IntroductionLanthanide-doped upconversion phosphors are capable of absorbing low-energy photons to emit high-energy photons through a nonlinear anti-Stokes process1, providing immense value for applications in bio-imaging2,3, optogenetics4, laser modulation5, solar cells6 and infrared vision7,8. Typically, high luminous intensity upconversion phosphors are prepared by combining Yb3+ ions with large absorption cross sections as the sensitizers to absorb infrared light, Er3+ ions with multiple energy levels as the activators to emit visible light and low phonon energy NaYF4 as the substrates9. However, the inherent low optical absorption and response of Yb- and Er-doped NaYF4 make exciting upconversion require a powerful light source to enable adequate amounts of dopant jumping from the ground state to the excited state10. Multilayered core-shell structures11,12,13,14, dye sensitization15,16 and surface passivation17,18 have been proposed to improve the sensitizer-to-activator energy transfer efficiency and address the high doping concentration quenching effect, improving the luminous intensity by several orders of magnitude, but still remain the challenge of high excitation thresholds. Recent studies demonstrated promising approaches to lower the excitation threshold by employing externally arranged nano- and micro- assembled structures, such as plasma-coupled nanocavities19, dielectric nanoantennas20 and photonic crystal microspheres21,22. Meng et al. achieved detected upconversion luminescence of individual upconversion nanoparticle (UCNP) under 0.45 W/cm2 excitation via coupling plasmonic nanocavity23. Liang et al. improved the luminous intensity up to 5 orders of magnitude by modulating the optical field of UCNPs through an array of dielectric microbeads, and realized imaging at 0.012 W/cm2 infrared light21. However, these external enhancement units limited the application scenarios and adaptability of upconversion phosphors, and inevitably incurred optical scattering and photoenergy dissipation, which were detrimental to the efficient utilization of photons, especially in low-power stimulation.Ma and co-workers inventively injected UCNPs for direct connection with retinal photoreceptor cells to empower mammals with infrared vision through converting invisible infrared light into visible light8. This proximity stimulation of photoreceptor cells diminished the energy loss during upconversion fluorescence transmission and effectively utilized the optical sensitivity of retinal rod cells, thereby reducing the upconversion luminescent photons required for infrared vision in the mammalian eyes. The approach showed the feasibility of upconversion-driven infrared vision under 1.62 mW/cm2 infrared light, however, the light utilization efficiency was 10-5 less than visible vision due to the quantum yield below 0.5% and its application was limited by injection surgical risks such as infection, inflammation, or retinal damage24. Commonly, inadequate retinal photoreception induced dull vision or even night blindness under dark conditions25,26. For certain irreversible retinal degenerative diseases in which partially functional retinal rod cells and photoreceptor neurons remain, it is desirable to realize revolutionary therapeutic strategies beyond gene and stem cell therapies27,28. The development of enhanced absorption and more efficient upconversion materials is essential and urgent for the opportunities presented by visual restoration and night vision in mammals. Considering that light exposure above a power of 0.3 mW/cm2 could impair the retina by thinning the outer nuclear layer (ONL) which contains photoreceptor cells29,30, it is necessary to further lower the excitation threshold and improve the upconversion efficiency.Here, we report a directional upconversion microsphere with consecutive β-phase NaYF4:Yb, Er shell, which was incorporated into a photonic crystal contact lens amplifier to achieve an ultralow excitation threshold for night blindness treatment. Using upconversion microsphere (UCM) as the heart of this device, we developed an approach that exploited the resonant cavity based on the NaYF4:Yb, Er shell and the optical convergence effect for directional upconversion enhancement. The core-shell structure is constructed with photoactive NaYF4:Yb, Er exposed on the outside and inert SiO2 as the inner core (Fig. 1a), as opposed to the conventional inert SiO2 layer on the outside17,31. This designed UCM achieved bright upconversion luminescence under low-power excitation by promoting Yb3+ absorbing infrared photons and transferring energy to enable sufficient quantity of Er3+ jumping to the excited state, thus increasing the radiative jump (Fig. 1a). The excitation spectrum exhibited broadband upconversion with substantially pure green light emission in the range of 800-1100 nm in the simulated solar spectrum (Fig. 1b). Moreover, as the particle size increased, finite-difference time-domain (FDTD) analysis of the UCMs revealed that 980 nm infrared light gradually to produce directionality and an enhanced photofield and electric field (Fig. 1c, Supplementary Fig. 1). The upconversion luminescence intensity of 500 nm UCM in the forward direction of the incident light is approximately 150 times greater than that in the backward direction (Fig. 1d) and three orders of magnitude greater than that of upconversion nanoparticles (Supplementary Fig. 2). To obtain a wearable efficient upconversion device, we designed an upconversion contact lenses (UCCL) with a 4-layer structure consisting of an antireflective layer (ARL), an upconversion luminescence layer (UCL) contained UCMs, and green and infrared light reflective layers (GRL and IRL) assembled from photonic crystals (Fig. 1e, Supplementary Fig. 3). Under the illumination of infrared light retained from AM 1.5 G (Supplementary Fig. 4), the light modulation of UCCL significantly improved the image clarity compared to a conventional contact lens, demonstrating powerful upconversion capability.Fig. 1: Directional luminescence enhancement of upconversion core-shell microspheres and the application in a 4-layer upconversion contact lens.a Schematic illustrations of SiO2@NaYF4:Yb,Er upconversion core-shell microsphere (UCM) and upconversion luminescence (UCL) enhancement. As the UCM size became larger, Er3+ ions obtained more energy from Yb3+ ions under low-power infrared light, and the quantities of Er3+ ions jumping from the ground state (E0) to the intermediate state (E1) and then to the excited state (E2) greatly increased. The probability of radiative jump from (E2) to (E0) was greatly improved, thus enhancing the upconversion luminescence. b Comparison of the excitation and emission spectra of the 500 nm UCM with the solar spectrum (AM1.5 G). c Photofields and enhancement factors of electric field of 300, 400, 500 nm UCM simulated by finite difference time domain (FDTD) for 980 nm input. Direction of light incidence is from left to right. Enhancement factor (Eef) is calculated as the ratio of the electric field intensity of the UCMs and upconversion nanoparticle (UCNP). The scale bar represents a length of 100 nm. d Directional upconversion emission spectrum of 500 nm UCM under 980 nm (1.5 mW/cm2) excitation. e Schematic illustration of the upconversion contact lens (UCCL) composed of microspheres. f Comparison of UCCL and conventional contact lens (CL) imaging performance. The scale bar represents a length of 2 cm.Full size imageResultsMechanistic investigationSiO2@NaYF4:Yb,Er microspheres was conceptualized to increase the infrared absorption by lengthening the travel length of light within the shell through the reflection-absorption effect. As light travelled from the light-dense NaYF4:Yb,Er shell32 into the light-sparse medium of SiO2 core33,34, the differential refractive indices and the shell shape induced multiple internal reflections (Fig. 2a). The NaYF4:Yb, Er shell acts as both a resonant cavity and an upconversion layer, and the infrared absorbance of the 500 nm microspheres increased 8-fold over that of the nanoparticles due to the multiple reflection-absorption-upconversion effects (Fig. 2b). In addition, the long path of the infrared light within the upconversion shell, enhanced the interaction of the light with the photoactive ions (Yb3+ and Er3+) and improved the energy transfer efficiency between Yb3+ and Er3+ ions, as manifested by an increased fluorescence lifetime (Fig. 2c). This is the reason that UCM exhibited broadband absorption property under infrared light and significantly enhanced green luminescence by transferring more energy to the higher energy levels of 4I11/2 and 4F7/2 of Er and then improving the probability of the radiative jump of 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 (Fig. 2d). Surprisingly, the UCM exhibited a significant directional upconversion luminescence, especially under low power excitation (Fig. 2e). FDTD simulations revealed that the UCM accumulated infrared flux on one side of the microsphere to generate a strong electric field, which was enhanced by 1200-fold of 500 nm UCM compared to 40 nm UCNP along the direction of incident light (Fig. 1c, Supplementary Fig. 1). This strengthened electric field environment dramatically increased the incidence of electronic energy level radiative jumps of lanthanide atoms under low-power light, raising the enhancement factor up to three orders of magnitude (Fig. 2f). Moreover, we additionally synthesised β-phase NaYF4: Yb,Er block with the size of about 500 nm, however, the luminous intensity is much lower than that of 500 nm UCM, confirming that the size effect was not the main reason for the upconversion enhancement (Supplementary Fig. 5). To integrate UCM into wearable device and further enhance the directional upconversion luminescence, we designed sandwich-shaped photonic crystal structures to increase the utilization efficiency of infrared light (Fig. 2g). The IRL with high reflectivity for infrared light at approximately 1000 nm facilitated secondary utilization of the incident infrared light by the UCL, and propagated the generated green light along the incident direction with 32% synergistic reflection of GRL (Fig. 2h). With the cooperative efforts of the upconversion core-shell microsphere-induced directional convergence, multiple reflection-absorption-upconversion by the NaYF4:Yb, Er shell resonant cavity and photonic crystal amplifiers, 4L-UCCL achieves an ultralow threshold of 0.0025 mW/cm2 at 980 nm excitation (Fig. 2i).Fig. 2: Mechanism of upconversion luminescence enhancement and directionality.a Schematic illustration of multiple reflection-absorption-upconversion of the NaYF4:Yb, Er shell. Refractive indices (n) of SiO2 and NaYF4:Yb,Er were 1.46 and 1.48, respectively. b Absorptivity of infrared light. c Fluorescence lifetimes of different-diameter UCMs under 980 nm excitation with an emission wavelength of 542 nm. d Proposed upconversion energy transfer paths of SiO2@NaYF4:Yb, Er microspheres under 800-1100 nm excitation. The black solid, dashed, colored solid, and curved lines indicate absorption, energy transfer, emission, and multiphoton relaxation, respectively. Black and gray represent high and low incidence, respectively. e Directional factors for upconversion emissions (542 nm) under infrared excitation (980 nm) of different powers. Directional factor is calculated by the ratio of the forward and backward upconversion emission intensity. Data are presented as mean with standard deviation of n = 3 independent samples. f Enhancement factors of UCMs for upconversion emissions (542 nm) under infrared excitation (980 nm) of different powers. Enhancement factor is calculated as the ratio of the upconversion luminescence intensity of the UCM and UCNP along the direction of incident light. Data are presented as mean with standard deviation of n = 3 independent samples. g Schematic of photonic crystal amplifiers for enhanced directional upconversion. h Reflectivity of light incident from the front and back of the 4-layer contact lens (4L-UCCL). i Excitation threshold of 4L-UCCL is 600 times lower than that of UCNP under 980 nm irradiation.Full size imageCharacterization and propertiesUnlike adsorption-assembled UCNPs on the surface of microspheres35, we constructed a continuous-phase dense shell generated at high temperatures and pressures with high structural stability (more details in Methods section). By using polyethylene glycol as a ligand between SiO2 and rare earth elements in a hydrothermal method, we successfully prepared core-shell microspheres with particle sizes of about 300, 400, and 500 nm (Fig. 3a, Supplementary Fig. 6,7). Transmission electron microscopy (TEM) and elemental mapping images revealed core-shell structure of the SiO2@NaYF4:Yb,Er microspheres, with a dense shell layer of about 50 nm after NaYF4:Yb,Er deposition (Fig. 3a, Supplementary Fig. 8,9). As shown in Fig. 3b, the crystalline phases of the microspheres changed from bare amorphous silica to the β-NaYF4:Yb,Er which was an efficient upconversion crystal structure36 before and after the growth of NaYF4:Yb,Er. Fourier transform infrared spectroscopy (FTIR) indicated that the polyether as a ligand and the absence of vibrational peaks of harmful hydroxyl group (Fig. 3c) that cause non-radiative upconversion interferences6. The ultradepth-of-field micrograph clearly showed the green upconversion fluorescence of the shells of UCMs under weak infrared light irradiation (Fig. 3d). The 500 nm SiO2@NaYF4:Yb,Er microspheres, poly(hydroxyethyl methacrylate-methyl methacrylate, HEMA-MMA) microspheres and SiO2 microspheres were synthesized (details in Methods section), and utilized to prepare UCL, GRL and IRL by dispersing in acrylate monomer mixing solutions and then thermally curing in disposable cast contact lens molds (Fig. 3e). The molded UCL was sandwiched with reflectance-verified GRL and IRL (Supplementary Fig. 10), and ARL was applied to increase the overall transmittance by reducing light reflection from the surface (details in the Methods section). The images demonstrated the compact attachment of the four layers and four types of microspheres in each layer, including hollow silica microspheres, upconversion microspheres in polymer, well-arranged PHEMA-MMA and SiO2 microspheres (Fig. 3f, Supplementary Fig. 11). Due to the similar refractive indices of the microspheres and the acrylate matrix, upconversion contact lenses have the high transmission of visible light without affecting normal vision (Fig. 3g). Eventually, the quantum yield of the multi-enhanced 4-layer UCCL is 30-fold greater than that of upconversion nanoparticles and reached 9.4% under ultralow excitation power (Fig. 3h). We validated the significant visible light benefits from the upconversion contact lenses due to the efficient infrared upconversion, and the advantages enhanced with the higher ratio of infrared to visible light (Fig. 3i, Supplementary Fig. 12). The infrared vision strategy of UCCL represents great potential for desirable improvement in night blindness.Fig. 3: Characterization and properties of upconversion core-shell microspheres and 4-layer upconversion contact lens.a Transmission electron micrographs (TEMs) and elemental mapping images of 500 nm SiO2@NaYF4:Yb,Er upconversion core-shell microsphere. The experiment was repeated three times with similar results. b X-ray diffraction patterns of UCMs and bare SiO2 microspheres. c Fourier transform infrared spectroscopy of UCMs. d Ultradepth-of-field micrograph of 500 nm UCM illuminated by infrared light (980 nm, 1.5 mW/cm2). The experiment was repeated three times with similar results. e Schematic illustrations of fabricating 4-layer upconversion contact lens. Mold sandwich-structural contact lens consisting of upconversion luminescence layer (UCL), green and infrared light reflective layers (GRL and IRL); form antireflective layer (ARL) by ultraviolet (UV) light curing; demold 4-layer upconversion contact lens. f Microstructure of the 4-layer upconversion contact lens and its interior microspheres. The images on the right from top to bottom showed TEM of the hollow SiO2 microspheres of the ARL, scanning electron micrograph (SEM) of the PHEMA-MMA photonic crystal structure of the GRL layer, SEM of the UCM in polymer of the UCL layer, and the SEM of the SiO2 photonic crystal structure composing the IRL layer, respectively. The experiment was repeated three times with similar results. g Optical transmittances of contact lenses. h Upconversion luminescent quantum yields. Data are presented as mean with standard deviation of n = 3 independent samples. i Total light power after contact lenses modulation under combined light irradiation with 0.2 mW/cm2 532 nm light and 980 nm infrared light of different power. Data are presented as mean with standard deviation of n = 3 independent samples.Full size imageInfrared vision application for the treatment of night blindnessWe established a retinal degeneration (RD) model via light-induced retinal damage in rabbit eyes37 and restored vision through wearing upconversion contact lenses (Fig. 4a). Hematoxylin-eosin staining revealed that intense light damaged the retinal rod cells associated with dark-light vision perception (Fig. 4b), which caused night blindness in dark lighting conditions38. The significant reduction in the outer nuclear layer (ONL) confirmed the successful establishment of the retinal degeneration model (Fig. 4c). Pupillary light reflex (PLR) experiments39 were recorded under an infrared camera (Fig. 4d). Under the same light conditions, the normalized pupil areas of the control and RD rabbits before and after wearing the UCCL were calculated and plotted (Fig. 4e). The pupils of the control rabbits strongly contracted upon 532 nm exposure; however, the pupil contraction of the RD rabbits was inconspicuous, which was attributed to diminished retinal photoreceptor function. Owing to the upconversion of infrared to visible light with the UCCL, the light response of the RD group significantly reverted to no less than that of normal eyes. Furthermore, long-term wear of the UCCL (7 days) had no effect on the corneal epithelium, ensuring safety (Fig. 4f). We tested in vivo electroretinogram (ERG) responses to analyze infrared vision (Fig. 4g). The ERG response to 532-nm light stimulation in the eyes of rabbits with RD was much lower than that in the eyes of normal rabbits and lacked a 980-nm light stimulus response. After wearing the UCCL, RD rabbit eyes acquired light-sensitive vision far beyond that of normal eyes under low-power dual stimulation at 532 and 980 nm (Fig. 4h). To further investigate the practicality and benefit of infrared vision by UCCL, the behavioral cognition was constructed by exposing rabbits with a green light and feeding them timothy hay40. The rabbits were not given food or water for 24 hours prior to testing and were kept in a dark environment. Different light conditions were administered in a partitioned room, and the required time was recorded that the rabbits reached the identical-distance location from a fixed starting point (Fig. 4i). Due to the retinal weak ability to perceive light, rabbits with retinal degeneration exhibited longer arrival times than normal rabbits, which could be significantly shortened by wearing the UCCL (Fig. 4j). The infrared visual superiority endowed by the UCCL consistently improved by increasing the optical power ratio of infrared to visible light, even exceeding normal rabbit visual performance (Fig. 4k). Benefiting from versatile structural-induced optical modulation and functional lens designs, UCCL realized a reversible non-invasive upconversion infrared vision with ultralow excitation threshold, efficient utilization of photons, practicality and safety. Moreover, as a sensitive and effective alternative to regulate light stimulation, UCCL has the potential to facilitate other phototherapeutic applications41, such as improving night vision in patients with congenital stationary night blindness42 and providing certain frequencies of light to Alzheimer’s disease43.Fig. 4: Treatment of retinal degeneration in rabbit eyes with upconversion contact lenses.a Schematic illustration of the construction of a photodamage-induced retinal degeneration model in rabbits and treatment with upconversion contact lenses. b Hematoxylin-eosin (HE) staining of retinas at the posterior pole of untreated eyes and photodamaged eyes. GCL ganglion cell layer, INL inner nuclear layer, ONL outer nuclear layer, PR photoreceptors, RPE retinal pigment epithelium. The scale bar represents a length of 50 μm. c Thickness of the ONL was measured on the basis of photographs of HE-stained sections. Data are presented as mean with standard deviation of n = 6 independent ONL samples. d Images showing pupil constriction from the control, retinal degeneration (RD) and RD with UCCL rabbits under no light, 532 nm and 980-nm light stimulation (40 s). The scale bar represents a length of 200 μm. e Dose-response curves of normalized pupil constriction. Data are presented as mean with standard deviation of n = 3 independent experimental rabbits (***p = 0.00062 for RD group without vs. with UCCL under the stimulation of 532 nm and 980 nm light). Significant difference was set at ***p