Fig. 1. Operation schematic of four-channel metasurface. The metasurface is composed of Si nanobricks with five different structures and different orientation angles. The four images are showcased on two separate channels. Channel 1 exhibits a continuous-brightness nanoprinting image and a holography image, both of which are acquired via LP light. Channel 2 displays a binary-brightness nanoprinting image and a holography image acquired via CP light. The polarizer is used to derive LP light for channel 1, whereas a combination of a polarizer and a QWP is utilized to extract CP light for channel 2.

Fig. 2. Working principle of displaying dual-channel nanoprinting images. The intensity of the output light can be modulated by two different modulation functions, namely, I₁ and I₂. To attain a specific intensity modulation for nanoprinting target 1, four orientation options are available for each nanobrick. The selection of the appropriate orientation is based on whether the intensity of nanoprinting target 2 is lower or higher than 0.5, whereby only two out of four orientations are deemed eligible.

Fig. 3. The design process for a four-channel metasurface. First, in order to achieve a specific intensity modulation of the nanoprinting target 1, one may choose from four different orientation angles. Second, by selecting two orientation angles based on whether the intensity of nanoprinting target 2 is lower or higher than 0.5, two-step geometric phase encoding can be attained. Subsequently, the propagation phase can be combined with the geometric phase to generate different phase delays for both YLP and LCP incident light. Finally, the GS algorithm can be employed to calculate the target phase. From the 10-step phases mentioned above, select the one closest to the two holographic images.

Fig. 4. (a) Schematic of a subwavelength-spaced Si nanobrick on a SiO₂ substrate; (b) dimensions of the selected five nanobricks and their cross-transmittance and propagation phase.

Fig. 5. (a) Simulated and theoretical intensity. (b) Simulation results of the metasurfaces for dual-channel nanoprinting images. The second column shows the nanoprinting images under YLP incident light. The fourth column shows the nanoprinting images under LCP incident light.

Fig. 6. (a) Design flow of four-channel metasurface; (b) simulation results of the metasurfaces for four-channel image displays. The first column and the second column, respectively, show the nanoprinting images and holographic images in the near field and far field under YLP light. The third column and the fourth column, respectively, show the nanoprinting images and holographic images in the near field and far field under LCP light.

Fig. 7. Simulation results of broadband response. The different columns show the simulation results at different wavelengths (1340, 1370, 1400 nm).

Fig. 8. Simulation results of bidirectional nanoprinting and bidirectional holography. When the direction of incidence polarization states is changed, bidirectional nanoprinting and bidirectional holography can be achieved.