevector of the guided mode.