Quasi-phase-matching (QPM) has become one of the most common approaches for increasing the efficiency of nonlinear three-wave mixing processes in integrated photonic circuits. Here, we provide a study of dispersion engineering of QPM second-harmonic (SH) generation in stoichiometric silicon nitride ( Si 3 N 4 ) waveguides. We apply waveguide design and lithographic control in combination with the all-optical poling technique to study the QPM properties and shape the waveguide dispersion for broadband spectral conversion efficiency inside Si 3 N 4 waveguides. By meeting the requirements for maximal bandwidth of the conversion efficiency spectrum, we demonstrate that group-velocity matching of the pump and SH is simultaneously satisfied, resulting in efficient SH generation from ultrashort optical pulses. The latter is employed for retrieving a carrier-envelope-offset frequency of a frequency comb by using an f − 2 f interferometric technique, where supercontinuum and SH of a femtosecond pulse are generated in Si 3 N 4 waveguides. Finally, we show that the waveguide dispersion determines the QPM wavelength variation magnitude and sign due to the thermo-optic effect.

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 (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.

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 (Si 3 N 4 ) 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 Si 3 N 4 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 Ezgi Sahin and Boris Zabelich contributed equally to this work.

Stoichiometric silicon nitride (Si 3 N 4 ) constitutes a mature platform for integrated photonics. Its pertinent properties, including wide transparency window from the visible to the mid-IR, low propagation loss, and high third-order nonlinearity, are exploited in many linear and nonlinear applications. However, due to the centrosymmetric nature of the Si 3 N 4 , the absence of the second-order susceptibility (χ (2) ) impedes a realization of three-wave mixing processes as well as the linear electro-optic effect, relevant for many applications on an optical chip. Here, we implement the electric-field poling technique to induce the effective χ (2) inside a Si 3 N 4 waveguide, thus enabling the linear electro-optic modulation. Using numerical simulations, we estimated the concentration and the diffusion coefficient of the charges responsible for the space-charge electric field formation. In addition, the DC third-order susceptibility of Si 3 N 4 previously unknown in the literature is measured using a free-space Mach-Zehnder interferometer.