The design parameters of the metasurface and the fabrication process are presented in Supplementary Information Section 2. Figure 3a shows the scanning electron microscope (SEM) image of the part of the metasurface. To visualize the hidden image in the polarization topology of the laser beam, an analyzer (linear polarizer) is used to reveal the grayscale of the image. Thus, we do not directly observe the spatially variant polarization profile of the laser beam but indirectly confirm its existence through the intensity profile (grayscale image) behind the analyzer. For this metasurface, the additional phase difference between neighboring pixels along the x direction is π/5, where the corresponding reflection angle is 12.2° (see Supplementary Information Section 1). The experimental setup is shown in Figure 3b. An objective with a magnification of 10× was used to expand the image for visualization with a charge-coupled device (CCD) camera. Figure 3c and 3d shows the simulation and experimental results. As shown by the numerical calculation, no image is observed in the intensity profile of the beam (Figure 3c, left). The experimental result on the right side of Figure 3c confirms that the image-hidden functionality is unambiguously realized. A high-quality image is revealed (Figure 3d, right) with the analyzer whose transmission axis along the vertical direction, which is notably consistent with the simulation result (Figure 3d, left). Here, the transmission axes of the polarizer and the analyzer are along horizontal and vertical directions, respectively. The incident light beam for the simulation is a plane wave with uniform intensity, whereas the incident beam for experiment is a collimated laser beam with a Gaussian profile. The varied intensity of the incident light causes a slight discrepancy between the experimental and simulation results. Another reason for the discrepancy is the imperfection of the linear polarizer and fabrication error. Because of the off-axis design, another identical image is also observed in the reflected beam on the other side with respect to the surface normal. The clear image of the mustache, eyeball and eyebrow indicates the ultrahigh resolution of the proposed approach. To further analyze the performance of our approach, the dependence of simulated and measured results on the direction of the transmission axis of the analyzer is shown in Figure 3e. The results at 0°, 45°, 90° and 135° show the consistency between experimental and simulation results. Interestingly, the two images for the analyzer with orthogonal directions of the transmission axis (for example, 0° and 90°, 45° and 135°) are complementary grayscale images, that is, the brightest area becomes the darkest area and vice versa. The evolution process of the revealed images is clearly observed by gradually rotating the analyzer (see the video in the Supplementary Information).

Figure 3 Fabricated metasurface, experiment setup and metasurface device characterization. (a) SEM image of the fabricated metasurface. The scale bar is 500 nm. (b) Experimental setup. The collimated light beam with the required linear polarization is generated using a linear polarizer (LP) and is incident on the metasurface, which is mounted on a 3D translation stage. An objective with a magnification of 10× is used to collect and expand the resultant beam. The images are captured by a CCD. The analyzer, which is a linear polarizer, is placed in front of the CCD to reveal the hidden image. The simulated and experimental results without (c) and with the analyzer (d). Note that the direction the transmission axis is along the vertical direction. (e) The simulated and experimental results for the analyzer with various directions of transmission axis. The results at 0°, 45°, 90° and 135° are given. The scale bar is 500 μm. Full size image

To better understand the image-hidden approach, we also studied the dependence of the image on the incident polarization state and the transmission axis of the analyzer. Although our design is based on linear polarization, the device also works for elliptical polarization because an elliptically polarized light can be decomposed into LCP light and RCP light with different components. However, the image quality will be reduced. The numerically calculated and experimentally observed images hidden in the laser beam are given in different circumstances (see Supplementary Information Section 3). The measured hidden images with various combinations of the linear polarizer and the analyzer are also shown in Supplementary Information Section 3.

Using the broadband nature of the geometric metasurface, the developed device can operate in a broad wavelength range. Images at different wavelengths were captured and shown in Figure 4a–4f. The experimentally revealed clear images at the wavelengths of 500, 550, 575, 600, 640 and 700 nm unambiguously show the operation of the developed device in the broad spectral range. The uniqueness of our approach lies in the encoding process of a high-resolution grayscale image onto the polarization profile of the laser beam. The image can be hidden and carried by a laser beam during light propagation and revealed by an analyzer. We explore the performance of the approach for long-distance propagation in free space (Figure 4g). Figure 4h shows the measured images after a 4-m propagation in free space. The image remains clearly observed, although there is a slight reduction change in image quality.

Figure 4 Broadband performance and robustness of the proposed approach. Experimental results at the wavelengths of (a) 500 nm, (b) 550 nm, (c) 575 nm, (d) 600 nm, (e) 640 nm and (f) 700 nm. (g) Experimental setup to characterize the hidden image for long-distance propagation. (h) Obtained image after propagating 4 m in free space. Scale bar: 500 μm. Images a–e were captured by a color CCD with 1024 × 768 pixels. Images f and h were captured by a monochrome CCD with 1280 × 1024 pixels. Full size image

The obtained image shows how the electric field is oriented in the beam profile of the laser beam. These hidden images demonstrate the rich polarization structure that a light beam can have at subwavelength scales. The conversion efficiency is calculated by the power of two resultant off-axis beams divided by that of the incident light. The efficiency in the wavelength range of 640–960 nm is shown in Supplementary Fig. S7, and the maximum conversion efficiency is 60% at the wavelength of 820 nm (see Supplementary Information Section 4).