image: Figure | Working principle of the folded-path metasurface. a, Schematic of a folded-path metasurface that can achromatically deflect and focus broadband light for two orthogonal polarization (or spin) states independently. Polarization-dependent local interference leads to different light paths for incident light with orthogonal polarization states. b, Diagram of metasurface array (red dashed box) consisting of four anisotropic nanopillars. c, Comparison of the phase spectra obtained from the multiple reflection model and full-wave simulation, verifying the tunability of dispersion based on the phase differences between subcells A and B. d-f, Phase spectra for different orthogonal polarization combinations. Blue and red curves indicate the results for two orthogonal polarization states. Solid and dashed curves correspond to the results obtained by two different supercells shown in the inset.
Credit: Fei Zhang, Hanlin Bao et al.
Spin photonics is a rapidly advancing and innovative field that harnesses the spin and polarization properties of photons for sophisticated information processing and transmission, with spin-decoupled metasurfaces playing a pivotal role in complex optical field manipulation. Current spin-decoupled metasurfaces are constrained to narrow-bandwidth operation due to insufficient dispersion control—existing techniques either affect both spin states equally or work for single spin states only. Crucially, achieving independent dispersion engineering for opposing spins at subwavelength scales remains elusive, blocking progress toward broadband decoupling and integrated devices.
In a new paper published in Light: Science & Applications, a team of scientists from National Key Laboratory of Optical Field Manipulation Science and Technology, Institute of Optics and Electronics, Chinese Academy of Sciences, have developed a folded-path metasurface platform that enables independent dispersion and phase control of two opposite spin states, effectively overcoming the limitations of spin photonics in achieving broadband decoupling and higher integration levels. Based on the metasurfaces, they showcase several capabilities that were previously unattainable with traditional metasurfaces, such as the realization of achromatic focusing and achromatic photonic spin Hall effect, and the generation of spatiotemporal vector optical fields with a single metasurface. The metasurface platform opens new avenues for a wide range of innovations, from dynamic control of light-matter interactions to the development of next-generation spin-photonic devices.
The key is to modify the equivalent path length by folding the light path through local interference at subwavelength scales. By carefully engineering polarization-decoupled interference, it is demonstrated to achieve independent dispersion control and versatile wavefront shaping for any pair of orthogonal states of polarization. These scientists summarize the operational principle of the proposed metasurfaces:
“More specifically the key points from our study are: (1) The first demonstration of simultaneous and independent control of both dispersion and phase for two opposite photon spin states using a single metasurface; (2) The first realization of the broadband achromatic photonic spin Hall effect; (3) The first achievement of broadband achromatic metalenses based solely on rotating meta-atoms, without requiring modifications to structural geometries; (4) The first generation of a spatiotemporal vector optical field using a single metasurface.”
“Our dispersion engineering via folded light paths represents a paradigm shift from conventional metasurface approaches relying on structural geometry modifications for effective refractive index tuning” they added.
“This metasurface platform is expected to unlock possibilities for compact spin-multiplexing devices for various applications, such as broadband polarization optics, information encoding, and spatiotemporal optical field manipulation” the scientists forecast.
Journal
Light Science & Applications
Article Title
Lanthanide-based metal halides prepared at room temperature by recrystallization method for X-ray imaging