Mode-locked mid-infrared frequency combs in a silicon microresonator

Mid-infrared (mid-IR) frequency combs have broad applications in molecular spectroscopy and chemical/biological sensing. Recently developed microresonator-based combs in this wavelength regime could enable portable and robust devices using a single-frequency pump field. Here, we demonstrate a mode-locked microresonator-based frequency comb in the mid-IR spanning 2.4–4.3 μm. We observe high pump-to-comb conversion efficiency, in which 40% of the pump power is converted to the output comb power. Utilizing an integrated PIN structure allows for tuning the silicon microresonator and controlling cavity soliton formation via free-carrier detection and control. Our results significantly advance microresonator-based comb technology toward a portable and robust mid-IR spectroscopic device that operates at low pump powers.