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arXiv · 2609.23328

$f$-2$f$ self-referencing for silicon nitride photonics via a heterogeneously integrated lithium niobate layer

Weichen Fan·Bastian Ruhnke·Camiel Op de Beeck·Giulio Tavani·Anton Vasiliev·Franz Kärtner·Tobias Herr
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Abstract

Heterogeneous integration combines complementary material properties in a single photonic platform. Here, we demonstrate on-chip $f$-2$f$ self-referencing in a heterogeneously integrated Si$_3$N$_4$/LiNbO$_3$ platform, in which supercontinuum generation in a Si$_3$N$_4$ waveguide and second-harmonic generation in a LiNbO$_3$ layer are spatially separated and linked by adiabatic escalator couplers. Pumped by a 1560 nm mode-locked laser with 75 pJ on-chip pulse energy, we detect the carrier-envelope offset frequency with 30 dB signal-to-noise ratio in 300 kHz resolution bandwidth. A comparative measurement confirms that the second harmonic originates in the LiNbO$_3$ layer and indicates opportunity for further improvements through periodic poling. These results establish heterogeneous Si$_3$N$_4$/LiNbO$_3$ integration as a viable building block for self-referenced ultrafast pulse sources in low-loss Si$_3$N$_4$ photonics.

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Weichen Fan, Bastian Ruhnke, Camiel Op de Beeck, Giulio Tavani, Anton Vasiliev, Franz Kärtner, Tobias Herr. 2026-09-20. $f$-2$f$ self-referencing for silicon nitride photonics via a heterogeneously integrated lithium niobate layer. https://arxiv.org/abs/2609.23328

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