The nonlinear response of any medium can be enhanced by focusing the light to a high intensity while at the same time, increasing the length of interaction between the light and the medium. The hollow core of these photonic band-gap fibers is an attractive site for placing nonlinear materials in order to attain maximum light-matter interaction. This can be beneficial in two ways: 1) it can evoke a stronger response from weak nonlinear materials and 2) for strong nonlinear materials it can push the power thresholds for nonlinear interactions towards lower limits. The figure on the right illustrates the advantage such waveguide geometries have over bulk geometries using a focusing lens. For a review of nonlinear optics in hollow-core band-gap fibers [Opt. Express 16, 5035-5047 (2008)].