From Deep-Sea Glimmer to Scorching Sun

From the faint glow of the deep sea to the scorching plains under the sun, life constructs its perception of the world by capturing photons. However, humanity's proud visual system is actually a "sensory cage." In the vast electromagnetic spectrum, the "visible light" we can see occupies only a tiny fraction, with wavelengths limited to between 380 and 700 nanometers. Beyond this threshold, near-infrared light (wavelengths greater than 700 nm), which accounts for more than half of the sun's radiant energy, remains an eternal darkness to humans. Imagine if one day, simply by wearing a special pair of contact lenses, we could perceive near-infrared information like superheroes in science fiction movies, and even see "infrared colors" never seen before. This would not just be an extension of the senses, but a tremendous expansion of human cognitive boundaries.

Humans Trapped in Visible Light

To understand how to break the sensory shackles, we must first examine the human visual system, which is remarkably precise. The formation of vision is a relay race spanning physics, chemistry, and bioelectrical signals. When external light passes through the transparent cornea and lens, is refracted by them, and precisely projects onto the retina at the back of the eye, the journey has only just begun. The retina, a thin layer of neural tissue, acts like a highly complex biological chip, translating the light and shadow of the physical world into electrical signals the brain can understand.

Distributed across the retina are tens of millions of photoreceptor cells—rod cells act as "night scouts," extremely sensitive to faint light but unable to distinguish colors; cone cells are "color masters," painting the colorful world through precise division of labor for red, green, and blue wavelengths. The reason these cells can perceive light lies in the photosensitive proteins they express internally. The photosensitive protein wraps around a molecule called retinal, which acts as an extremely sensitive microscopic switch. When a visible light photon hits retinal, the molecule undergoes "photoisomerization"—instantly changing from a twisted cis structure to a straight all-trans structure. This tiny movement is like knocking over the first domino, triggering a cascade of biochemical reactions and generating electrical current signals. Ultimately, these electrical pulses travel along the optic nerve into the cerebral cortex, piecing together the shapes, depth, and colors we perceive.

However, this precise system appears "powerless" in the face of near-infrared light. This limitation is not accidental but the result of a game between biological evolution and physical laws. From an energy perspective, the photoelectric conversion process is like "shooting a basketball": a photon must have enough energy to cross the potential barrier and activate the photosensitive protein. Near-infrared photons have longer wavelengths and lower energy; when they hit the photosensitive protein, it's like a player with too little strength throwing the ball—it cannot reach the height of the basket, naturally cannot score, and thus cannot initiate subsequent visual reactions.

The most fatal limitation comes from thermodynamics. Biological bodies have temperature; according to physical laws, any object with temperature radiates infrared rays outward. If human senses were extremely sensitive to this low-energy infrared light, the thermal motion of the photosensitive proteins themselves would continuously activate photoreceptor cells like background noise. At that point, our eyes would no longer see a clear world but a chaotic "light noise" generated by our own thermodynamic motion. These thermodynamic noises would severely interfere with true visual signals. To ensure the signal-to-noise ratio of vision, evolution ultimately chose to sacrifice infrared perception in exchange for extremely high resolution and clarity of daylight vision. Humans are thus trapped in the "narrow window" of visible light, missing the bands rich in information in the solar spectrum.

Equipping the Eye with a Nano "Translator"

Since we cannot modify the photosensitive proteins inside the eye, scientists have conceived a completely new approach: at the moment light enters the eye, equip it with a "translator" to translate it into a language the retina understands. Upconversion nanoparticles perfectly fill this role. These are tiny substances with magical properties that, like "energy batteries," sequentially absorb two or more low-energy long-wavelength photons (near-infrared) and, through stepwise transitions of internal energy levels, ultimately emit a high-energy short-wavelength photon (visible light). This physical phenomenon of turning low into high is called "anti-Stokes shift luminescence." In this nanoscale system, scientists have meticulously constructed a collaboration network of sensitizers and activators. The sensitizer acts like an efficient "energy collection antenna," responsible for capturing free near-infrared light in the environment. Subsequently, it transfers energy in the form of resonance to the activator, which finally emits visible light. The fascinating aspect of this technology is that it is completely "passive," requiring no bulky batteries or complex electronic circuits, relying solely on natural light energy conversion. It essentially establishes a miniature "frequency converter" on the ocular surface, capturing infrared rays and converting them into visible light that the retina can capture.

With the ideal "translator" in hand, the next challenge is seamlessly integrating it into daily life. Scientists have set their sights on a tool worn by over 150 million people worldwide—contact lenses. However, embedding nanoparticles into contact lenses is not easy. The core material of contact lenses is typically hydrophilic hydrogel, while upconversion nanoparticles are essentially inorganic salt crystals. When the two are mixed, the first major problem is "scattering." Due to the mismatch in refractive index between nanoparticles and hydrogel materials, light passing through undergoes severe scattering, causing the lens to become blurry and cloudy, like frosted glass. If the wearer cannot see the original visible-light world clearly, the practicality of this technology will be greatly compromised. To overcome this challenge, the research team at the University of Science and Technology of China developed a precise refractive index matching strategy. Through screening and fine-tuning various polymer formulations, they finally locked onto a specific polymer base material whose refractive index is almost identical to that of the nanoparticles.

At the same time, the research team also performed surface chemical modification on the nanoparticles—the original particle surface was covered with a layer of hydrophobic oleic acid ligands, which easily aggregate in hydrogel. Through ligand exchange technology, researchers removed the oleic acid, allowing the particles to distribute evenly in the polymer network. The final upconversion contact lenses maintain over 90% transparency under normal light, with wearing comfort comparable to ordinary commercial lenses. While ensuring the clarity of original visible-light vision, it successfully overlays a functional layer for infrared conversion on the ocular surface.

"Seeing" Images in Pitch Darkness

After completing the synthesis and optical optimization of the contact lenses, scientists first began physiological validation on laboratory mice. To verify the effect of the lenses, researchers carefully placed these specially made, thin upconversion contact lenses on the surface of the mice's eyeballs. After confirming they would not cause damage to the mouse eyes, they began exploring near-infrared vision in the mice:

When the laboratory entered a complete "infrared mode"—visible light extinguished, only invisible near-infrared light turned on—a miracle occurred. Through electroretinogram (ERG) detection, the electrophysiological curve, originally a straight line, showed clear infrared-induced signals. These characteristic waveforms clearly indicated that the photoreceptor cells on the retina had received visible light signals converted by the contact lenses and successfully transformed them into neural electrical impulses.

Subsequent pupil light reflex experiments provided more intuitive evidence. Under near-infrared light irradiation, the pupils of mice wearing upconversion contact lenses showed obvious, repeatable contraction responses. This proved that infrared signals had entered the central nervous system and triggered physiological reactions. In behavioral tests, mice wearing the lenses exhibited active avoidance or tracking of infrared light sources, while control mice were at a loss in the dark. These experiments powerfully demonstrated that without any intraocular surgery, relying solely on a thin film on the ocular surface, life can cross the evolutionary gap and directly sense the existence of another spectral dimension.

The success in mice paved the way for human experiments. When human subjects first wore these contact lenses and looked into the infrared world, the challenge was how to evolve from "seeing light" to "seeing images clearly." Initially, because the converted light from the contact lenses scattered in all directions, subjects could only perceive a blurry green halo. To achieve precise imaging, the research team built an auxiliary optical imaging framework. This system uses precision lens groups to refocus the faint light emitted by the contact lenses, ensuring that spatial information in the infrared band is precisely projected onto the fovea of the retina. Through this optical optimization, human subjects achieved astonishing spatial resolution under near-infrared light. They could not only identify obstacles in the environment but also read extremely small letters and numbers projected by infrared lasers.

In addition to spatial precision, infrared vision also demonstrated extremely high "temporal sensitivity." Since the upconversion process is almost instantaneous, subjects could recognize infrared-encoded information flickering at specific frequencies. This means a completely new, highly covert visual channel has opened—while others see pitch darkness or an unchanging background, the wearer can receive complex instructions or data streams in real time through infrared strobing. This dual mastery of spatiotemporal information marks the formal transition of infrared vision from primitive light perception to advanced image cognition.

Furthermore, when human subjects wore these contact lenses, infrared vision exhibited many wonderful characteristics beyond traditional senses. The most striking is the "closed-eye perception" achieved through the strong penetrability of near-infrared rays. In physics, the longer the wavelength, the less easily it is scattered by matter. Compared to visible light, which is easily blocked, near-infrared rays can easily pass through objects that appear "opaque" visually, such as smoke, thick dust, and even biological tissues. In experiments, subjects found that even with their eyes closed, because near-infrared light can penetrate eyelid skin like a ghost, the contact lenses attached to the ocular surface can still capture these infrared photons and convert them into visible light. This means that for the first time, humans can perceive the flow of light and information changes in the external world even with their eyes closed. This wonderful experience completely breaks the traditional visual dependence on the act of "opening eyes," revealing the unique application potential of infrared vision in special environments (such as dense smoke, heavy fog, or resting with eyes closed).

Future Humans May Gain Night Vision

If seeing infrared images is a "qualitative change" in vision, then seeing infrared colors is a "dimensional upgrade." In nature, the infrared spectra reflected by different objects are actually as rich as visible light. Inspired by the "red-green-blue" trichromatic principle of the human retina, researchers proposed a forward-looking innovative concept: reconstructing a "three primary colors" system in the infrared band. At this point, the researchers did not simply mix three different nanoparticles but independently synthesized an extremely precise core-multishell upconversion nanoparticle.

This nanoparticle is like a multi-layered "energy hotel." By constructing multiple functional shells within a single particle, different energy conversion processes are spatially isolated. The greatest advantage of this structure is its highly "orthogonal" conversion process. Specifically, this core-multishell particle can independently convert three different wavelengths of near-infrared light (such as 808 nm, 980 nm, and 1532 nm) into green, blue, and red visible light, with almost no crosstalk between them. This orthogonality is far superior to simple physical mixing, as it not only greatly improves conversion efficiency but also ensures color purity.

This is akin to rebuilding a precise "RGB three primary colors" system in the infrared world. Through this innovative color conversion, humans can distinguish infrared light sources that originally looked completely identical with the naked eye, achieving color conversion and mixing of multiple near-infrared lights into visible light. This not only gives subjects an extremely rich color perception of infrared space but also means we can distinguish substances of different compositions through color differences, achieving true multispectral cognition. This breakthrough expands the spectral range of human vision without electronic assistance, with unprecedented depth and precision.

The boundaries of human understanding of the world depend largely on the boundaries of our senses. From the initial instinctive reaction to light to today's ability to perceive the colorful infrared world across the spectrum, every step represents science's deciphering of nature's deep codes.

Although this technology cannot yet give humans night vision based on infrared vision under natural nighttime light, there are still some bottlenecks in the sensitivity and conversion efficiency of upconversion nanomaterials. However, this already points to a whole new path—without relying on bulky optoelectronic devices, a thin, lightweight ocular device alone is expected to rewrite the spectral boundaries of biological perception. The limits of the senses are never the endpoint but the starting point of scientific exploration. In the future, humanity hopes to achieve the leap from "invisible" to "visible" for the near-infrared world.