Abstract
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.
Highlights
Silicon nitride, with tunable material composition [1] and well-developed material processing techniques [2], has been experiencing a surge of interest for both linear and nonlinear photonics [3, 4]
We demonstrated here near- and non-degenerate Difference-frequency generation (DFG) on the Si3N4 platform relying on alloptical poling performed using telecommunication band sources
The χe(2ff) grating, optically written to automatically satisfy QPM for the C/L band poling wavelength and its second harmonic (SH), can be efficiently leveraged for DFG towards the middle infrared (mid-IR) given that the adequate dispersion relation is satisfied
Summary
With tunable material composition [1] and well-developed material processing techniques [2], has been experiencing a surge of interest for both linear and nonlinear photonics [3, 4]. The current value of CE is measured in the %/W scale, and our results could be further improved through more advanced dispersion engineering as to enable the use of a lower photon energy pump This first demonstration shows a way of designing new types of integrated sources in silicon photonics. Near-degenerate DFG where ωP,dfg ≈ ωsh, and ωs ≈ ωP,sh satisfies Eq (1) with an idler generated close to the signal (ωs ≈ ωi) and can serve as a confirmation of the possibility to realize reverse processes in optically poled waveguides. The performed work indicates the possibility of the idler generation in the region 1.9 μm using the pump near 840 nm and signal in the C-band range Such dispersion engineering and numerical optimization within a broader search space will unlock a wider band of operation for DFG. Overall, rethinking how dispersion can be utilized as to satisfy multiple processes simultaneously represents a new way of designing mid-IR sources based on DFG while solely relying on waveguides all-optically poled using standard telecommunication laser sources
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