Spin photonics, an innovative field that merges concepts from spintronics and traditional photonics, is poised to revolutionize optical technology by enabling the precise manipulation of photon spin states. While significant strides have been made, particularly with spin-decoupled metasurfaces for intricate optical field manipulation, a persistent challenge has been achieving broadband spin decoupling and higher integration levels. This limitation stems from the difficulty of independently controlling both the dispersion (how light of different wavelengths behaves) and phase of two opposite spin states across a broad spectrum. However, a recent breakthrough introduces a novel concept: folded-path metasurfaces, which promise to unlock previously unattainable capabilities in spin photonics.
Traditional methods for engineering dispersion in optical metasurfaces typically involve altering the physical geometry or arrangement of subwavelength nanostructures. This folded-path metasurfaces approach modifies the effective refractive index experienced by light. While effective, such methods often lead to complex designs, simulations, and fabrication processes. More critically, existing dispersion control techniques are generally either independent of spin or limited to a single spin state. This is because the dispersive propagation phase of light is spin-independent, while the geometric phase, often used for spin manipulation, is spin-conjugate (meaning it acts oppositely on opposite spins) and typically dispersion-free. This fundamental challenge has historically hampered the development of compact, broadband spin-multiplexing devices. For instance, manipulating spatiotemporal vector optical fields, which involves simultaneously controlling light’s spatial and temporal properties for different polarizations, usually requires bulky and cumbersome setups with multiple components.
The groundbreaking innovation of folded-path metasurfaces introduces an entirely new paradigm for dispersion engineering. The core idea is not to alter the physical structure’s geometry to change its inherent refractive index, but rather to modify the equivalent path length that light effectively travels. This “folded path” is created through highly localized interference phenomena at the subwavelength scale, effectively acting as a virtual reflective surface within the metasurface. By meticulously engineering polarization-decoupled interference within these virtual folded paths, the researchers have demonstrated an unprecedented ability to independently control both the dispersion and the wavefront shape for any pair of orthogonal polarization states. This includes linear, circular, or elliptical polarizations, offering remarkable versatility.
This folded-path metasurfaces breakthrough has profound implications for a wide range of applications in optics and photonics. For example, it enables the realization of achromatic focusing, meaning the metasurface can focus different colors (wavelengths) of light to the exact same point – a significant challenge for conventional lenses. It also allows for the achromatic photonic spin Hall effect (PSHE), where light with different spins can be deflected consistently across a broad range of wavelengths. Furthermore, this new metasurface platform makes it possible to generate complex spatiotemporal vector optical fields using just a single metasurface, eliminating the need for the bulky, multi-component setups previously required.
This advancement substantially broadens the potential of metasurface-based spin photonics, extending its applications beyond simply manipulating light in the spatial domain to controlling its properties simultaneously in both space and time. This new metasurface platform is expected to unlock possibilities for highly compact spin-multiplexing devices, paving the way for innovations in areas such as broadband polarization optics, advanced information encoding techniques, and sophisticated spatiotemporal optical field manipulation for future technologies.
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