Hybrid light-matter quasiparticles, or polaritons, are foundational to next-generation photonic and quantum technologies; however, their practical application is strictly limited by a fundamental trade-off between interaction strength and optical loss. Here, we overcome this limitation using a scalable semiconductor platform that achieves ultrastrong coupling (USC) between surface plasmons and lattice phonons with exceptional coherence. By fabricating epsilon-near-zero (ENZ) nanocavities directly into high-quality crystalline III–V semiconductors, we preserve the material’s intrinsic long-range atomic order, enabling a pristine phononic response. This “crystal advantage” yields a normalized coupling strength (g/ω0) of up to 0.28 and a cooperativity (2g/γavg) exceeding 14─a 7-fold improvement over previous systems based on disordered materials. Our top-down fabrication approach compresses the optical mode volume by 5 orders of magnitude below the diffraction limit while remaining compatible with mature 80 nm-node semiconductor manufacturing processes. Furthermore, we demonstrate the platform’s versatility by realizing the simultaneous ultrastrong coupling of a single plasmon mode to multiple distinct phonon modes within a semiconductor heterostructure. This work establishes a new paradigm for low-loss on-chip polaritonics, opening avenues for mass-producible terahertz devices and the exploration of many-body quantum phenomena.
Yu et al. (Wed,) studied this question.