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Abstract
Augmented reality (AR), a technology that superimposes virtual information onto a user's direct view of real-world scenes, is considered one of the next-generation display technologies and has been attracting considerable attention. Here, we propose a flat optic AR system that synergistically integrates a polarization-independent metalens with micro light-emitting diodes (LEDs). A key component is a meticulously designed metalens with a numerical aperture of 0.25, providing a simulated focusing efficiency of approximately 76.5% at a wavelength of 532 nm. Furthermore, the laser measurement system substantiates that the fabricated metalens achieves a focusing efficiency of 70.8%. By exploiting the reversibility of light characteristics, the metalens transforms the divergent light from green micro-LEDs into a collimated beam that passes through the pupil and images on the retina. Monochromatic pixels with a size of 5×5 µm2 and a pitch of 10 µm can be distinctly resolved with a power efficiency of 50%. This work illustrates the feasibility of integrating the metalens with microdisplays, realizing a high-efficiency AR device without the need for additional optical components and showcasing great potential for the development of near-eye display applications.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The implementation of augmented reality (AR) primarily relies on see-through head-mounted displays (HMDs) for users. This technology is revolutionizing the way users interact with information by overlaying computational virtual data onto real-world scenes, applications of which extend to entertainment, education, healthcare, and the military [1–3]. Various eyepieces, such as free-form optics [4,5], diffractive optical elements (DOEs) [6,7], and holographic optical elements (HOEs) [8,9], have been proposed to facilitate see-through near-eye displays. Nonetheless, these solutions continue to confront significant hurdles in achieving the portability and practicality of AR glasses, primarily due to the inherent limitations of conventional optical elements. Specifically, the bulky size and low efficiency of the existing AR system lead to excessive power consumption, problematic heat dissipation, and discomfort in terms of user wearability. Consequently, developing an optical system with a compact form and high operational efficiency is crucial for realizing practical AR displays.
Metasurfaces are two-dimensional elements composed of artificially fabricated subwavelength scatters termed meta-units, enabling the arbitrary manipulation of the phase, amplitude, and polarization of light waves by modifying the parameters of these nanostructures [10,11]. Through algorithmically arranging meta-units, metasurfaces have demonstrated a unique ability to manipulate light for diverse applications, including beam steering [12–14], holography [15,16], and polarization conversion [17–19], and so on. A metalens is a type of metasurface primarily used to manipulate light wavefronts, enabling the focusing and attainment of specific optical characteristics through the application of specialized phase modulations. The flat structure of the metalens allows it to overcome the limitations associated with miniaturizing optical elements, a challenge encountered in flat optics when employing traditional refractive and diffractive optics [20]. Several excellent studies have reported metalenses made of different materials, including those with metal structures [21–23] and others constructed solely from dielectrics. However, metallic metalenses suffer from low operating efficiency in the visible due to their strong intrinsic absorption at optical frequencies. In contrast, dielectric metalenses are recognized for their potential to achieve high transmission efficiency with significantly reduced reflection in the visible spectrum. Various materials have been employed for dielectric metalenses, including silicon (Si) [24], titanium oxide (TiO2) [25], and gallium nitride (GaN) [26]. Among these materials, GaN stands out as one of the most promising options owing to its wide bandgap, high refractive index, and low absorption at visible wavelength. Given its high-performance capabilities and compact design, the GaN-based metalens shows tremendous potential in the realm of flat optics, especially for applications in near-eye displays.
Micro-LEDs (µ-LEDs) constitute to a display technology characterized by the arrangement of micron-sized LEDs, each with a dimension smaller than 50 µm, in specific array configurations. GaN-based µ-LEDs, with outstanding characteristics, such as excellent luminescence, high efficiency, long-term stability, and fast response time, have emerged as next-generation displays [27–29]. Benefiting from versatile fabrication approaches, µ-LED exhibits high flexibility across diverse applications, including augmented reality/virtual reality microdisplays, flexible/transparent displays, and extra-large displays [30–32]. Recently, the demand for high-performance microdisplays has been increasing, driven by the emergence and rapid development of wearable devices. However, in pursuing practical AR HMDs, the inherent limitations of LCD and OLED technologies make it challenging to meet the stringent requirements and criteria of microdisplays [33]. µ-LED microdisplay technology is considered the most promising candidate for achieving the goals of AR due to its distinctive features.
In this work, we propose a compact AR system utilizing a metalens to manage the light signals generated from µ-LEDs. The dielectric metalens is specifically designed for 532 nm incident wavelength by means of ray tracing and wave optics simulations. To prevent efficiency degradation in the system, we implement the polarization-independent metalens, structured through the arrangement of symmetric rectangular meta-units. These meta-units, composed of GaN nanopillars, aim to achieve high transmittance and undergo computational optimization to ensure the necessary phase coverage ranging from 0 to 2π. By employing the near-to-far field method, we obtain the characterization of the simulated metalens, showcasing an estimated focusing efficiency of 76.5% and an FWHM that closely matches theoretical predictions. After undergoing e-beam lithography and a series of fabrication processes, the metalens is validated through laser measurement systems, experimentally confirming a focusing efficiency of 70.8%. On the other hand, we design and fabricate a green light µ-LEDs with a size of 5 × 5 µm2 and a pitch of 10 µm to function as the light source within this optical system. As a self-emissive display, the passive-matrix (PM) µ-LEDs can realize a fine-pitch display with high transparency for this system. For the proof of AR concept, the metalens and µ-LEDs are aligned and integrated using the high-precision six-axis stage and specially designed fixtures, allowing for meticulous control over their relative positioning. By exploiting the reversibility of light characteristics, the metalens transforms the divergent light from µ-LEDs into collimated beams, realizing a direct near-eye see-through system with a power efficiency of 50%. We believe the development of this ultra-thin metalens integrating µ-LED display with such attractive optical properties would pave the way towards realizing next-generation applications for near-eye displays.
2. Design and simulation
Compared to conventional lenses that focus light relying on gradual phase accumulation within refractive media, metalenses accomplish focusing via abrupt phase modulation across subwavelength meta-units. Figure 1(a) illustrates a schematic of a focusing metalens with an incident beam along the z-axis, where x and y coordinates are aligned with the plane parallel to the metalens. To design a metalens functioning like a spherical lens, hyperbolic phase profiles can be modeled regarding optical phase difference [25] as follows.
Fig. 1. (a). An illustration of an on-axis focusing metalens. (b) Configuration of the near-eye AR system based on the propagation-type metalens, with target images generated by the designed µ-LED display.
Some previous studies have reported that the ZEMAX optical software can model diffractive optical elements and optimize the desired phase profiles for metalenses using ray tracing methods [35,36]. The phase profile of the metalens is defined as follows.
where M is the diffraction order, N is the number of the polynomial term, Ai represents the coefficient within the extended polynomial term toward optimizing the smallest root mean square (RMS) spot size using the damped least-squares method, and ${E_i}({x,y} )\; $ signifies the phase distribution of the metalens as a polynomial expansion in the spatial coordinates of x and y. According to this equation, we can calculate and simulate the ideal phase profile for the target metalens with the ZEMAX software. Figure 2(a) presents the ideal target phase distribution of the designed metalens, displaying a symmetric concentric circular contour that signifies a 2π phase shift variation on the subwavelength structures. The plot below depicts the 2π phase shift in the x-direction.Fig. 2. Design and simulation of the metalenses. (a) The ideal target phase profiles. (b) The schematic diagram of a subwavelength structure within a unit cell. (c) Data plots comparing 2π phase-shift and transmittance with varying dimensions. (d) Intensity distribution of a 500 µm-diameter metalens on the y-z plane. (e) Intensity distribution of a 45 µm-diameter metalens on the x-y plane. (f) Cross-sectional intensity profiles of the simulated results for the 45 µm-diameter metalens along the x and y directions compared with the theoretical curve.
We have chosen the propagation-type metasurface for this AR system. Under such cases, each dielectric meta-atom acts as a truncated dielectric waveguide with top and bottom interfaces of low reflectivity. Desired induced phase-shift modulations can be obtained by tuning the parameters of meta-units, including the height, cross-sectional dimension, lattice spacing, and effective refractive index. Taking into account the use of a light source with a wavelength of 532 nm, GaN is chosen as the material for the metalens due to its high refractive index of 2.31 and a significant energy bandgap of 3.4 eV, effectively avoiding interband transitions across the visible spectrum. Considering the potential for mass production, it is crucial to assess the influence of fabrication accuracy on performance. Therefore, a compromise is made in selecting subwavelength structure compositions, opting for square nanopillars, as depicted in Fig. 2(b), in which the symmetric structure is suitable for near-eye displays without specific polarized light. Next, we build a structural phase library that correlates the phase shift and transmittance of meta-units with their geometric parameters, including span, pitch, and height, using the S parameter extraction method in finite-difference time-domain (FDTD) simulation from Lumerical Inc. The optimized nanostructures with high transmittance are utilized to cover the entire 0 to 2π phase shift and achieve good efficiency. We demonstrate the variation in dimensions of these square columnar structures with a height of 650 nm, pitch of about 260 nm, and span ranging from 50 nm to 200 nm, along with their corresponding phase shift and transmittance, as shown in Fig. 2(c). Notably, 2π phase accumulation is achieved by arranging the cross-sectional dimensions of nanopillars within the range of 50 to 196 nm, and all these square structures exhibit excellent transmittance above 93%.
The metalens can be constructed by combining the library of phase accumulation with meta-unit dimensions and the phase profile. We validate the characteristics of the metalens regarding a plane wave with a wavelength of 532 nm, utilizing the FDTD method to perform the numerical simulation. The near-to-far field transformation [37] is then applied to capture the intensity distribution at the focal plane. Given the excessively extensive computational requirements of a 500 µm-diameter metalens, we focus the analysis on the y-z plane, as shown in Fig. 2(d). The simulated focusing position and condition align well with our designs, confirming the realization of a 1 mm focal length for the focusing metalens. To further investigate the characterization of this metalens, we additionally design a scaled-down metalens with a diameter of 45 µm, employing identical simulation procedures while maintaining the same NA value. The 45 µm-diameter metalens, with identical parameters of meta-units, can be regarded as the proportionally reduced version that is not constrained by computational limitations, allowing for comprehensive data collection and characterization analysis. Figure 2(e) shows the intensity distribution of the electric field at the x-y plane. The focusing efficiency is calculated as 76.5%, determined by conducting an integration over the focal plane, specifically within a spot size equivalent to three times the radius of the Airy disk’s full width at half maximum (FWHM), divided by the overall intensity of the light source [38]. Furthermore, the optical performances can be evaluated by comparing the intensity pattern with the theoretical values. The intensity of the Airy pattern follows the Fraunhofer diffraction formula of a circular aperture, which is expressed as the squared modulus of the Fourier transform of the said aperture, given by:
where $I(\theta )$ represents the diffraction intensity at a point within the observation plane, ${I_0}$ is the maximum intensity of the pattern at the Airy disk center, ${J_1}$ is the Bessel function of the first order, k is the wavenumber, a is the radius of the aperture, and $\theta $ is the incident angle. To clearly verify the optical performance of the metalens, the cross-sectional intensity profiles of the simulated results for the 45 µm-diameter metalens along the x and y axes are compared with the theoretically calculated curve from the equation, as shown in Fig. 2(f). The intensity distributions of the simulated data closely match the basic principles of Fraunhofer diffraction. Moreover, the FWHM values in both the x and y axes are approximately 1090 nm, demonstrating a numerical error of less than 1% when compared to the ideal FWHM. We take the proportionally reduced version as a reference for the 500 µm-diameter metalens, obtaining estimated efficiency and demonstrating that metalenses designed through these simulation steps exhibit characteristics consistent with the theory.3. Fabrication of the metalens and µ-LEDs
3.1 Metalens
The metalens is fabricated on a 630 µm-thick sapphire substrate, epitaxially grown with a 650 nm-thick layer of undoped GaN using metal-organic chemical vapor deposition (MOCVD). A 150 nm-thick positive photoresist, AR-P 6200, is spin-coated at 7100 rpm onto the substrate and undergoes a soft bake at 140 °C for one minute. Subsequently, an e-beam writing system is employed for the exposure step with an electron beam current set at 100 pA, creating nanoscale structures with dimensions ranging from 50 to 196 nm. After that, the sample is transferred to the E-gun evaporation machine for deposition of 50 nm-thick nickel (Ni) metal and followed by a lift-off process, serving as an etching hard mask. Then, the GaN film with the patterned Ni hard mask undergoes etching through inductively coupled plasma-reactive Ion etching (ICP-RIE). Finally, the target sample will be obtained following the removal of the residual Ni mask using the piranha solution.
3.2 µ-LEDs
In this research, we have developed and fabricated a µ-LED array intended for integrating with the metalens to showcase the imaging and see-through capabilities of our AR optical design. The PM µ-LED display not only features a cost-effective and uncomplicated fabrication process but also attains remarkable luminosity, fine-pitch resolution, and outstanding transparency, rendering it well-suited for application in this system. We set the total emissive area of the µ-LED array within 200 × 200 µm2, with each µ-LED of an emissive area of 5 × 5 µm2 and a pitch of 10 µm.
Figure 3 depicts the schematic diagram of the process flow of the µ-LED device. A commercial InGaN-based green LED wafer is grown on an 800 µm-thick planar sapphire substrate using the MOCVD system. The epitaxial layers include 2.4 µm-thick undoped-GaN (u-GaN), 2.3 µm-thick Si-doped GaN (n-GaN), a multiple quantum wells (MQWs) active region consisting of 16 periods of InGaN/GaN wells, and 300 nm-thick Mg-doped GaN (p-GaN) epilayer. The fabrication processes are as follows: an initial deposition of a 100 nm-thick indium tin oxide (ITO) layer is performed through electron-beam evaporation, followed by annealing at 500°C for 2 min to establish a p-ohmic contact. Notably, the ITO layer simultaneously functions as a current spreading layer and provides a transparent emission window for the device. ICP-RIE is then performed to achieve a MESA etching with a depth of 1.1 µm, defining the emissive area of the device. The subsequent step is additional isolation etching, reaching the sapphire layer to form isolated trenches between the rows of µ-LEDs, allowing the n-GaN layer to serve as the n-interconnect channels within each row. Subsequently, a SiNx insulating layer is deposited using plasma-enhanced chemical vapor deposition (PECVD) at 300°C. The SiNx layer is selectively removed via dry etching with the patterning photoresist, exposing p-type and n-type electrodes. Next, the p-electrode lines are deposited with metals (Ti/Al/Ti/Au, 200/800/200/600 nm) through electron-beam evaporation, spanning the isolation structures. Finally, a 500 µm-diameter black photoresist is patterned using photolithography, exposing only the specific light-emitting areas. In such structures, the black matrix effectively mitigates unintended illumination caused by total internal reflection in either n-GaN or sapphire, thereby reducing optical crosstalk and improving the contrast ratio. As a result, the top-emissive PM µ-LED device is obtained and can be driven by applying current to the peripheral p- and n-electrodes through the power supply.
Fig. 3. The process flow of the PM µ-LEDs, including (a) MESA, (b) isolation, (c) p-metal, and (d) black matrix.
4. Results and measurement setups
4.1 Metalens
To experimentally validate our AR system, we initially verify the fabrication condition and characterization of the metalens. Figure 4(a) reveals the pattern observed using an optical microscope (OM) following the fabrication of the metalens with a diameter of 500 µm, which is helpful for preliminary confirmation of the fabrication condition. To ensure the process effectively preserves meta-units with dimensions from 50 to 196 nm, the corresponding scanning electron microscope (SEM) image of the metalens is presented in Fig. 4(b). The top view image shows well-defined nanopillars, demonstrating high fidelity to our designed dimensions. Additionally, the SEM image captured at a 45-degree angle is shown in Fig. 4(c). The measurement reveals GaN nanopillars with a height of 650 nm and nearly vertical sidewalls, achieving a high-aspect-ratio metalens.
Fig. 4. (a) OM image of the fabricated metalens with a diameter of 500 µm. (b-c) SEM images of the GaN nanopillars forming the metalens.
After careful scrutiny to ensure that the fabrication dimensions and specifications meet our designs, we set up a series of optical elements to assess the performance of the metalens. Figure 5(a) schematically illustrates the experimental setup for the metalens characterization. The optical path of the laser measurement system is as follows: A 532 nm laser passes through a spatial filter, a long focal length convex lens, the metalens, an objective lens, and finally, the image is captured on a complementary metal-oxide-semiconductor (CMOS) camera. The inset of Fig. 5(b) displays the measured focus spot of the metalens at the x−y plane under illumination with a collimated 532 nm laser at an incident angle of 0°. To further assess the focusing characteristics of metalens, we compare the normalized intensity distribution of the measured focal spot along the x-axis to the ideal Airy function, as shown in Fig. 5(b). The measured FWHM is 1240 nm, closely approaching the theoretically diffraction-limited value of 1090 nm, experimentally demonstrating outstanding performance in light convergence. Furthermore, the measured focusing efficiency can reach up to 70.8%, with efficiency defined as the ratio of the optical power of the focused beam relative to the optical power of the incident beam. When compared to the simulated efficiency of 76.5%, the discrepancy in experimental efficiency can be attributed to the intrinsic loss of GaN and process errors.
Fig. 5. (a) Measurement setup for the metalens characterization. (b) Cross-section of the measured intensity distribution, with the dashed line representing the normalized ideal Airy function. The inset figure shows the measured focal spot at the wavelength of 532 nm.
4.2 AR system
Upon verifying the functional characteristics of the metalens, we integrated it with µ-LEDs to achieve the objective of AR display. Figure 6(a) illustrates the experimental setup of the AR system. Given the imperative need for precise alignment, the E2200B six-axis stage plays a crucial role in this integration, featuring coarse adjustment at 10 µm per increment and fine adjustment at 0.5 µm per increment for the x, y, and z axes, along with 30 arcseconds per scale for the θx, θy, and θz axes. The fabricated µ-LED component is mounted on the stage, and a specially designed fixture ensures stability at the rotational centers of the stage, thereby enabling meticulous displacement. Simultaneously, the metalens is secured in a fixture to maintain a stationary position. Importantly, both fixtures are engineered with a hollow structure to permit light transmission, enabling the realization of a see-through device. A camera is strategically positioned in front of the metalens to monitor the alignment condition between the µ-LEDs and the metalens. This not only ensures precise alignment but also allows for the acquisition of the final imaging results. Figure 6(b) provides a detailed cross-sectional diagram of the components in this AR system. Due to the precise alignment of the µ-LED with the metalens, the 500 um-diameter light-blocking area we designed in the µ-LED device matches the diameter of the metalens, ensuring no compromise in the transmittance of the AR device. By exploiting this feature, preliminary alignment is facilitated by aligning the two identical circular shapes on the x-y plane, while the final alignment is achieved by fine-tuning the z-axis to achieve a focal distance of 1 mm. Through illuminating specific pixels to generate the ‘NTU’ pattern, the correctly aligned µ-LEDs emit divergent light that passes through the metalens, transforming into parallel beams and imaging on either the camera or the retina, allowing the virtual image to be augmented on the real-world scene, as shown in Fig. 6(c). The polarization-independent metalens optimally transforms the non-polarized light from µ-LEDs, achieving a power efficiency of 50%. The efficiency is defined as the optical power received by the pupil at a 2 cm eye relief distance divided by the entire optical power output of the µ-LEDs. The measurement setup and details are available in Supplement 1. Furthermore, the field of view (FOV) of this AR system is 11.42°, with an angular resolution of 34.26 arcmin. Notably, the design ensures that the real environment behind the µLED area is imaged with clarity. This is made possible as light can bypass shaded regions and enter the human eye, taking into account a pupil size of approximately 5 mm. More details and analysis of the see-through transmission in this system are available in Supplement 1. Furthermore, the measured FWHM of the metalens's focal spot is 1240 nm, representing the critical resolvable dimension. Hence, each fine-pitch µ-LED with a size of 5 × 5 µm2 can be clearly imaged.
Fig. 6. (a) Experimental setup for the AR system. (b) The detailed cross-sectional schematic illustration of the components in this system. (c) Augmented image captured by the camera.
5. Conclusion
In summary, we have demonstrated the feasibility of the proposed AR system by integrating the polarization-independent metalens with the designed µ-LEDs. The dielectric metalens has been numerically and experimentally verified to achieve high focusing efficiency at the incident wavelength of 532 nm. A green µ-LED array is designed and fabricated to serve as the light source, with the metalens collimating the divergent light for precise imaging on the retina. The discernible resolution of µ-LEDs featuring a pixel size of 5 × 5 µm2 is attributed to the metalens, characterized by an FWHM of the focal spot at 1240 nm, delineating the minimum pixel critical dimension required for imaging. We experimentally demonstrate a high power efficiency of 50% in the proposed system, realizing a highly efficient direct near-eye see-through display without the incorporation of supplementary optical components. At the current stage, the field of view FOV and resolution are not high enough for AR glasses due to the utilization of a single metalens and a µ-LED array. To meet the criteria for AR display, the future optical design system will involve designing a metalens array with corresponding µ-LED arrays and employing image stitching techniques. In this work, we preliminary confirm the viability of this prototype system and propose an innovative way to improve the performance of existing AR displays. It broadens the applications of metalenses in AR wearable devices, providing a new avenue for the industry and scientific research to advance AR technologies.
Funding
National Science and Technology Council (111-2221-E-002-083-MY3).
Acknowledgment
The authors would like to thank Electronic and Optoelectronic System Research Laboratories, Industrial Technology Research Institute (ITRI) for contributions and useful discussions.
Disclosures
The authors declare no conflicts of interest.
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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