Millimetre-wave comb generated by an optical microcomb.

Millimetre-wave baseband combs are relevant to wireless links, sensing, and device characterisation. Direct broadband generation from an optical microcomb has remained elusive because of challenges in photoconductive conversion. Here, we demonstrate a 50 GHz repetition-rate millimetre-wave baseband comb covering the sub-THz region, generated by photoconductive conversion of a laser-cavity-soliton microcomb. The microresonator-filtered source combines low free-running phase noise and high spectral efficiency, enabling coherent conversion without additional optical amplification to single-cycle, carrier-envelope-offset-free waveforms with multiple harmonics directly observed in the converted output. The source supports free-running time-domain spectroscopy over optical path delays above 8 m, and its lines are driven by the optical repetition-rate harmonics with traceable improvement of synchronisation performance upon external locking. In addition, multisoliton operation reshapes the harmonic envelope while preserving the common line spacing, providing a simple route to source-level spectral control in the sub-THz region.