Unlocking dual-spin achromatic meta-optics with hybrid-phase dispersion engineering

Broadband achromatic wavefront control, a cornerstone of next-generation photonic systems that supports full-color imaging, multi-spectral sensing, has seen important progress reported in PhotoniX by the research group led by Professor Yijun Feng and Professor Ke Chen from Nanjing University. They proposed a hybrid-phase cooperative dispersion-engineering strategy that combines Aharonov–Anandan (AA) and Pancharatnam–Berry (PB) geometric phases in a single-layer metasurface to unlock independent, dual-spin achromatic wavefront control.

Dispersion is an intrinsic property of electromagnetic waves. As both a blessing and a curse, it enables useful wavelength-dependent functionalities yet also introduces chromatic aberrations that make steering angles drift, focal spots shift, and spatial fidelity degrade as bandwidth expands. Metasurfaces, consisting of planar arrays of engineered subwavelength meta-atoms, have emerged as a powerful platform to reshape wavefronts. However, most achromatic metasurface strategies remain effectively confined to a single spin channel, or they treat the two spin channels as independent, forcing them to share the same dispersion response. Consequently, truly independent dual-spin control of phase and group delay within a single, compact platform has remained elusive, despite its clear importance for multi-channel integration and functional multiplexing.

To tackle this bottleneck at the meta-atom level, the research group developed a hybrid-phase framework with separated roles: AA phase provides the “spin unlocking,” while PB phase provides “phase extension.” Specifically, asymmetric current inside the meta-atom enables right- and left-handed circularly polarized (RCP and LCP) waves to follow distinct reflected pathways, decoupling their phase and dispersion. Resonant-strength engineering then independently tailors the group delay for each spin, while frequency tuning and local structural rotation set the phase with minimal crosstalk. PB phase, introduced via global rotation, expands the phase coverage toward full 2π without significantly perturbing the group delay design, together forming a practical single-layer design space for dual-spin achromatization.

The research group experimentally validated two representative device classes in the 8–12 GHz band: spin-unlocked achromatic beam deflectors with stable spin-separated steering, and achromatic metalenses that assign different focal functions to RCP and LCP while maintaining robust focusing across the band. They further present designs extending the same principles into the 0.8–1.2 THz terahertz range, underscoring that the method is not tied to a specific frequency band but instead constitutes a transferable dispersion-engineering paradigm.

Overall, this work advances achromatic metasurfaces from single-channel correction to independently designable dual-spin meta-optics, enabling compact multi-functional systems where the two spin channels act as genuinely independent degrees of freedom. Looking ahead, the hybrid-phase design principle could be extended to the visible regime for polarization-multiplexed imaging and broadband integrated meta-optics. Inverse-design tools such as genetic algorithms and deep learning may further accelerate practical device optimization and system-level deployment.

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