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Abstract
Augmented reality (AR) display, as a next-generation innovative technology, is revolutionizing the ways of perceiving and communicating by overlaying virtual images onto real-world scenes. However, the current AR devices are often bulky and cumbersome, posing challenges for long-term wearability. Metasurfaces have flexible capabilities of manipulating light waves at subwavelength scales, making them as ideal candidates for replacing traditional optical elements in AR display devices. In this work, we propose and fabricate what we believe is a novel reflective polarization multiplexing gradient metasurface based on propagation phase principle to replace the optical combiner element in traditional AR display devices. Our designed metasurface exhibits different polarization modulations for reflected and transmitted light, enabling efficient deflection of reflected light while minimizing the impact on transmitted light. This work reveals the significant potential of metasurfaces in next-generation optical display systems and provides a reliable theoretical foundation for future integrated waveguide schemes, driving the development of next-generation optical display products towards lightweight and comfortable.
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1. Introduction
Augmented reality (AR) display technology greatly enhances our perception and interaction with the world by overlaying computer-generated virtual information onto real-world scenes [1–5]. In recent years, AR display devices have found extensive applications in various fields such as healthcare, education, entertainment, engineering, and aviation [6–9]. Different application solutions, including prisms, birdbath, freeform and holographic lenses have been used for implementing AR displays. In these schemes, the use of traditional optical elements still results in bulky and cumbersome optical systems, which is not ideal for long-term wearability. Besides, the newly proposed waveguide-based approach offers advantages in terms of imaging clarity, field of view (FOV) and portability, making it the mainstream optical display solution for AR displays [10–12]. However, this approach also faces the issue of low optical efficiency which results in low brightness of single point output, low contrast ratio which causes poor visual experience, and the diffractive dispersion which leads to “rainbow” phenomenon in the image. Therefore, there is an urge requirement for compact and efficient AR elements.
Metasurfaces, as novel ultrathin planar optical elements, offer an innovative platform for flexibly manipulating the fundamental properties of light [13–16], including amplitude, [17,18] phase, [19,20] polarization, [21,22] and orbit angular momentum [23,24]. By employing diverse meta-atoms, metasurfaces provide a high degree of freedom to control the wavefront of light with subwavelength resolution, breaking the limitations of traditional diffraction and refraction optics. This excellent ability to modulate light has led to extensive applications such as metalenses, [25–27] beam shaping, [28–30] nonlinear optics, [31,32] holography, [33–37] information encryption, [34,38,39] and beam deflection [40,41]. Among these applications, beam deflectors, typically realized by using grating structures, serve as basic optical elements that efficiently deflect light beams in specific propagation directions, playing indispensable roles as optical combiners in AR display systems [42,43]. In recent years, due to the inherent ohmic losses of metallic materials, the utilization of all-dielectric materials has gradually replaced metallic materials for manufacturing beam deflectors, aiming to achieve higher deflection efficiency [44–46].
Due to their thin form-factor, excellent modulation capabilities, and high deflection efficiency, beam deflectors composed of all-dielectric meta-atoms have become key components in near-eye displays, particularly in AR display applications. For example, Liu et al. designed a compact stereo waveguide display system employing a metagrating in-coupler paired with two diffractive grating out-couplers, achieving a binocular stereo AR display effect [47]. Boo et al. developed an ultracompact focal-free AR waveguide display system by designing a freeform lense as a waveguide coupler, which achieved a high-resolution full-color AR display effect [48]. Tang et al. proposed and implemented a dynamic AR holographic display system based on a layer-folded metasurface integrated with an electrically driven liquid crystal (LC) platform, allowing for the active switching of independently encoded holographic images in real-time scenes [49]. These research works have paved the way to realize ultracompact AR waveguide display solutions based on metasurfaces. However, further explorations and improvements are still highly desired in terms of diffraction efficiency, display quality, and diffraction chromatic aberration.
For the situation where transmission and reflection share a common information channel, it inevitably results in crosstalk which may lead to a poor visual experience for users in practical applications. Additionally, when using reflective optical combiner to superimpose virtual and real information, it is crucial to delicately consider the balance of exit pupil brightness between the virtual images and the real-world scenes. Generally, the optical combiners require higher transmittance, as it is challenging to modulate the energy of light from the real-world scenes. However, the higher transmittance inevitably reduces the brightness of the virtual images reflected to the human eyes. How to reduce crosstalk between transmission and reflection channels and balance the intensity between the two becomes an essential consideration for further enhancing the display quality of AR devices.
In this paper, we aim to further modulate the transmission and reflection through different polarization channels to achieve better display performance and propose an optical see-through reflective dielectric gradient metasurface for AR waveguide display solutions. The designed metasurface provides efficient beam deflection for x-polarized reflected light at $\lambda $=800 nm, while allowing high-transmission unmodulated y-polarized transmitted light to pass. This design enables an ultra-thin, efficient, and high-resolution on-chip AR waveguide display element. Further, we can integrate this element with the existing waveguide scheme as integrated waveguide system to achieve efficient waveguide propagation and low crosstalk imaging.
As shown in Fig. 1(a), the AR waveguide display system consists of three main parts: a microdisplay, a glass waveguide, and optical combiners using metasurface optical elements. The x-polarized light emitted from the microdisplay is coupled into the waveguide through an in-coupling metasurface optical element (In-MOE) and propagates forward via total internal reflection (TIR). When it reaches the out-coupling metasurface optical element (Out-MOE), it will be coupled out of the waveguide. Simultaneously, the y-polarized light from the real world also passes through the Out-MOE and combines with the emitted light from the microdisplay, enabling AR display functionality. The Out-MOE plays a crucial role as the optical combiner to integrate virtual with real information, which is the focus of our design.
Fig. 1. Simplified schematic diagrams of an AR display system using the polarization-multiplexing meta-combiner. (a) The near-eye AR waveguide display scheme. The Out-MOE is the foremost element to combine virtual images and real-world scenes. (b) The AR display scheme which uses a metasurface as a single-piece proof-of-concept for the Out-MOE in integrated waveguide schemes. (c) The schematic diagram to derive generalized Snell’s laws of reflection.
For the In-MOE part, it only requires achieving efficient deflection of the incident beam to couple with the waveguide. While for the Out-MOE, it is necessary to simultaneously consider the different modulation effects for transmitted and reflected light in order to achieve a better visual experience. So we design and fabricate the foremost Out-MOE based on the generalized Snell's law [50] as a single-piece proof-of-concept for integrated waveguide schemes. As shown in Fig. 1(b), the designed metasurface acts as an optical combiner to merge the virtual images emitted from the microdisplay with the light from the real-world scenes, imaging at the viewer's eyes simultaneously. It significantly reduces the complexity and weight of wearable near-eye AR display devices, providing users with a more comfortable experience. Additionally, the designed meta-combiner has polarization multiplexing capabilities, which can efficiently modulate x- and y-polarized light, simultaneously. This ensures high display brightness for both virtual images and real-world scenes, which greatly enhances the imaging quality of the final image presented to the user with negligible crosstalk.
2. Principle and result
Figure 1(c) illustrates the reflection case of generalized Snell’s laws, where a plane wave is incident on the interface at an angle of ${\theta _i}$. The generalized Snell’s laws suggest that one can control the reflection direction by designing the phase gradient along the interface. In particular, when the medium is air (${n_i} = 1$) and the incidence is normal (${\theta _i} = 0$), the angle of reflection is determined exclusively by the incident wavelength and the phase gradient
To realize the effect of polarization multiplexing, the electromagnetic resonance provided by dielectric nanofins is used to control the reflection and transmission of light, respectively. The unit cell of the metasurface, depicted in Fig. 2(a), is consisted of a simplest rectangular nanofin with a tailorable cross-section but fixed period and height, which is made of amorphous silicon (α-Si) and fabricated on a fused silica (SiO2) substrate. To understand the working principle of such a polarization multiplexing metasurface, we use Jones matrix method for explanation. For a nanofin without any rotation, the Jones matrix of its transmission and reflection can be described as:
Fig. 2. Simulation results and schematic diagrams of the polarization multiplexing reflective gradient metasurface. (a) Schematic diagram of a unit cell of the meta-combiner, consisting of an amorphous silicon nanofin positioned on the glass substrate with fixed period (P = 500 nm) and height (H = 600 nm). (b) Simulation results for the amplitude and phase distribution of the reflection coefficient ${r_{xx}}$ and transmission coefficient ${t_{yy}}$. The final selected nanofins are marked as black dots in the figure. (c) The reflection coefficient ${r_{xx}}$ and transmission coefficient ${t_{yy}}$ of the final selected nanofins, along with the geometric parameters of the six nanofins. (d) Simulation results for the polarization multiplexing meta-combiner under different polarized incidences. (e) Simulated far-field normalized diffraction efficiency of each diffraction order for the polarization multiplexing meta-combiner.
Consider the transmission of y-polarized incident light and the reflection of x-polarized incident light, the output light can be calculated as:
By judiciously designing the structural parameters of the nanofins, we can select suitable amplitude and phase terms to realize different polarization modulation for reflected and transmitted light, respectively. Specifically, we select the length and width of the nanofins within a supercell with six phase levels to obtain high efficiency of 1st diffraction order in reflection and high efficiency of 0th diffraction order in transmission for the normal incident light at 800 nm. This requires the nanofins to have an amplitude term close to 1 for transmission and reflection under the corresponding polarization, while providing a certain phase gradient for the reflected light and maintaining uniform phase modulation for the transmitted light.
The rigorous coupled wave analysis (RCWA) method is utilized to calculate the amplitude and phase distribution of the $\mathrm{\alpha }$-Si nanofins in different polarized incidences as a function of length L and width W in the range of 80 nm - 460 nm and the final selected nanofins are marked, as shown in Fig. 2(b). After careful evaluation of the amplitude and phase of the reflection coefficient ${r_{xx}}$ and transmission coefficient ${t_{yy}}$ for each one, we optimized and selected six nanofins for composing a supercell, as illustrated in Fig. 2(c). Numerical simulation confirms that the x-polarized reflection amplitudes of the nanofins are nearly equal and exceed 0.75, with π/3 phase increment, which guarantees a consistent phase gradient for the reflected light to deflect it. According to Eq. (1), the reflection angle for normal incidence is around 15.7°. As for the y-polarized transmitted light, the transmission amplitudes of the nanofins are still uniform and exceed 0.9. The maximum phase difference between the transmission phases of each nanofins is within π/8 to ensure a constant phase distribution and achieve modulation-free effects. The geometric parameters of each selected nanofins are illustrated in the diagram below Fig. 2(c).
After determining the geometric parameters, we utilized finite-difference time-domain (FDTD) method to conduct full-wave simulations on the periodically arranged selected nanofins. This allowed us to calculate the near-field and far-field electromagnetic responses of the designed metasurface for transmission and reflection under different polarized light illuminating. The near-field phase distribution in the x-z plane is presented in Fig. 2(d). At an incident wavelength of 800 nm, it is evident that the reflected light ${r_{xx}}$ from x-polarized incident light experiences a significant deflection angle of approximately 15.7°, while the transmitted light ${t_{yy}}$ from y-polarized incident light continues to propagate in the original direction. Noting that the transmission phase distribution exhibits slight curvature, which is attributed to the transmission phases for y-polarized incident light of the selected nanofins are close but not identical, consequently, a small portion of energy still deflects to other diffraction orders. Figure 2(e) illustrates the normalized efficiency of each far-field diffraction order of our designed metasurface. For ${r_{xx}}$, over 75% of the energy in output light is concentrated in the +1 order, while for ${t_{yy}}$, over 85% is concentrated in the 0 order. Additionally, the total reflectivity of ${r_{xx}}$ is 56.1% and the total transmittance of ${t_{yy}}$ is 77.3% according to the simulation. By conducting full-wave simulations using FDTD, we have demonstrated that the polarization multiplexing metasurface can efficiently deflect the reflected light from x-polarized incident light, while allowing nearly modulation-free and low-loss transmission of the transmitted light from y-polarized incident light.
3. Results and discussion
Thanks to its unique transmission and reflection characteristics, we replaced the conventional optical combiner in the AR display system with our ultrathin meta-combiner and conducted imaging experiments to further validate the feasibility of our design. Figure 3(a) shows a scanning electron microscope image of the fabricated sample which possesses an area of 500 $\mathrm{\mu}$m × 500 $\mathrm{\mu}$m (1000 × 1000 units). The dielectric amorphous silicon metasurface is fabricated on a fused quartz substrate based on electron beam lithography. Figure 3(b) shows the main experimental setup for implementing the AR display system (additional experimental setups can be found in Supplement 1). In our experiment, we used a linear polarizer (LP) to set the polarization of the incident light to x-polarization at $\lambda $=800 nm. A customized resolution plate is used to generate the desired virtual images and a filter is employed to adjust the intensity of virtual images for the optimal display effect. Later the virtual images are downsized through a lens (f = 50 mm) and a microscope objective (10×, NA = 0.25) to only image within the effective region of the sample. Acting as the meta-combiner, the sample reflected the virtual images while simultaneously transmitting the real-world scenes behind it, thus enabling AR display imaging. To facilitate recording, we used a charge-coupled device camera (CCD) as a substitute for human eyes to observe.
Fig. 3. Experimental setup and scanning electron microscope images of the fabricated sample. (a) Scanning electron microscope images of the fabricated amorphous silicon sample shown with a top and side view. (b) Schematic diagram of the experimental optical setup for the AR imaging system.
As the experimental validation, we first imaged and recorded a resolution plate illuminated by the y-polarized light at $\lambda $=800 nm as the real-world scene by home-made synthetic optical path (more details are provided in Supplement 1). By using the meta-combiner, both the virtual images and real-world scenes were simultaneously observed at the camera, as depicted in Fig. 4. The left side displayed the transmitted digits on resolution plate under y-polarized illumination, while the right side showed the reflected resolution plate pattern under x-polarized illumination. With the change of the polarizer direction, the image intensity of different polarization states continuously varies, proving that the transmitted real scene and the reflected virtual pattern represent different polarization states, respectively (see Visualization 1).
Fig. 4. The imaging results of using the resolution plate pattern as the real-world scene under laser illumination at 800 nm. The left side surrounded by blue dash line displays the transmitted patterns under y-polarized illumination, while the right side surrounded by red dash line shows the reflected patterns under x-polarized illumination.
Next, we changed the real-world scenes to photos projected by a phone, as shown in Fig. 5(a). We utilized a custom resolution plate with etched patterns such as “π,” “flower,” and “tree"(the school badge of Beijing Institute of Technology (BIT)) to generate virtual images, while projecting photos like the “office desk,” “cola bottle,” “the central teaching building of BIT,” “the stone with BIT’s motto,” and “campus street of BIT “ (from top to bottom in sequence) for AR display imaging. When removing the sample, the virtual image disappeared, which demonstrates that the virtual image is generated by the reflection of the sample rather than the substrate (see Visualization 2). Finally, we validated our system with real objects. The two images in the left side of Fig. 5(b) shows the AR display results using different virtual images, with a Rubik's Cube and two aviation dolls as the real-world scenes. And the two images in the right side of Fig. 5(b) demonstrate that the virtual images would remain clear and visible at various focal depths. The upper right corner photo focuses on a front whiteboard with a scale with a focal depth of 25 cm, while the lower right corner photo focuses on the back figurine with a focal depth of 40 cm. This implies that our system provides a high-definition viewing experience for AR display, even when the human eyes look at objects at different depths, which is crucial for future AR display devices. The FOV of our AR display system is approximately 14°, which is is mainly limited by the size of the metasurface and the numerical apertures (NA) of the optical imaging system used. While it can be increased by designing larger area metasurface and utilizing optics with larger NA, without any theoretical limitations.
Fig. 5. The imaging results of the AR system using meta-combiner. The leftest side shows different projection methods for the real-world scenes in AR display. (a) The imaging results of using the photos projected by a mobile phone as the real-world scene. Three different virtual images and six different mobile phone photos are used for AR display. (b) The imaging results of using the real object as the real-world scene. The left set of images shows the results of different virtual images under the bright environment, while the right set of images shows the results of imaging at different focal depths in the dark environment. The images are respectively focused at 25 cm (whiteboard) and 40 cm (figurine).
Based on the experimental results, whether the real-world scenes were the patterns projected by the laser, the photos projected by a smartphone, or real display objects, our AR display system all demonstrated efficient and high-definition display effects. It is worth mentioning that although the transmission function of our meta-combiner is designed for y-polarized light at 800 nm, during the experiment we found that even when observing the transmitted real-world scenes which were colored images under natural light, the camera was still able to capture high-color-fidelity and high-definition images without crosstalk. It should be noted that the deflection angle we designed is smaller than the TIR condition. While for integrated waveguide schemes, larger deflection angle can be realized by designing larger phase gradient accordingly.
We believe that the designed reflective polarization-multiplexing optical see-through gradient metasurface can serve as an ideal substitute to the optical combiner devices in current AR display systems. It greatly reduces the complexity and weight of the system, which improves the user's wearing experience, and provides the reliable support for achieving ultrathin, high-efficient, and high-definition AR waveguide display schemes in the future.
4. Conclusion
In summary, we propose and demonstrate a polarization multiplexing optical see-through reflective gradient metasurface as an ideal substitute for the bulky optical combiners used in traditional AR display systems to reduce the complexity and weight. In contrast to other designs of reflective meta-combiners, we have taken into consideration the impact of the metasurface on transmitted light and utilized polarization multiplexing method to efficiently deflect the reflected light while leaving the transmitted light unmodulated. This enables high-fidelity and high-energy utilization imaging of both real-world scenes and virtual images, enhancing the user's visual experience and immersion. As a single-piece proof-of-concept, we develope an AR optical display system to experimentally demonstrate the capability of the designed metasurface in achieving efficient and high-definition near-eye AR display functionality. Further improvements are desirable to realize ultracompact waveguide AR display systems combined with lens phase and achieve colorful AR display utilizing dispersion engineering. We believe that the lightweight and ultrathin meta-combiner with such appealing functionality will pave the way for the next generation of wearable AR display devices and hold remarkable potential in fields such as biomedical imaging, medical devices, and imaging multiplexing.
Funding
National Key Research and Development Program of China (2021YFB2802200); National Natural Science Foundation of China (No. 92050117, No. U21A20140); Beijing Outstanding Young Scientist Program (BJJWZYJH01201910007022)); Science and Technology Innovation Program of Beijing Institute of Technology (2021CX01008); Beijing Municipal Science and Technology Commission, Administrative Commission of Zhongguancun Science Park (No. Z211100004821009).
Acknowledgements
This work was supported by the Synergetic Extreme Condition User Facility (SECUF). We acknowledge the fabrication and measurement service in the Analysis & Testing Center, Beijing Institute of Technology. We also sincerely appreciate Professor Jingbo Sun from the School of Materials Science and Engineering, Tsinghua University for providing the spectral testing device.
Disclosures
The authors declare no conflicts of interest regarding this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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