Ultrafast broadband soliton microcomb laser

Abstract: Optical frequency combs, defined by their ultrafast timescale and profound coherence, have become a foundational technology in science and technology. Their chip-scale miniaturization offers great promise for refining a wide range of applications from metrology to advanced computing. Existing microcomb technologies, however, are constrained by fundamental limitations inherent to their mode-locking mechanisms. Specifically, the carrier dynamics of saturable absorption in semiconductor mode-locked lasers and the external laser pumping required for dissipative Kerr soliton generation lead to degraded comb performance and significant operational complexity. Here, we introduce an on-chip mode-locking approach that resolves these challenges. By leveraging the intriguing property of a thin-film lithium niobate photonic circuit integrated with semiconductor optical gain, the chip-scale soliton laser directly emits background-free, ultrashort soliton pulses with a 3-dB bandwidth exceeding 3.4THz ($<$90fs pulse width) and an ultra-narrow comb linewidth down to 53 Hz. Our approach dramatically simplifies bright-soliton generation, featuring fully electric pumping, stable turn-key operation, near-unity optical efficiency, superior long-term stability, and remarkably low soliton generation threshold (1V, 75mA) that could be powered with a standard alkaline battery. The demonstrated soliton laser represents a paradigm shift in chip-scale frequency comb technology, which is expected to have profound impacts across a wide range of microcomb applications.