Passive organic upconversion and nanophotonic enhancements convert weak infrared light into visible images, enabling passive visible imaging without electrical power or lasers.
(Nanowerk Spotlight) Night vision often brings to mind the green tinted scenes seen in movies, where the world is made visible through specialized optical goggles. The real devices behind those images are built around electronic amplification. Image intensifiers increase the signal from incoming near infrared light using high voltage. Semiconductor cameras based on materials such as indium gallium arsenide convert infrared photons into electrical signals that are read as images. Nonlinear optical crystals can shift the frequency of light without electronics, but they work only under intense laser illumination with power densities on the order of 10⁶ W cm⁻². These systems function in demanding environments but depend on electrical input or coherent illumination.
Photon upconversion offers a different approach. It relies on materials that combine the energy of two low energy photons into one higher energy photon. Rare earth ions in solids can perform this through atomic transitions. Organic semiconductors can do it through a mechanism known as triplet-triplet annihilation.
In this method, molecular states temporarily store energy from absorbed photons and then combine that energy to emit higher energy light. These strategies are attractive because they can operate with incoherent light such as ambient near infrared and do not require external electrical power or optical pumping. A challenge has been building a complete imaging system that uses these materials in low illumination conditions while maintaining resolution.
A study in Advanced Functional Materials (“Photonic Engineering Enables All‐Passive Upconversion Imaging with Low‐Intensity Near‐Infrared Light”) presents a passive near infrared imaging system based on organic upconversion. The device converts incoherent near infrared light in the range of 700 nm to 930 nm into visible emission around 610 nm. It operates without external electrical power input and can function at input intensity levels near 1 × 10⁻⁶ W cm⁻² across an aperture with a diameter of 23 mm. These operational conditions are within two orders of magnitude of natural nightglow irradiance reported in the absence of moonlight or other sources.
NIR-to-visible upconversion imaging using a bulk heterojunction (BHJ) thin film. a) The upconverting device consists of a BHJ of organic
molecules Y6 and DBP-doped rubrene, sealed between two glass pieces using epoxy glue to prevent oxygen ingress. b,c) Molecular diagram (b) and energy diagram (c) of TTA upconversion in the Y6/rubrene/DBP BHJ. Y6 absorbs incident NIR photons to generate excited singlets (S1), which diffuse to the interface between Y6 and rubrene. At this interface, singlets transform into free charges and recombine to form charge-transfer states at the interface, which can then transfer to rubrene as molecular triplets (T1). These triplets combine via TTA in rubrene to produce a high-energy singlet, which transfers into DBP and then emits a high-energy photon via photoluminescence. d) Images of the upconverting thin film under ambient light are shown with (bottom) and without (top) NIR laser (𝜆 = 852 nm) illumination. e) The upconverter is placed at the intermediate focal plane of a Keplerian lens system. This configuration preserves the relative angles between rays, 𝜃, across the upconversion process, and therefore enables upconversion imaging. f) Schematic of the imaging setup used for transmission-mode imaging of an Air Force resolution target. g) Image captured using the setup in (f) with 1.17 magnification between the object and the image, using incoherent broadband illumination. Resolution groups 4 and higher are highlighted on the right. The scale bars indicate the dimensions of the image on the upconverter. (Image: Reproduced from DOI:10.1002/adfm.202515334, CC BY) (click on image to enlarge)
The core of the system is an organic film made from three molecular ingredients. Y6 absorbs the incoming infrared light. Rubrene stores the absorbed energy in a form that lasts long enough to combine with other stored energy. DBP releases the visible light. The film is about 100 nm thick and sealed between glass to protect it from oxygen, because rubrene becomes damaged when exposed to air.
Light conversion in this material follows a sequence of energy transfers. When Y6 absorbs an infrared photon, it forms an energized state that migrates to the border between Y6 and rubrene. There it briefly becomes electrical charges, which recombine into an intermediate state at the interface. Rubrene accepts this energy and stores it as what scientists call a triplet. A triplet is an energized version of the molecule that does not emit light right away. Instead, it remains in place until it meets another triplet, and the two then merge. When two triplets merge, their combined energy forms a higher energy state. That energy moves to the DBP dopant, which emits a visible photon centered near 610 nm. Two infrared photons have therefore been converted into one photon of visible light.
A single conversion layer cannot form an image because the visible photons leave in many directions. To maintain detail, the material is placed between two optical subsystems. A near infrared lens focuses incoming light from a scene onto the film so each direction corresponds to a specific position on the material. A visible lens then takes the light coming from each position and directs it outward in the correct direction.
Because the electrons and excitations in the film travel less than 40 nm before producing light, they do not smear the image formed by the lenses. The smallest focused spots from the optics are about 10 μm, much larger than this molecular motion. Under broadband infrared illumination between 750 nm and 1000 nm, the system can resolve about 100 to 110 line pairs per mm on the film.
The brightness of the unmodified film is low. The study reports external quantum efficiency values of about 0.02–0.04 % at λ = 808 nm, decreasing at longer wavelengths. That output is insufficient for imaging at night or in other low light environments. The researchers therefore add two photonic enhancements to increase usable light.
The first enhancement is a thin multilayer coating placed behind the organic film. It passes infrared light into the conversion layer but reflects visible light toward the viewer. Because emission from the organic film spreads in all directions, half of the visible photons normally travel backward and are lost. By reflecting that portion forward, the coating increases the collected emission. Measurements show roughly a 2.5 times increase in visible light without noticeable loss of sharpness. The coating is less than 1 μm thick, which helps prevent blurring or internal reflections.
The second enhancement improves how much infrared light the film absorbs. Beneath the conversion layer, the researchers arrange a regular array of nanometer scale gold pillars. These structures support localized resonances that concentrate infrared light around them. Concentrated light increases the chance that Y6 absorbs photons. A thin insulating layer separates the gold from the organic film to prevent energy loss caused by direct contact with metal.
In measurements, samples with these nanostructures produce about twice as much visible light as samples without them. The structures also reduce the intensity at which the upconversion process transitions from low output to more efficient operation. In the patterned regions this threshold decreases from about 1.3 × 10² mW cm⁻² to about 9.0 × 10¹ mW cm⁻².
The full device places the conversion layer between the nanostructured base and the reflective coating. This arrangement increases forward visible light by about a factor of four compared with the untreated film. In laboratory measurements, when illuminated with infrared intensities around 8.54 × 10² nW cm⁻² and beam areas below 1 mm², the integrated device produces visible output above 2 nW cm⁻².
The authors note that some baseline noise and filter leakage contribute to readings at very low intensities, but the enhancement trend remains consistent. If the visible beam produced by the system were expanded to match the diameter of a dilated human pupil, about 0.5 cm², the expected brightness would be approximately 0.04 nW cm⁻², which falls within the level that the eye can detect at night when it relies on rod cells, a condition scientists call the scotopic range.
The work shows that passive upconversion imaging can preserve spatial detail under weak and incoherent illumination conditions. By combining molecular upconversion, nanophotonic structures, and conventional lenses, the system converts low intensity near infrared light into visible images without external power. This approach lays a foundation for optical devices intended for night vision, inspection, and other applications in environments where electrical amplification or active illumination is impractical.
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