Second-order nonlinear silicon photonics

Abstract

Silicon photonics is a relatively recent research field1–4 that aims to substitute electrons with photons as the carriers of information in devices such as planar lightwave circuits. The technology marries the device electronic performance of traditional semiconductor materials with the speed and bandwidth offered by light. Although in the linear regime a silicon photonic circuit can efficiently guide, modulate, and detect IR light, in the nonlinear (i.e., ultrafast, ultrahigh capacity) regime the situation is more complex. In particular, nonlinear second-order optical phenomena (e.g., the electro-optic effect) are needed to generate, convert, and modulate light at the desired speeds inside complex networks of silicon wires. Nonlinear silicon photonics has so far exploited the natural third-order optical nonlinearities— .3/—of silicon (e.g., optical-field-induced birefringence) for use in security, military, and biosensing applications. Consequently, substantial research effort has gone into enhancing these properties and to applying them to novel all-optical photonic devices.5 A few years ago, a second-order nonlinear optical susceptibility— .2/—was reported for a so-called Mach-Zehnder interferometer based on a strained silicon photonic band-gap waveguide.6 While .2/ is prohibited by the centro-symmetry of the silicon crystal itself, being able to induce it in silicon would provide access to a wide range of nonlinear optical devices at power densities intrinsically lower than those allowed by mere third-order effects. Our recent contribution7 to this field has unambiguously shown, both theoretically and experimentally, that silicon can be efficiently stressed to become a good second-order nonlinear crystal. We computed the second-order nonlinear optical response from strained bulk silicon ab initio using time-dependent Figure 1. Simulated second-order nonlinear optical susceptibility— .2/—for weakly strained silicon (Si) (black), medium-strained Si (yellow), and heavily strained Si (red). The hierarchy of the increasing strain in the Si models is indicated quantitatively by the values of pressure (HY) and shear stress (SS) as reported in the legend. w: Energy of the pump photons.