Gallium nitride (GaN) based light-emitting diodes (LEDs) have garnered significant attention due to their extensive applications in solid-state lighting, flat-panel displays, and imaging^[1,2]. Notably, polarized GaN-based LEDs demonstrate substantial potential across various sectors such as machine vision, LED illumination, display technology, and medical imaging^[3–7]. For instance, in machine vision inspection, the utilization of polarized light sources enhances diffuse reflection and light penetration, thereby significantly improving image clarity and precision for subsequent processing and analysis^[8]. For manufacturers of virtual reality (VR) and augmented reality (AR) headsets, polarized light-emitting devices facilitate faster product iteration, reduce dependence on filtered polarizers, minimize device size and weight, and simplify the manufacturing process^[9]. In medical detection systems, high linear polarization from LEDs mitigates interference from non-polarized background radiation, improves energy efficiency, and enables precise polarization control, thus enhancing imaging diagnostics^[5].

Several methods can achieve polarized light emission from GaN-based LEDs. Using nonpolar or semipolar GaN substrates typically results in low polarization extinction ratios (ERs) and high costs^[10,11]. Conventional GaN-based LEDs on c-plane sapphire substrates emit unpolarized light. Alternative methods, such as integrating nanostructures like photonic crystals, patterned thin films, and metal gratings, have been explored^[12–18]. Studies show that metallic nanogratings in InGaN/GaN diodes can produce highly linearly polarized light but block at least 50% of the emitted energy by reflecting transverse electric (TE) polarized light. Efforts to improve transverse magnetic (TM) polarized light extraction efficiency have been limited. Wang et al. reported a highly efficient linearly polarized green LED with a metasurface structure on the sapphire substrate, but its practicality is limited by a complex two-step lithography and bonding process^[14].

In this paper, we propose and demonstrate a highly efficient linearly polarized GaN-based LED grown on a sapphire substrate featuring metallic nanogratings on both the top and bottom surfaces. The bi-layered metallic nanograting (BMNG) metasurfaces integrated on the top emitting surface generate strong TM-polarized light emission, while the asymmetric metallic nanograting (AMNG) metasurfaces at the bottom function as a half-wave plate to enhance the extraction efficiency of the TM-polarized emission. All metallic nanogratings are fabricated using nanoimprint lithography (NIL), significantly reducing manufacturing complexity.

Figure 1(a) illustrates the cross-sectional structure of the proposed GaN-based LED featuring AMNG metasurfaces on both sides. The LED comprises a sapphire substrate, an n-GaN layer, InGaN/GaN multiple quantum wells (MQWs), and a p-GaN layer. A BMNG structure with a dielectric transition layer is deposited on the top GaN emitting surface, while aluminum (Al) AMNGs functioning as a half-wave plate are embedded at the bottom of the sapphire substrate. Figure 1(b) depicts the propagation and polarization conversion processes of both TM- and TE-polarized components within the LED structure.

First, the polarization component of the TM light emitted directly from the quantum well (QW) is transmitted through the BMNGs on the upper surface. Meanwhile, the majority of the TE polarization component (denoted as TE-1) is reflected back into the GaN-QW-GaN structure. The back-reflected TE polarization component passes through the QW layer and is incident on the back metal grating surface. After reflection by the metal grating at the rear, the polarization state of the incident TE component is converted to the TM component due to the wave plate functionality designed into the AMNGs. The converted TM polarization component subsequently traverses the QW structure and exits through the top BMNGs. Due to the imperfect TE-to-TM conversion efficiency of the bottom metal grating metasurface and the scattering/absorption effects within the QW, a small portion of the residual TE component will pass through the top BMNGs, partially reflect back into the structure, and repeat the process. The bottom AMNG recycles previously unemitted TE polarization components, thereby significantly enhancing the extraction efficiency of TM-polarized light emission while preserving a high degree of polarization.

The overall performance of the integrated device structure, specifically the extraction efficiency of polarized emission and the degree of polarization, is primarily determined by two key components. The top BMNG layer dictates the degree of polarization, while the bottom AMNG metasurface facilitates polarization conversion, thereby enhancing the extraction efficiency of polarized emission. The designed BMNG structure is illustrated in the upper part of Fig. 1(a). It consists of a dielectric grating (H2=80nm, W1=75nm, and P1=150nm) and two interleaved metallic gratings made of Al (H3=55nm, W1=75nm, and P1=150nm), all situated on a low-refractive-index dielectric transition layer (H1) atop the GaN emitting surface. The incorporation of this low-refractive-index dielectric transition layer significantly enhances TM transmission and extraction efficiency from the high-refractive-index GaN substrate due to the Fabry–Perot (F–P) effect between the BMNG and the GaN substrate. This F–P coupling between the two metal grating layers effectively relaxes the stringent requirements on the grating dimensions (both period and height), thereby substantially reducing fabrication complexity.

The BMNG structure parameters employed at the top are based on our previous design^[14]. The detailed optimization process will not be reiterated here; instead, we present only the relevant parameters and results. The grating period P1=150nm, H1=55nm, H2=80nm, and H3=180nm. The simulation results demonstrate that within the spectral range of green light, the TM transmission exceeds 85%, while the ER [ER = 10log(TMT/TET)] surpasses 38 dB. Notably, both the TM transmission and ER maintain high levels across a wide angular range of ±60°. The polarization performance of the BMNGs with a transition layer at the top fundamentally determines the degree of polarization emitted by the integrated device.

The structural parameters of the bottom AMNGs are also specified in Fig. 1(a). The nanogratings are composed of Al, with a period denoted as P2, a width as W2, and a height as H4. By adjusting the width and height of the AMNGs, various wave plate functionalities can be achieved. The characteristics of the AMNGs were designed and calculated using the finite-difference time-domain (FDTD) method. To simulate the performance of the AMNG structure, the XYZ coordinates and relative orientations of the structural elements were defined. Periodic boundary conditions were applied at the X and Y boundaries, while perfectly matched layer (PML) conditions were used at the Z boundary. The incident field was assumed to be linearly polarized light at an angle of 45° relative to the X and Y axes, consistent with the typical application scenarios of conventional wave plates. Due to the breaking of azimuthal symmetry, the AMNGs can support both even and odd resonator modes when the electric field components are aligned along the X and Y axes. With appropriate design parameters for P2, W2, and H4, the phase delay induced by the resonator mode in two orthogonal directions can rotate the polarization direction of the reflected light by 90° compared to the incident polarized light reflected from the metasurface.

To achieve the functionality of the AMNGs half-wave plate, it is necessary to achieve a phase difference of π and an amplitude ratio of 1 between the electric fields in two orthogonal directions, as defined for a half-wave plate. We have analyzed the variations in phase difference Δφ and amplitude ratio |Ex|/|Ey| with respect to width W2 and height H4, as illustrated in Fig. 2. From the phase difference results depicted in Fig. 2(a), it can be observed that, to achieve a phase difference of π, the grating width should not exceed 100 nm and the height should not exceed 170 nm. According to the electric field amplitude ratio shown in Fig. 2(b), an amplitude ratio of 1 can be achieved for any line width when the grating height is within the range of 90–145 nm.

To further refine the parameters of AMNGs, we conducted an analysis of their transmittance at varying heights and widths, as illustrated in Fig. 2(c). The results indicate that the transmittance of the AMNGs exhibits a decreasing trend when the height exceeds 130 nm. Taking into account the AMNG characteristics and the allowable preparation tolerance, we selected W2 in the range of 70–80 nm and H4 between 100–130 nm.

To investigate the characteristics of the designed AMNGs across a broad spectrum of green light, we computed the phase difference and amplitude ratio for the AMNG structure with parameters P2=250nm, W2=75nm, and H4=110nm over the wavelength range of 500–550 nm, as illustrated in Fig. 3(a). The figure reveals that within the wavelength interval of 520–540 nm, the variations in phase difference and amplitude ratio remain within ±5% of the desired π and 1, respectively, thereby achieving an effective half-wave plate function. Additionally, the corresponding reflectance is consistently maintained at approximately 66.5%, as shown in Fig. 3(b). The approach of enhancing polarization extraction efficiency through the design of asymmetric metal nanograting metasurfaces on both the upper and lower surfaces of LEDs can be generalized to other wavelength bands (e.g.,  blue light, red light), as well as various types of light-emitting devices. This concept may further serve as a valuable reference for the investigation of multi-parameter light field control.

The preparation of the top BMNG structure involves the following steps: A 250 nm thick PMMA layer is coated on the emission surface of the GaN LED. Subsequently, an NIL process is employed to imprint the PMMA grating structure over an area of 1cm×1cm. Figure 4(a) illustrates the scanning electron microscope (SEM) image of the BMNGs with a period of 150 nm and a line width of approximately 75 nm (FEI Quanta 400 FEG). The height of the PMMA grating is 80 nm (as shown in the inset figure), while the remaining PMMA thickness serves as a transition layer at 170 nm. Finally, a 55 nm thick Al layer is deposited on the PMMA surface via magnetron sputtering.

For the bottom AMNG metasurface, PMMA is spin-coated onto the sapphire substrate located on the backside of the LED. The PMMA grating structure is then imprinted using NIL over an area of 1.5cm×1.5cm, resulting in a final grating structure with a period of 250 nm, a width of 175 nm, and a height of 200 nm. The residual PMMA at the bottom is subsequently removed using a glue remover, followed by etching of Al2O3 via an inductively coupled plasma (ICP) etch machine. The final etch depth is approximately 124 nm, with a width of 78 nm. Figure 4(b) shows the top view and cross-sectional view of the sapphire structure after etching. Finally, a 2 µm thick Al layer is deposited on the alumina grating surface to ensure complete filling of the grooves with metallic aluminum. It should be noted that the optimized structure of the AMNGs is achieved by preparing the medium grating on the sapphire substrate and subsequently depositing Al, so the provided structural parameters are opposite to those of the optimized metal grating.

The measurement principle diagram illustrating the transmission and polarization characteristics of the integrated GaN-based LED device is presented in Fig. 5(a). The setup consists of a linear polarizer at the front end of the sample and a polarization measuring instrument detector (Thorlabs PAX5720VIST) at the back end. Polarized light detection is achieved by rotating the linear polarizer. Figure 5(b) displays the electroluminescence (EL) spectrum at room temperature for GaN-LED devices with the AMNG structure on the top emission surface and the BMNG metasurface structure on the sapphire substrate’s backside, under a forward current of 100 mA. The emission wavelength is 533 nm, with a full width at half-maximum (FWHM) of 42 nm. Even if there is a minor shift in the central spectrum, it will not substantially affect device performance, as clearly demonstrated by the optimization results presented in Fig. 3. The inset in Fig. 5(b) shows images of the polarization emission after rotating the integrated GaN LED with a linear polarizer in different directions (θ=0°, 90°). From the series of optical microscope images, it is evident that the emitted light from the LED is wavelength or FWHM, indicating that the designed nanopattern maintains uniform transmission efficiency within the green spectral range, exhibiting neither wavelength modulation nor filtering effects. Figures 5(c) and 5(d) present the test results for the transmission efficiency and polarization characteristics of a GaN-based LED incorporating AMNG metasurfaces integrated on both sides. Figure 5(c) illustrates the EL green luminescence intensity of GaN-based LEDs featuring asymmetric metasurfaces integrated on both upper and lower surfaces at various angles θ. To evaluate the improvement in extraction efficiency provided by the integrated asymmetric metasurfaces on both surfaces, two comparative samples were examined: one featuring a BMNG metasurface on top but with an untreated sapphire substrate at the bottom, and another with the same BMNG metasurface along with a uniformly vapor-deposited Al film on the back of the sapphire substrate. The experimental findings, as depicted in Fig. 5(c), indicate that, within a wide angular range of ±60°, the polarized light extraction efficiency of the device with integrated asymmetric metasurfaces on both upper and lower surfaces increases by an average of 50% compared to devices with unstructured sapphire substrates at the bottom. Furthermore, this efficiency is enhanced by an average of 32% over the same angular range when compared to devices with Al films on the bottom of the sapphire substrate. The comparison reveals that TE-polarized light reflected by the integrated BMNG structure at the top can be converted into TM-polarized light via the bottom AMNG wave plate structure and repeatedly emitted through the top metasurface. In contrast, the TE-polarized light component in devices with an Al film vapor-deposited on the back remains confined within the device and contributes negligibly to the TM-polarized light output. Notably, the extraction efficiency of TM-polarized emission in integrated devices falls short of the corresponding theoretical predictions. By comparing the extraction efficiencies of LEDs without a bottom structure and those with a bottom Al film, we can understand the reduction in extraction efficiency. The Al film on the backside of the sapphire substrate serves to reflect light emitted from the rear toward the top, allowing it to exit through the surface. Under ideal conditions, such as no absorption in the quantum well structure, equal radiation intensities at both the front and back surfaces of the quantum well, and 100% reflectivity of the Al film, the TM extraction efficiency of an LED with a bottom Al film should be double that of an LED with a bare sapphire substrate. However, experimental findings show that the presence of an Al film only results in a 20% increase in efficiency compared to devices without it.

This discrepancy suggests several potential mechanisms affecting light propagation in multi-layer devices. For instance, the intensity of light emitted from the rear of the quantum well may be lower than that emitted from the front. Additionally, the imperfect reflection of the aluminum film, which does not reach 100%, diminishes its contribution to the overall TM emission intensity. More critically, we propose that significant absorption occurs as light traverses the multi-layer quantum well structure, likely serving as the primary mechanism for reduced TM emission extraction efficiency. When the AMNGs is integrated into the back of the device, TE-polarized light reflected from the top surface will be converted into TM-polarized light and emitted through the BMNGs. Simultaneously, as TM- and TE-polarized light propagates downward through the quantum well, it undergoes conversion into their respective complementary polarization states (i.e., TM-polarized light converts to TE-polarized light and TE-polarized light converts to TM-polarized light) before being reflected. During this process, TM-polarized light is radiated outward via the top BMNGs, while TE-polarized light is reflected back into the device and undergoes a secondary conversion through the AMNGs at the bottom. Ultimately, it is emitted in the form of TM-polarized light through the top grating. Given that this portion of light experiences two transmission processes, some absorption loss inevitably occurs, resulting in its relatively smaller proportion in the total output light. In addition, manufacturing imperfections, including size variations, roughness, and defects in the metal grating, could also lead to the conversion of TE mode to TM mode, further decreasing extraction efficiency. For example, the width and height of the fabricated AMNGs exhibit certain deviations from the theoretical design values. As shown in Fig. 2, the simulation results indicate that minor variations in width and height have a limited effect on the amplitude ratio and transmittance but can induce phase changes. The primary function of AMNGs is to achieve the half-wave plate effect, which involves converting TE-polarized light into TM-polarized light. This conversion requires a phase difference of π. If the phase is altered, the incident TE-polarized light cannot be fully converted into TM-polarized light during reflection, resulting in a residual TE light component. Consequently, this reduces the polarization conversion efficiency and further affects the intensity of the top BMNGs emitting polarized light.

Figure 5(d) presents the ER results for the integrated device and two comparative devices. The experimental data reveal that, at θ=0°, the ER value of the LED featuring asymmetric metal nanograting metasurfaces on both the top and bottom surfaces reaches approximately 25 dB. Over a 60° angular range, the integrated device exhibits an average ER value exceeding 20 dB. Conversely, the LEDs with configurations of a top BMNG combined with either a sapphire substrate or a uniform Al film at the bottom demonstrate ER values of approximately 20 dB. This similarity in ER values is attributed to the fact that the degree of linear polarization is primarily determined by the top BMNG structure. The bottom AMNG functions as a half-wave plate, converting the TE-polarized component into the TM-polarized component, thereby enhancing the extraction efficiency of TM-polarized light emission.

To more effectively highlight the advantages of the designed asymmetric metal nanograting metasurface in high-efficiency polarized light LEDs, we provide a detailed comparison of the ER, extraction efficiency, and fabrication process for LEDs with various polarization structures, as presented in Table 1. Through comparative analysis of the data, it is evident that implementing a double-layer metal grating structure on top enables favorable polarization characteristics across a wide angular range. However, due to the inherently unpolarized nature of the LED light source, the efficiency of polarized light obtained remains relatively low. Theoretically, using only the top double-layer metal grating would result in over 50% loss of polarization energy. To further enhance the extraction efficiency, the asymmetric metal nanograting metasurfaces we designed exhibit remarkable advantages in both polarization-dependent efficiency extraction and fabrication preparation.

Subsequently, we employed the prepared polarization light source to attempt the verification of the micro-LED electrode detection. A micro-LED with a size of 20 µm and its electrodes were evaluated using a custom-designed polarized light source. For comparative purposes, Fig. 6(a) illustrates the irradiation results obtained with non-polarized light. As observed from the figure, the high reflectivity of the anode causes significant glare, which adversely impacts the accuracy of electrode detection. The integration of a polarizing light source with a polarizer enables selective filtering of specular reflections while retaining diffuse reflections, thereby markedly enhancing image contrast, as demonstrated in Figs. 6(b) and 6(c). In Fig. 6(b), the anode highlighted within the red box exhibits distinct bright spots. Conversely, Fig. 6(c) presents the polarized light imaging outcome following device failure. At this point, the electrode region within the red box has failed, exhibiting black edges and an absence of bright spots. This phenomenon occurs because polarized light is sensitive to internal material stress, residual stress, or micro-cracks in the metal electrode that can be identified by analyzing changes in the polarization state of reflected light, aiding in pinpointing potential failure locations of the electrode. These experimental findings may significantly contribute to the rapid detection of micro-LED electrodes and analysis of device failures, potentially improving overall device inspection efficiency.

We have designed and fabricated a polarized LED based on asymmetric nanograting metasurfaces, characterized by distinct nanograting structures on the top and bottom surfaces. Experimental results demonstrate that this LED achieves an average 34.66% enhancement in polarization light extraction efficiency within a ±60° angular range compared to traditional structures with a metal Al film evaporated on the sapphire substrate’s bottom. Within a 60° angular range, the measured average ER of the integrated device exceeds 21.62 dB. Particularly, at 0°, the degree of polarization attains 24.9 dB. By incorporating a half-wave plate function at the bottom using AMNGs, this highly efficient linearly polarized LED not only simplifies the fabrication process but also reduces complexity while further enhancing the output efficiency of linearly polarized light. The design concept of converting the emission of LED devices from non-polarized light sources to polarized light sources with high extraction efficiency through the synergistic effect of asymmetric metal nanograting metasurfaces is not only applicable for multi-parameter light field control but also demonstrates substantial potential in enhancing the wavefront shaping capability of various light-emitting devices. The design and fabrication of this device offer a novel solution for the application of polarization LEDs in micro-LED failure analysis, advanced optical display technologies, optical communication systems, and photonic computing.