Extreme-bandwidth valley photonic metamaterials approaching Dirac velocity.

Valley photonics supports edge transport robust to sharp bends and disorder. However, conventional valley states, typically developed within a quantized valley Chern number framework, remain limited group velocity and bandwidth, restricting their applicability in high-speed, high-capacity on-chip transport. By relaxing the use of a sizable valley Chern number as the primary constraint in performance optimization, we leverage Dirac-mass engineering and electromagnetic mode control to simultaneously enhance the bandwidth and group velocity of valley transport. By combining extreme inversion-symmetry breaking with effective suppression of mode mixing, the platform restores an approximately scale-invariant interfacial mass profile, enabling both a maximized valley bandgap and a near-Dirac group velocity. Near-field measurements directly confirm unidirectional valley propagation with deep-subwavelength confinement and robustness against sharp bends. The system supports estimated error-free transmission at 2 Gbps and an open eye at symbol rates approaching the carrier frequency, with a carrier-normalized symbol rate exceeding those of previously reported valley photonic platforms by more than one order of magnitude. These findings establish a practical design route for high-performance valley photonics, offering ultrabroadband, high-velocity, and compact signal transport with potential applications in high-capacity photonic interconnect.