Recently,emerging ultracompact planar photonic devices have been developed using a plurality of metasurfaces with excellent performance in light manipulation. In particular,the versatility of metasurfaces in pixel-level high-resolution phase control has prompted the emergence of single-channel metasurface holograms that simplify the computer-generated hologram (CGH) production procedure. Subsequently,the progressive advancement of multi-channel metasurface holograms with polarization-multiplexed control opens a new direction for potential applications such as optical encryption and storage. However,most optical holographic or storage metasurfaces are currently static and lack mechanisms for active regulation or dynamic anticounterfeiting. In this scenario,to further strengthen the applicability and security level of optical encryption and storage devices,this study proposes a type of actively switchable or “erasable” metasurface hologram with multi-channel polarization-dependent multiplex control. Although diverse mechanisms for active control are available,such as liquid crystals,semiconductors,conductive oxides,varactors,and device architecture design based on micro-electromechanical system,the phase change materials integrating scheme transpires to be a solution with a practical trade-off owing to its advantages of nonvolatile and high-speed switching control,and a significant number of reversible phase transitions.
In this study,by introducing the phase-change dielectrics of GeSbSeTe with almost no loss into the multi-channel holographic metasurface architecture,a type of phase-change dielectric meta-hologram concurrently with actively “erasable” control is achieved for the incidence of circularly-polarized (CP) waves. In order to achieve the two-dimensional (2D) isotropic phase encoding with “erasable” control,the GeSbSeTe elliptical pillars with varied diameters are screened out initially via a rigorous process of numerical calculations to produce a phase map that covers the full range of 0-2π. Subsequently,according to the near-field phase profile immediately behind the meta-hologram to be designed for the far-field holographic images,all GeSbSeTe elliptical pillars are determined to construct the metasurface hologram once their diameters are customized by fully retrieving the phase map. To enable multi-channel polarization-dependent phase control for CP incidence herein,both the geometrical and propagation phases along one axis are treated in the phase profile coding process for holographic imaging.
After numerical simulations,a group of pillars with optimized copolarized transmittance and cross-polarization efficiency are selected for as-required phase profile encoding. For efficient phase encoding,a series of 36 pillars are screened in the 36-level phase sampling process to construct the metasurface hologram. Consequently,when the phase-change coupling layer or GeSbSeTe pillar is in an amorphous state,well-defined metasurfaces exhibit multi-channel polarization-independent phase control,assuming different holographic images or focal points under different CP inputs. When the phase-change layer becomes crystalline,the cross-polarization that determines the phase modulation is turned off such that the corresponding phase encoding is deactivated and the far-field images disappear,i.e. they are “erased”. The erasable polarization-multiplexing holographic metasurface proposed in this study provides a new degree of freedom in potential applications,such as optical anti-counterfeiting,encryption,and storage.
In summary,this study introduces a new degree of freedom for actively switchable control into recent multi-channel polarization-multiplexed metasurface holograms,and a scheme for the erasable meta-hologram is demonstrated by integrating the phase-change material GeSbSeTe into the metasurface design. Elliptical GeSbSeTe or silicon pillars are customized to construct a metasurface with the intended near-field phase profile for holographic imaging. Finally,the numerical results confirm that at the CP wave incidence with opposite helicities,different far-field images can be created in the GeSbSeTe amorphous state. However,upon the GeSbSeTe phase transition to the crystalline state,the cross-polarization efficiency is minimized,the geometrical phase deactivates,and the far-field images in both channels are erased. As verified by the numerical results,a 36-level phase-sampling procedure completes phase-profile encoding to efficiently construct the intended erasable meta-hologram,which feasibly poses a new degree of freedom for applications such as optical anti-counterfeiting and encryption.
