As the sunset sinks into the horizon, the world gradually fades in the eyes of humanity. However, in the perception of snakes, thermal radiation outlines the clear contours of prey; In the compound eyes of fireflies, flowers reflect ultraviolet patterns that are invisible to humans. Beyond the visible spectrum, nature always presents a silent drama of light and shadow, while humans have become absent from this feast due to the limitations of the retina. Now, there is a possibility of breaking through this visual limitation. Recently, the research results of the collaboration between the University of Science and Technology of China and Fudan University and other domestic and foreign research institutions were published in the journal "Cells" under the title "Upconversion Contact Lenses Empowering Human Near Infrared Vision". This study innovatively combines a rare earth particle containing multiple fluorescent emissions with contact lenses, allowing humans to perceive multidimensional information such as time, space, and color of near-infrared light in a wearable form, providing new solutions for the treatment of visual diseases such as color blindness. The essence of infrared light that most mammals cannot recognize is a continuous electromagnetic spectrum, and the wavelength difference determines whether humans can perceive its existence. There is a type of cone cell on the human retina that can specifically respond to the three primary colors of red, green, and blue. The ratio of activation of three types of cells determines the color that humans see. Throughout history, the wavelength range of light visible to humans has been limited to 400 nanometers (purple light) to 700 nanometers (red light). If one could perceive a wider range of near-infrared wavelengths (700-2500 nanometers), it would surpass the limits of human vision. In nature, visible light that can be perceived by the human eye only accounts for a small part of the electromagnetic spectrum. Due to the lower energy carried by infrared light compared to visible light, this characteristic makes it unable to excite the photoreceptor cells in the human retina and difficult to be directly perceived by the visual system of the vast majority of mammals. ”Professor Xue Tian from the University of Science and Technology of China said. In nature, a few organisms have evolved a special ability to perceive infrared radiation. Some snake species such as rattlesnakes and pit vipers have specialized cheek socket organs that can detect long wave infrared radiation emitted by warm blooded animals. Some insects, such as some beetles, can perceive light in the near-infrared range. But this is essentially a thermal perception, not a true 'visual imaging'. However, for the vast majority of mammals such as humans and mice, infrared radiation has always been a blind spot in the sensory world. How to break through the natural barriers of mammalian vision by turning special materials from invisible to visible? In the preliminary research, Xue Tian and the team led by Ma Yuqian, a special professor at the University of Science and Technology of China, as well as other collaborators, injected an upconversion nanoparticle that can convert near-infrared light into visible light into animal retina, achieving for the first time the naked eye near-infrared image visual ability of mammals. In 2019, this research paper was published in the journal Cell. However, the application of intraocular injection in the human body is limited, and how to achieve near-infrared vision through non-invasive means is the key challenge for the practical application of this technology. The flexible transparent contact lenses prepared from polymer materials provide a wearable solution. But there are two prerequisites for preparing near-infrared light upconversion contact lenses: efficient upconversion ability and good optical performance. ”Xue Tian said. To this end, researchers have surface modified upconversion nanoparticles to improve their uniform dispersion in polymer materials. At the same time, polymer materials that match the refractive index of upconversion nanoparticles have been screened, and near-infrared upconversion contact lenses with high doping ratios and high transparency of upconversion nanoparticles have been prepared. Researchers have introduced that rare earth elements have unique optical properties and can convert light of different colors through near-infrared light excitation. The human body can identify the near-infrared "color" by judging the wavelength of near-infrared light that is invisible to the naked eye through the fluorescence color of rare earth nanoparticles. Researchers will precisely integrate thin films containing highly efficient upconversion nanoparticles into conventional soft contact lens matrices, ensuring their high transparency and comfortable wearing. When wearing these glasses, external infrared light passes through the lens, is captured by the nanocrystals, and is converted into visible light in real time. These newly generated visible photons immediately enter the human eye and are normally received and processed by the photoreceptor cells on the retina. The visual cortex of the brain ultimately interprets these signals, allowing the wearer to "see" the previously invisible infrared world. During experimental verification, mice wearing this type of contact lens were able to distinguish near-infrared light information at different time frequencies and orientations. It is worth mentioning that human volunteers wearing this contact lens can not only see a certain range of near-infrared light intensity, but also accurately recognize the time encoded information of near-infrared light. In addition, researchers have developed a wearable frame glasses system with built-in near-infrared light upconversion contact lenses, allowing human volunteers to obtain near-infrared image vision with the same spatial resolution as visible light vision, accurately identifying complex near-infrared patterns. In addition to temporal and spatial information, visual perception can also convey rich information in the color dimension. Researchers have replaced traditional upconversion nanoparticles with tri color orthogonal upconversion nanoparticles to prepare tri color upconversion contact lenses, which can convert near-infrared light of three different spectra into visible light of the three primary colors of red, green, and blue. The experimental results show that by wearing tricolor upconversion contact lenses, human volunteers can effectively recognize three wavelengths of near-infrared light and perceive multiple near-infrared colors. This indicates that tri color upconversion contact lenses can effectively achieve human near-infrared color image vision. There is still room for further improvement in this technology. For example, the current conversion efficiency of the contact lens is relatively low, and it still requires infrared light source assistance to enable human volunteers to recognize near-infrared light; In addition, if the nanoparticles in upconversion contact lenses can achieve directional output of emitted light, it may not rely on the frame optical system and directly achieve fine near-infrared graphic vision mediated by contact lenses. All of these rely on interdisciplinary collaboration between visual physiology, materials science, and optics. ”Xue Tian said. The breakthrough technology of opening up new directions for visual restoration brings far more than a novel experience in the laboratory, and its application prospects are broad and profound. For example, compared to traditional infrared night vision devices that are bulky, power consuming, have limited field of view, and only provide monochrome images, this contact lens solution will provide unprecedented non-invasive "super vision" tools for security personnel to operate in complete darkness or smoke, rescue team members to search for heat sources, engineers to conduct equipment thermal checks, and even ordinary consumers to engage in nighttime activities. This technology also shows potential in the field of information transmission: it can use specific infrared light sources that are invisible to the human eye but can be converted by glasses to design highly covert communication or encryption systems. For example, infrared signals flashing at specific locations can transmit secret instructions to specific wearers in a crowd; The infrared anti-counterfeiting mark on the product can only be displayed when wearing these glasses, greatly improving the reliability of anti-counterfeiting; Museums or confidential areas can use infrared beams to demarcate invisible warning lines, which can be seen when wearing these glasses. More importantly, this technology will bring new hope to visual restoration and enhancement. About 1 in every 12 males and 200 females worldwide have color vision defects, mostly red green color blindness. The root cause lies in the absence or dysfunction of cone cells in the retina, which are responsible for sensing specific wavelengths of red or green. For people with color blindness, color weakness and other color vision abnormalities, this technology provides highly imaginative intervention directions, which can help them break through the natural visual limitations and perceive richer color information through spectral "re encoding". The silent flicker of nanocrystals on contact lenses is a gentle reconstruction of the natural sensory boundaries by scientific wisdom. When infrared spectra cast the first visible glow on the human retina, people were able to glimpse another possibility set by evolution beyond millions of years. Science endows us not only with tools, but also with new senses and languages for understanding the vast universe. (New Society)