Periodically-Poled Silicon

As a centrosymmetric crystal, bulk silicon lacks the crucial second-order nonlinearity chi⁽²⁾ - the cornerstone of parametric light conversion. Although it has been demonstrated that mechanical stress can create chi⁽²⁾ effects, efficient parametric chi⁽²⁾ processes require a means to achieve phase matching. A powerful approach for efficient parametric conversion is quasi-phase matching (QPM) by periodic poling. However, conventional poling methods are not applicable to silicon because it lacks an intrinsic dipole moment. This paper investigates through numerical simulations that periodic stress gradients along a silicon waveguide can be realized by integrated thin films. Also the stress gradient create chi⁽²⁾ and modulate its sign in a periodic fashion. The structure can lead to efficient generation of mid-wave infrared (MWIR) from commonly available near-infrared (NIR) laser sources. This so-called periodically poled silicon (PePSi) brings the powerful periodic poling capability to silicon, where the excellent material´s properties and mature processing technology can be exploited for chi⁽²⁾ processes. The PePSi design consists of a channel waveguide integrated with two types of silicon nitride stressed films: one with tensile stress and another with compressive stress, periodically deposited along the waveguide. Stress-induced chi⁽²⁾ is found to have oscillatory values from -15 pm/V to +6 pm/V in one period. The QPM-DFG process is numerically investigated using nonlinear Schrodinger equations which incorporates the induced chi⁽²⁾ effect, chi⁽³⁾ effects including Kerr effect and two photon absorption (TPA) and the TPA associated free-carrier plasma effects with its wavelength dependence.