Dispersion-engineered spin photonics based on folded-path metasurfaces

Abstract

Spin photonics revolutionizes photonic technology by enabling precise manipulation of photon spin states, with spin-decoupled metasurfaces emerging as pivotal in complex optical field manipulation. Here, we propose a folded-path metasurface concept 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. This advanced dispersion engineering is achieved by modifying the equivalent length of a folded path, generated by a virtual reflective surface, in contrast to previous methods that depended on effective refractive index control by altering structural geometries. Our approach unlocks previously unattainable capabilities, such as achieving achromatic focusing and achromatic spin Hall effect using the rotational degree of freedom, and generating spatiotemporal vector optical fields with only a single metasurface. This advancement substantially broadens the potential of metasurface-based spin photonics, extending its applications from the spatial domain to the spatiotemporal domain.

Fig. 1. Principle of dispersion control based on folded-path metasurfaces.

a Schematic of a folded-path metasurface that can achromatically deflect and focus broadband light for two orthogonal polarization (or spin) states independently. b Diagram of polarization-dependent light path folding through a virtual reflective surface generated by local interference. The left and right panels represent the cases of two orthogonal incident polarizations. The top and bottom panels represent different cases

Fig. 2. Demonstration of independent dispersion control on an arbitrary pair of orthogonal states of polarization.

a Diagram of supercell array (red dashed box) consisting of four anisotropic nanopillars. b 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. c–e 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

Fig. 3. Broadband achromatic metalens simply by rotating nanopillars.

a Phase distribution for the metalens at the maximum and minimum incident frequencies. The rotation angle θ and included angle α are used to control reference phase φ and phase dispersion Δφ, respectively. b Simulated reference phase and phase dispersion for all combinations of α and θ. c Group delay as functions of α and θ. d Spatial distribution of the rotation angle θ. e Spatial distribution of the included angle α. f Optical and electron microscopy images of the sample. g Simulated and measured the intensity on the xoz plane at LCP incidence. h Measured focal length. The line segment represents the FWHM of the focal depth. i FWHM of the focal spots. Scale bar: 20 μm. FWHM full width at half maximum

Fig. 4. Broadband achromatic PSHE simply by rotating nanopillars.

a Comparison among classical chromatic PSHE (left), single-polarization achromatic deflection (middle), and broadband achromatic PSHE (right). b Simulated far-field intensity profiles at various incident frequencies. The reflection angles keep at around −10 and +10 degrees at LCP and RCP incidence, respectively. c Captured intensity images of scattering light for different incident frequencies

Fig. 5. Spatiotemporal vector optical field generated by a single metasurface carrying azimuthal-varied vortex phase and radial-varied dispersion.

a Phase distributions of LCP and RCP at the center frequency. b Group delay distributions of LCP and RCP at the center frequency. c, d Spatiotemporal wave packet of E_x and E_y components. The plot shows the iso-intensity profile at 10% of the peak intensity. Inset: instantaneous intensity distribution at three time delays t = −30 fs, −20 fs, and −10 fs. e Evolution of the optical vector fields over time. Right-handed, left-handed, and near-linear polarizations are represented by red, blue, and green colors. Scale bar: 2 μm