Researchers devise a full-color night vision goggle - Ars Technica
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Human eyes don’t register the infrared portion of the light spectrum because infrared photons don’t carry enough energy to trigger the signaling pathway inside our light-sensing cells. But we’ve been able to make devices that give us a visual representation of what’s happening in the infrared.
A team at the Beijing Institute of Technology, led by Xin Tang and Ge Mu, has now built a device that lets people see infrared in a new way. Instead of just translating it to visible shades of green as it’s done in standard night-vision goggles, it translates different infrared wavelengths into distinct parts of the visual spectrum, giving the eye something closer to natural vision.
Researchers achieved that by combining mercury telluride colloidal quantum dots, which absorb infrared light, and a dual-layer OLED, which converts that absorbed energy into visible color. Stacked together with the right internal wiring, they make incoming infrared radiation come out the other side as an ordinary-looking, full-color image.
Invisible light
Vision begins when a photon strikes a light-sensitive pigment in the retina and causes it to change shape. That alters a protein that the pigment is embedded in, starting a nerve impulse that eventually contributes to an image in the brain. The molecular shape change that starts it all needs a minimum amount of energy, roughly 1.6 electron volts; infrared photons with wavelengths longer than 700 nanometers don’t carry enough energy to do it. That leaves over half of the Sun’s radiant energy outside our visual reach, along with anything that emits or reflects heat rather than visible light.
Most existing devices that allow us to see infrared use IR photodetectors wired to visible-light LEDs. This approach makes infrared light visible, but only as brightness: A warmer object glows a bit more, a cooler one glows a bit less, and we represent them all in the same (usually greenish) color. One problem with this is that human eyes are far better at distinguishing subtle differences in hue than they are at picking out brightness differences, so a device that only modulates brightness is leaving most of the eye’s sensitivity unused.
The solution Tang’s team arrived at starts with the way infrared light is registered in the first place.
Selective quantum dots
Bulk semiconductors routinely used in IR detectors have continuous energy bands, which means they absorb a broad, undifferentiated swath of the infrared spectrum. This doesn’t preserve much information about which specific wavelength arrived. The mercury telluride colloidal quantum dots the Chinese team picked for their device behave differently. Due to their tiny size, roughly 4 nanometers across, quantum confinement breaks their energy levels into discrete steps rather than a continuous chunk. Photons of different infrared wavelengths and intensities excite different electronic transitions, moving an electron between discrete energy levels within the dot, rather than just producing more or less of a single signal.
At wavelengths of around 2 micrometers, the longest the team tested, incoming photons have just enough energy to bump an electron across the dot’s fundamental bandgap, producing a modest number of charge carriers, either negatively charged electrons or positively charged holes that move around the dot.
At shorter infrared wavelengths, photons carry more energy per particle, and they can access additional, higher-energy electronic transitions inside the dot. In some cases the excess energy in a carrier can be enough to kick loose more than one electron-hole pair per photon. These processes open extra channels for generating charge carriers. The effect is that shorter infrared wavelengths and more intense infrared light push more positively charged holes out of the quantum dot layer and toward the OLED side of the device.
But getting a variable number of holes dependent on the wavelength and intensity of infrared light out of the detector was only half the problem. The other half was translating this signal into a color image at the other side.
The barrier
The team achieved this by building the OLED with two separate emissive layers stacked on top of each other. One layer, closer to where the holes enter, was doped with a red-emitting phosphor. The other one, positioned farther away, was doped with phosphor-emitting cyan light. The key component that made full-color vision work was an energy barrier of about 0.82...