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

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.