Nonlinearity, mode dispersion and periodicity are intimately tied with the spon- taneous generation of photon pairs in a nanowaveguide. We explore how the combination of mode hybridization and nonlinearity structuring can be used to satisfy phasematching and remove frequency entanglement from spontaneous down conversion states. The first device we study is a heralded single photon source based on the evanescent coupling of a silica microfibre and a lithium niobate waveguide. We predict this device to be capable of producing heralded single photons with a purity of 0.95 over a total device length of only 200μm. This is possible due the dispersion control introduced by the mode hybridisation and the nonlinear control introduced by an integrated, proton exchanged, channel. We show how, in a spontaneous four wave mixing source of single photons, effective modulation of the nonlinearity can be introduced using a two pump setup where one of the pumps undergoes discrete diffraction. Using an example system of a silicon rib waveguide we show how purities of 0.997 can be generated over a 1000μm device length. Nonlinearity engineering is not limited to localised modulation. Rapid peri- odic nonlinear modulation also has interesting consequences. We study how quasi-phasematched waveguides are an example of the more universal phe- nomenon of parametric resonance. We then develop a spontaneous down con- iiiversion model which can account for some of the more exotic features found in the classical Mathieu analysis known as 'Arnold' tongues and in so doing predict new regions of phasematching. Finally, we investigate how beating between phase-mismatched terms of dif- ferent pump modes at the same frequency can act like a nonlinear grating. In this case we predict geometric parametric resonance and a larger family of Arnold tongues. We then introduce our multimode scheme to two different types of real world waveguide and two types of mode superposition: different order modes of the same polarisation in a step index fibre and cross polarised modes in a rect- angular silicon waveguide. We find that while these parametric processes can be phasematched they are currently limited by the required continuous wave power requirements.