Tunable photo-induced second-harmonic generation in a mode-engineered silicon nitride microresonator.

All-optical poling enables reconfigurable and efficient quasi-phase-matching for second-order parametric frequency conversion in silicon nitride integrated photonics. Here, we report broadly tunable milliwatt-level second-harmonic generation in a small free spectral range silicon nitride microresonator, where the pump and its second-harmonic are both always on the fundamental mode. By carefully engineering the light coupling region between the bus and microresonator, we simultaneously achieve critical coupling of the pump as well as efficient extraction of second-harmonic light from the cavity. Thermal tuning of second-harmonic generation is demonstrated with an integrated heater in a frequency grid of 47 GHz over a 10 nm band.

Linear Electro-optic Effect in Silicon Nitride Waveguides Enabled by Electric-Field Poling.

Stoichiometric silicon nitride (Si3N4) is one of the most mature integrated photonic platforms for linear and nonlinear optical applications on-chip. However, because it is a centrosymmetric material, second-order nonlinear processes are inherently not available in Si3N4, limiting its use for multiple classical and quantum applications. In this work, we implement thermally assisted electric-field poling, which allows charge carrier separation in the waveguide core, leading to a depletion zone formation and the inscription of a strong electric field reaching 20 V/µm. The latter results in an effective second-order susceptibility (χ(2)) inside the Si3N4 waveguide, making linear electro-optic modulation accessible on the platform for the first time. We develop a numerical model for simulating the poling process inside the waveguide and use it to calculate the diffusion coefficient and the concentration of the charge carriers responsible for the field formation. The charge carrier concentration, as well as the waveguide core size, is found to play a significant role in determining the achievable effective nonlinearity experienced by the optical mode inside the waveguide. Current findings establish a strong groundwork for further advancement of χ(2)-based devices on Si3N4.

Difference-frequency generation in optically poled silicon nitride waveguides.

Difference-frequency generation (DFG) is elemental for nonlinear parametric processes such as optical parametric oscillation and is instrumental for generating coherent light at long wavelengths, especially in the middle infrared. Second-order nonlinear frequency conversion processes like DFG require a second-order susceptibility χ (2), which is absent in centrosymmetric materials, e.g. silicon-based platforms. All-optical poling is a versatile method for inducing an effective χ (2) in centrosymmetric materials through periodic self-organization of charges. Such all-optically inscribed grating can compensate for the absence of the inherent second-order nonlinearity in integrated photonics platforms. Relying on this induced effective χ (2) in stoichiometric silicon nitride (Si3N4) waveguides, second-order nonlinear frequency conversion processes, such as second-harmonic generation, were previously demonstrated. However up to now, DFG remained out of reach. Here, we report both near- and non-degenerate DFG in all-optically poled Si3N4 waveguides. Exploiting dispersion engineering, particularly rethinking how dispersion can be leveraged to satisfy multiple processes simultaneously, we unlock nonlinear frequency conversion near 2 µm relying on all-optical poling at telecommunication wavelengths. The experimental results are in excellent agreement with theoretically predicted behaviours, validating our approach and opening the way for the design of new types of integrated sources in silicon photonics.