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We present a photonically driven on-chip millimeter wave (mmWave) source enabled by the heterogeneous integration of a high-speed InGaAs/InP photodiode and silicon nitride (Si₃N₄) microcavity solitons. The chip delivers mmWaves with −18dBm of electrical power at a frequency of 98 GHz with kHz-class linewidth and low phase noise and marks a significant advancement in on-chip photonic mmWave source performance. This breakthrough not only demonstrates capabilities of heterogeneous photonic integration but also offers a compact and scalable solution for future low-noise mmWave applications in communications and sensing technologies. 

Abstract The generation of ultra-low-noise microwave and mmWave in miniaturized, chip-based platforms can transform communication, radar and sensing systems^1–3. Optical frequency division that leverages optical references and optical frequency combs has emerged as a powerful technique to generate microwaves with superior spectral purity than any other approaches^4–7. Here we demonstrate a miniaturized optical frequency division system that can potentially transfer the approach to a complementary metal-oxide-semiconductor-compatible integrated photonic platform. Phase stability is provided by a large mode volume, planar-waveguide-based optical reference coil cavity^8,9and is divided down from optical to mmWave frequency by using soliton microcombs generated in a waveguide-coupled microresonator^10–12. Besides achieving record-low phase noise for integrated photonic mmWave oscillators, these devices can be heterogeneously integrated with semiconductor lasers, amplifiers and photodiodes, holding the potential of large-volume, low-cost manufacturing for fundamental and mass-market applications¹³.