• A spatiotemporal QPM grating is shown to self-organize in silicon nitride microresonators through all-optical poling.
• The coherent photogalvanic effect mediates a traveling space-charge grating, impacting momentum and energy conservation.
• The observation of photoinduced spatiotemporal QPM broadens the understanding of phase-matching conditions in nonlinear photonics.

Researchers have discovered a novel phenomenon in silicon nitride microresonators: the self-organized formation of spatiotemporal quasi-phase-matching (QPM) gratings. This process, driven by all-optical poling and mediated by the coherent photogalvanic effect, results in a traveling space-charge grating. This grating simultaneously modulates both the spatial and temporal properties of light, leading to a Doppler-shifted second harmonic generation. This discovery expands the scope of phase-matching conditions in nonlinear photonics and provides new insights into resonant all-optical poling.

• What: Self-organized spatiotemporal quasi-phase-matching (QPM) in silicon nitride microresonators, resulting in a Doppler-shifted second harmonic.
• Who: Researchers from École Polytechnique Fédérale de Lausanne and LIGENTEC SA.
• When: Research published May 1, 2025, with data collected and analyzed prior to this date.
• Where: Experiments conducted in a laboratory setting using silicon nitride microresonators fabricated by LIGENTEC.

The study presents a significant advancement in nonlinear photonics by demonstrating the self-organized formation of spatiotemporal QPM in microresonators. The traveling χ(2) grating, resulting from the coherent photogalvanic effect, introduces a Doppler shift to the generated second harmonic. This phenomenon opens new avenues for controlling and manipulating light at the microscale, with potential applications in frequency conversion, quantum optics, and advanced photonic devices. The theoretical model developed in the study provides a foundation for understanding and optimizing these processes.

The research provides compelling evidence for the existence of self-organized spatiotemporal QPM in silicon nitride microresonators. The observed Doppler frequency shift confirms the traveling-wave nature of the photoinduced χ(2) grating. This discovery not only enhances the fundamental understanding of nonlinear processes in microresonators but also paves the way for novel applications in integrated photonics. Future work may focus on optimizing the interaction parameters and exploring the potential of this phenomenon in advanced photonic devices and systems.