Fig. 1. Heterogeneous, low-loss, Si₃N₄–LiNbO₃ photonic integrated platform for fast tunable self-injection-locked lasers.

a, Schematic illustration of the heterogeneous Si₃N₄–LiNbO₃ platform realized by heterogeneous integration of a 4″ (100 mm) thin-film LiNbO₃ wafer onto a 4″ Si₃N₄ wafer, with cross-sections of both wafers. b, False-colour SEM image of a heterogeneous Si₃N₄–LiNbO₃ waveguide cross-section. The original SEM image data are shown in Extended Data Fig. 1. Inset: a finite-difference time-domain simulation of the spatial distribution of the hybrid transverse electric mode’s electric-field amplitude with 12% participation in LiNbO₃, electric-field maximum is coloured in red and minimum in blue. c, Schematic illustration of the self-injection locking principle. The optical path is marked with the dashed red line. The red arrow shows the forward optical wave and the blue arrow shows the reflected optical wave from a microresonator. Laser wavelength tuning is achieved by applying a voltage signal (for example, a linear ramp) on the tungsten electrodes. The structures in yellow are the tungsten electrodes. d, Photo of the set-up with a DFB laser butt-coupled to a heterogeneous Si₃N₄–LiNbO₃ chip (sample D67_01b C16 WG 4.2). A pair of probes touch the electrodes for electro-optic modulation, and a lensed fibre collects the output radiation.