Nonlinear optics in photonics nanowires

Nonlinear optical processes benefit from high field intensities and long interaction lengths between light and the media within which it propagates. In bulk media, high optical intensities can be achieved by tightly focusing the light, but this leads to reduced interaction lengths, which usually results in no net efficiency increase. Waveguiding geometries, such as optical fibers, enable high optical intensities over extended interaction lengths as long as kmpsilas, but dispersion and loss can act to cap the effective interaction length for nonlinear processes. Integrated, sub-micron silicon waveguides, on the other hand, have been shown to enable nonlinear processes at the chip scale due to the fact that their engineered dimensions increases the effective nonlinearity. Such waveguides, when compared to other Si and silica waveguides, offer exceptional confinement significantly below the wavelength of light due to the high index contrast between the Si core and the SiO₂ cladding. As a result of the strong confinement and the large nonlinear refractive index of Si, the effective nonlinearity gamma_Si ~ 300 (Wmiddotcm)^-1 of Si nanowaveguides is extremely high and is typically more than 4,000 times that of conventional optical fiber. Driven by the 3 orders of magnitude increased effective nonlinearity, silicon structures can now be envisioned to encompass complete optical-domain processing functionalities within a single integrated platform, with complexities that were simply unimaginable in prior material systems.