Fig. 1: Concept of inducing a phase shift to the pump mode for power-efficient comb generation.

a, Soliton generation in a single microring resonator fabricated on-chip, whose separation between resonances is defined by the cavity length and the anomalous-dispersion of the waveguides (i). The microring is pumped with a CW laser (ii), with the laser frequency effectively red-detuned from cavity resonance, as illustrated by the dashed line in (i). Such configuration can exhibit the generation of a temporal soliton, whose power distribution is displayed on top of the microring. Due to the high red-detuning, much of the CW laser bypasses the cavity, resulting in an output spectral envelope (iii) with a strong CW component and low comb power¹⁷. The y-axes in (ii) and (iii) have 20 dB div^–1 scaling. b, The same microring now has a mechanism that causes a constant phase shift each roundtrip to the pump mode only, for example, by inducing a mode-crossing using linear coupling to a small auxiliary cavity. This causes the pump resonance to appear shifted while other resonances remain in the same place (i). The pump can now be shifted further towards the red side while still coupling ample amount of power into the cavity. This results in higher soliton power and comb power (iii) while operating with a CW laser. c, A representative image of a fabricated photonic molecule taken with a scanning electron microscope. The white tracks are metallic heaters. d,e, Illustration of the modal distribution of the two cavities, where the mismatch in FSR allows a strong mode-shift to be applied to one longitudinal mode at a time (d). This is reflected in the dispersion measurement (e), which shows a relatively smooth anomalous profile of the main cavity with a strong shift at the pump mode (marked by the arrow) induced by coupling to the auxiliary cavity. f, Shows that the mode-crossing induced by the auxiliary ring can be controlled by tuning the auxiliary heater voltage (V_aux) and through the main heater voltage (set to 0 V and 4 V).