As a fundamental advanced display technology of emerging spatial computing devices, near-eye display creates an immersive or perspective visual effect, demonstrating enormous potential for widespread applications. Nevertheless, the further integration of near-eye display systems into daily work and life of general consumers is influenced by the long-term wearing comfort and convenience. On the one hand, system weight is an important factor affecting wearing comfort [1,2]. Typically, high-capacity batteries accounting for over half of the total weight of current commercial near-eye display products, the weight of hundreds of grams, result in wearing pressure and structure design difficulties. On the other hand, the few-hour-long battery life requires frequent charging, making continuous use of the system troublesome or even impossible. Besides, the continuous heating of the battery during operation also negatively affects system performance and user experience, increasing the demand for effective thermal management.

Previous researchers have attempted to address the weight burden and endurance capability of near-eye display systems. To reduce the weight and volume of the main system involves three effective solutions, utilizing lighter image sources such as micro light-emitting diodes [3] and laser beam scanning [4], designing lighter optical combiners including waveguides [5,6], holographic optical elements [7,8], or micro- and nano-optical elements [9,10], as well as separating light sources from the main system [11,12]. Besides, in existing commercial products, Apple’s Vision Pro separates the battery from the helmet system, while Meta’s Orion puts the computing components in a separate box from the glasses body. While the above methods reduce system weight to a certain extent, the user experience is degraded by the decrease in display performance and inconvenience of separating modules. Moreover, heavy batteries in the existing optimized system fail to meet the increasing demand for lightweight design. To extend the battery life, researchers proposed two solutions: improving energy utilization efficiency through reasonable optical design [7,13] and reducing energy consumption by implementing mobile edge computing [14,15]. Obviously, the battery life is only slightly extended due to the lack of a real-time energy input solution, so frequent battery charging is still inevitable.

To satisfy the requirement of charging while consuming, researchers proposed some self-charging systems that eliminate the need for high-capacity batteries and additional charging devices. Currently, existing self-charging systems typically use biomechanical energy [16–18], thermoelectric energy [19,20], piezoelectric energy [21], and solar energy [22–25]. As a widely available clean energy, solar energy is a promising energy source for self-charging devices, but the acquisition and accumulation of solar energy need to add a complex structure, which is also a serious challenge for near-eye display systems. Overall, some methods were proposed to improve the comfort and convenience of near-eye display systems. However, there is a lack of fundamental solutions that comprehensively achieve lightweight design and continuous self-acquisition of clean energy from an optical perspective.

In this study, we present a lightweight holographic near-eye display system with self-charging capability by simply capturing solar energy. The utilization of solar capture holographic optical element (HOE) and waveguides enables the collection of solar energy over a wide range in a compact structure without affecting near-eye display, providing an excellent continuous energy input solution. This integration not only adds a self-charging function to the system, but also avoids the impact of continuous battery heating on user experience and heat dissipation design difficulties. Replacing heavy batteries with small-area solar cells significantly reduces the weight of near-eye display systems and promotes the revolutionary evolution of structure from a helmet system to a wireless glasses system.

Figure 1(a) illustrates the basic structure of the proposed system, which can be divided into two parts: solar power supply module and near-eye display module. The core component is an optical combiner consisting of a waveguide and three HOEs. Regarding one of the two symmetrical eyeglasses, the solar power supply module includes a solar capture HOE (HOE-1), a waveguide, and two solar cells. HOE-1 is anchored to the outer surface of the waveguide, and solar cells are located on both sides of the optical combiner above and below. The near-eye display module includes an image source, a coupling-in HOE (HOE-2), a coupling-out HOE (HOE-3), and a waveguide, where HOE-2 and HOE-3 are attached to the inner surface of the waveguide. It should be noted that the waveguides of the two modules are the same one and serve as a common carrier for collected solar energy and signal light. Compared with the traditional holographic near-eye display system, simply adding a thin film and small-area solar cells brings numerous benefits of self-charging capability.

Figures 1(b)–1(d) respectively illustrate the principles of solar energy collection, power supply, and near-eye display. Sunlight and signal light are separately coupled into the waveguide by HOE-1 and HOE-2, propagating through different total internal reflection directions along $z$-axis and $x$-axis, and finally coupled out at HOE-3 and the edges of the waveguide, respectively. The collected solar energy is received by solar cells and converted into electrical energy, which powers the image source to generate signal light. Because of the angle selectivity and wavelength selectivity of volume HOE, only light that meets the Bragg condition will be diffracted with high efficiency, so the optical functions of three HOEs in the two modules will not interfere with each other.

Specifically, the design parameters of the optical combiner are provided in Fig. 2. As shown in Fig. 2(a), in the near-eye display module, the signal light emitted from the image source is designed to vertically incident on HOE-2. The total reflection angle of the signal light within the waveguide is set at 60°, and HOE-3 couples the signal light vertically out of the waveguide into the human eye. It should be noted that this configuration represents only one example of a near-eye display module, while more complex forms (such as field-of-view expansion or exit-pupil expansion schemes) can also be integrated with the solar power supply module.

The collection characteristics of HOE-1 will directly affect the power supply efficiency. The design of HOE-1 involves two steps: partition design and recording condition design. On the one hand, the secondary diffraction effect will cause some of the collected light to be coupled out in advance and unable to propagate to the edge of the waveguide, thereby reducing the effective collection area. The partition design aims to mitigate the impact of secondary diffraction on the effective collection area, requiring comprehensive consideration of waveguide thickness and propagation angles of collected light within the waveguide to determine appropriate partition quantities and single-partition lengths. For HOE-1 recorded with a single wavelength, different wavelengths propagate at varying angles within the waveguide due to Bragg matching conditions. Insufficient partition quantities or excessively large single-partition lengths may allow residual secondary diffraction effects for light with smaller propagation angles. Therefore, the partition design must holistically address the secondary diffraction of the wide collected spectrum rather than solely focusing on the recording wavelength, maximizing suppression and its influence on the collection area.

On the other hand, as solar elevation angles vary across regions and throughout the day, the system requires wide-angle collection capability. However, the selectivity of HOE will lead to limitations in collection wavelengths and angles. In our system, HOE-1 is designed as a simple fabricating method using monochromatic plane waves, so the variable wavelength reconstruction condition is expected to overcome the above limitations as long as the recording wavelength and angle are designed reasonably. Generally, selecting a recording angle near the central angle within the target collection angle range and choosing a recording wavelength close to the central wavelength of the target spectrum help to optimally meet both angular and spectral requirements. The detailed analysis of the HOE-1 design is added to Appendix A.1. Ultimately, HOE-1 is designed with four partitions (each 12.5 mm), and each partition is designed with a different propagation angle within the waveguide. In addition, the recording wavelength is 532 nm and the recording angle in air is 30°, as illustrated in Fig. 2(b). Different from previous solar energy collectors, we fully utilize the flexibility and freedom of HOE and develop a sustainable real-time solar energy input solution suitable for near-eye display systems.

In order to verify the rationality and feasibility of the proposed system, we have experimentally fabricated a prototype of the optical combiner as shown in Fig. 3(a). HOEs are fabricated by holographic exposure as designed and attached to a glass substrate, which serves as the waveguide. The fabrication wavelength of HOE-1 is 532 nm, and the collected solar energy is designed to propagate in total internal reflection mode along the $z$-axis inside the waveguide. HOE-2 and HOE-3 are fabricated by synthetic white light formed by 457 nm, 532 nm, and 639 nm lasers, and the signal light is designed to propagate in total internal reflection mode along the $x$-axis. The detailed fabrication method is provided in Appendix A.2. Subsequently, we measure the light collection and near-eye display effects of the optical combiner. Since the optical combiners of the two eyeglasses are symmetrical, only one of them is used for measurement.

On the one hand, when using a flashlight with an LED light source to illuminate the optical combiner with white light at the recording incident angle, the collected light emitted from the edge of the waveguide is shown in Fig. 3(b). A portion of the collected light (represented by the green arrows) is reflected by the front surface for the last time inside the waveguide and tends to propagate to the left after coupling out from the edge of the waveguide, while another portion of the collected light (represented by the blue arrows) is reflected by the back surface and tends to propagate to the right. The separation of the two parts is relatively obvious under the divergent light of the flashlight, and this problem can be solved by designing the coupling-out surface as a curved surface in the future. Figure 3(c) illustrates a comparison of the transmitted light spectrum without and with the optical combiner. The difference indicates that the optical combiner collects part of the solar energy effectively. Define half of the maximum collection efficiency (the ratio of the collected light power to the incident light power) as the threshold for effective collection. At the recording incident angle, the center wavelength of the effective collection light is 532 nm and the spectral width is approximately 16 nm. Overall, the optical combiner can effectively accomplish the function of solar energy collection for recording wavelength at the recording incident angle. The collection of light that deviates from the recording conditions will be analyzed in Section 3.B.

On the other hand, using a laser projector as the image source, the full-color near-eye display effect is shown in Fig. 3(d), and the target image consists of the letters “R”, “G”, and “B” and their corresponding colors. Figure 3(e) indicates the impact of HOE-1 on the near-eye display effect, when displaying the Chinese characters “Beijing” in the physical environment as augmented reality. The left and right figures show the display effects without and with HOE-1, respectively. It is obvious that HOE-1 causes a slight decrease in the transmittance of the physical environment, but it hardly affects the display effect of the target information. Overall, the impact of solar energy collection on near-eye display is negligible as expected.

HOE has obvious wavelength and angle selectivity, so it can generally only be efficiently reconstructed under recording conditions, as demonstrated in Section 3.A. Nevertheless, by utilizing the grating merging effect, the effective collection wavelength and angle are not limited to the recording conditions any more (see Appendix A.1 for details). As a result, Figs. 4(a) and 4(b) show the definition of the vertical incident angle and the variation of the collected light at the upper and lower edges of the waveguide when white light is illuminated from different vertical incident angles. As shown in Fig. 4(b), the wavelength of the effective collection light is greatly expanded from the recording wavelength 532 nm to the entire visible spectrum. Figure 4(c) shows the detailed simulation and measurement results of the vertical incident angle and central collection wavelength for four different partitions of HOE-1. The experimental results are consistent with the simulation results of variable wavelength reconstruction based on k-vector analysis. In addition, broadening the collection spectrum will also lead to further expansion of the collection angle. Initially, the vertical angle width of effective collection is only about 3° for the recording wavelength of 532 nm. After collection wavelength expansion, the vertical angle width of effective collection increases up to 60°, as shown in Fig. 4(c). Besides, the optical combiner also has a larger horizontal collection angle of approximately 106°, thanks to the larger vertical selection angle of HOE (see Appendix A.1 for details). In summary, HOE-1 can collect solar energy within a large angle range of $106°\times 60°$, and the collection spectral band covers the entire visible spectrum.

In addition to the collection wavelength and angle mentioned above, the effective collection area is also an important indicator. Generally speaking, if the thickness of the waveguide or the total reflection angle is not large enough, the secondary diffraction effect will lead to a decrease in the effective area of HOE-1 due to the premature coupling-out of the collected light (see Appendix A.1 for further analysis). When 532 nm parallel light is incident on the optical combiner, the different results of secondary diffraction under different conditions are shown in Figs. 4(d)–4(f). Here, we demonstrate the impact of secondary diffraction on the effective collection area by displaying the transmitted light. The bright bars in the figures can be equivalently considered as invalid areas where light fails to be collected. As shown in Figs. 4(d) and 4(e), if HOE is not partitioned or the thickness of the waveguide is small (2 mm), the length of the effective collection area is about 17.5 and 15.5 mm, respectively. By comparison, as shown in Fig. 4(f), with a thick waveguide (5 mm) and partitioned HOE, the secondary diffraction effect is eliminated, which results in an increase in the length of the effective collection area to 25 mm. It should be noted that HOE-1 can collect solar energy to both upward and downward directions, so the total length is 50 mm, consequently covering the whole waveguide.

Finally, we conduct tests on the collection and power supply efficiency of HOE-1. The results indicate that the collection efficiency of HOE-1 exceeds 60% at all wavelengths of 457 nm, 532 nm, and 639 nm under their corresponding Bragg angle incidence. It should be pointed out that the collection efficiency is the ratio of the collected light power to the incident light power; therefore it is influenced by both the diffraction efficiency of HOE and the transmission efficiency of total reflection. Additionally, as shown in Fig. 5(a), two solar cells with an area of $6\mathrm{cm}\times 4\mathrm{cm}$ and an output voltage of 3 V, as well as two rechargeable batteries with a rated voltage of 1.2 V are utilized to assess the power supply efficiency under sunlight. The collected light on the bottom solar cell is shown in Fig. 5(b), and the bottom solar cell can charge 17 mAh in half an hour. The charged amount of rechargeable batteries is measured by discharge equipment as shown in Figs. 5(c) and 5(e). Besides, the collected light on the top solar cell is shown in Fig. 5(d), and the top solar cell can charge 11 mAh in half an hour. Overall, the two waveguides of the system can charge a total of 56 mAh in half an hour. The main reason why the bottom solar cell charges more than the top solar cell is that it not only receives the collected light, but also receives some direct sunlight, which can also be achieved in practical near-eye display devices by using transparent eyeglass frames. The stored electrical energy of our system for half an hour can also be calculated as $56\mathrm{mAh}\times 1.2\mathrm{V}=67.2\mathrm{mWh}$. The proposed system can fulfill the function of self-charging using solar energy with a lightweight structure.

Previous research has made many attempts to solve the weight burden and endurance capability of near-eye display systems, but mainly focused on retaining high-capacity batteries and reducing electricity consumption. In this paper, HOE is used to collect solar energy and power the system, and the problems of limited collection angle and secondary diffraction effect are solved. The results indicate that the system has the ability to charge 67.2 mWh in half an hour. Here, we define an indicator called “charging to consumption ratio”, which refers to the ratio of energy stored during charging to energy consumed during operation. Typically, the existing commercial intelligent glasses product Ray-Ban Meta has a rated battery capacity of 154 mAh 0.595 Wh, which can power the device for 4 h. The combination of our charging system and this product will result in a charging to consumption ratio of 0.9. Besides, some commercial AR glasses have a 0.85–1.7 Wh battery capacity that can power the devices for about 2.5–4 h. In this case, the charging to consumption ratio will decrease to 0.20–0.63. Additionally, accounting for charging efficiency, energy loss, variable weather conditions, and nocturnal usage constraints, the battery remains challenging to be fully replaced; nevertheless, the solar-based self-charging solution will inevitably prolong the operating time of near-eye display systems.

Furthermore, there will be many interesting studies in the future, given that the power supply efficiency of current prototypes needs further improvement. On the one hand, the optical multiplexing characteristics [26,27] and more flexible light modulation schemes based on spherical waves or freeform waves are anticipated to increase the collected solar energy. On the other hand, the collection of wasted signal light that is not received by users will also significantly improve power supply efficiency. Finally, the employment of solar cells with high conversion efficiency and power-per-weight [28,29], such as perovskite cells [30,31], will further enhance the output power. In the future, we believe that more efficient energy collection elements, solar cells with higher conversion efficiency, and the integration of lightweight solar cells and eyeglass frames such as temples will inevitably increase the “charging-to-consumption ratio” to one, or even exceeding it.

In summary, this paper proposes a lightweight self-charging holographic near-eye display system that collects sustainable solar energy over a large angle range of $106°\times 60°$. The functional multiplexing waveguide aids in achieving both the collection of sunlight and near-eye display in a compact structure. The self-charging capability effectively avoids many drawbacks of huge-capacity batteries, such as heavy weight, continuous heating, and frequent charging. These improvements also make it possible to miniaturize near-eye display devices. It is worth mentioning that the proposed self-charging method is not limited to the above-mentioned system, but is universally applicable. We believe that our contribution will help sustainable energy become the primary energy source for lightweight near-eye display systems, enabling all-day wear and facilitating its widespread application.