Ultra-broadband soliton microcombs in resonantly-coupled microresonators

Soliton microcombs -- equidistant spectral lines generated from nonlinear microresonators -- have endowed integrated photonics with unprecedented precision and parallelization. However, generating broadband microcombs is challenged by the required pump power that scales quadratically with the number of comb lines. Here, we demonstrate that incorporating a resonant coupler passively enhances the effective pump power by several orders of magnitude, thereby expanding the soliton microcomb's usable bandwidth by a factor of 3. Moreover, the resonant coupler enables unconventional tuning dynamics that facilitate thermally robust access to soliton states. Powered by an on-chip laser, the soliton microcomb delivers unprecedented spectral coverage. Our strategy overcomes the power constraints in generating broadband soliton microcombs and paves the way for their deployment in timing, metrology, and communications.