• Spatiotemporal QPM, involving both spatial and temporal modulation of nonlinear response, naturally emerges in silicon nitride microresonators through all-optical poling.
• The coherent photogalvanic effect mediates the formation of a traveling space-charge grating, leading to a Doppler-shifted second harmonic.
• This self-organized spatiotemporal QPM expands the understanding and application of phase-matching conditions in nonlinear photonics.

A recent study published in Nature Communications details the observation of self-organized spatiotemporal quasi-phase-matching (QPM) in silicon nitride microresonators. This phenomenon involves the natural emergence of a spatiotemporal QPM grating through all-optical poling. This grating, consisting of concurrent spatial and temporal modulation of the nonlinear response, results in a frequency shift of the generated light. The effect is mediated by the coherent photogalvanic effect, where a traveling space-charge grating self-organizes, affecting both momentum and energy conservation. This discovery expands the possibilities for phase-matching conditions in nonlinear photonics.

• What: Demonstration of self-organized spatiotemporal quasi-phase-matching (QPM) in silicon nitride microresonators using all-optical poling, leading to a Doppler-shifted second harmonic.
• Who: Researchers at École Polytechnique Fédérale de Lausanne and LIGENTEC SA.
• When: Published May 1, 2025; experiments conducted and data analyzed in 2024-2025.
• Where: Research conducted at École Polytechnique Fédérale de Lausanne, Switzerland; Microresonators fabricated by LIGENTEC SA, Switzerland.

The demonstration of self-organized spatiotemporal QPM in silicon nitride microresonators represents a significant advancement in nonlinear photonics. The ability to control and manipulate light at the microscale opens up possibilities for developing novel optical devices and technologies. The observed Doppler shift in the second harmonic provides concrete evidence for the spatiotemporal QPM. The findings contribute to the understanding of photoinduced nonlinear processes in resonant systems and may lead to optimization of frequency converters in amorphous materials.

The study successfully demonstrates the self-organized spatiotemporal QPM in silicon nitride microresonators, offering insights into the dynamics of AOP and its potential for advanced nonlinear photonic applications. The observed frequency shift and the comprehensive model developed provide a foundation for future research and development in integrated χ(2) frequency converters and related technologies. Further optimization can be envisaged by leveraging controls over interaction parameters.