Parasitic Photon-Pair Suppression via Photonic Stop-Band Engineering
  We calculate that an appropriate modification of the field associated with only one of the photons of a photon pair can suppress generation of the pair entirely. From this general result, we develop a method for suppressing the generation of undesired photon pairs utilizing photonic stop bands. For a third-order nonlinear optical source of frequency-degenerate photons we calculate the modified frequency spectrum (joint spectral intensity) and show a significant increase in a standard metric, the coincidence to accidental ratio. These results open a new avenue for photon-pair frequency correlation engineering. 
L. G. Helt, Agata M. Branczyk, Marco Liscidini, M. J. Steel
quant-ph physics.optics