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Optical parametric gain in CMOS-compatible photonic crystal waveguides

Main complementary metal-oxide-semiconductor (CMOS)-compatible photonics platforms, i.e., silicon and silicon nitride, rely on Kerr nonlinearities for near-instantaneous modulation of optical signals. Kerr nonlinearities emerge due to the third-order susceptibility (χ(3)) and lead to several nonlinear optical processes, including the four-wave mixing (FWM). FWM is the underlying process of many applications, such as parametric amplification, optical sampling, all-optical wavelength conversion, and demultiplexing and therefore has an indispensable role in optical signal processing. The full potential of on-chip nonlinear processes can be efficiently unlocked by tuning the light-matter interactions at a greater extent via manipulating both material and structural properties of integrated photonic devices. We employ photonic crystal waveguides (PhCWgs) to engineer the structural properties of a waveguide by introducing two-dimensional periodicity in the plane of light propagation. In addition to structural engineering, the chosen material platform aims to bypass the limitations of existing CMOS-compatible platforms: the compositionally engineered ultra-silicon-rich nitride (USRN : Si7N3), eliminates two-photon absorption at 1.55 μm and possesses an order of magnitude higher Kerr nonlinearity compared to stoichiometric silicon nitride at telecommunication wavelengths. Here, we present four-wave mixing in a USRN-based, CMOS-compatible, PhCWg leading to on/off optical parametric signal gain reaching 3 dB, and a large instantaneous idler conversion efficiency of −1 dB experimentally. Enhancement of the Kerr nonlinearity in the presence of a sizable and near-constant group index allows us to demonstrate a large on/off gain per unit length of 333 dB/cm on an ultra-compact, 97 μm-long PhCWg.