|Schematic showing how different colored light is absorbed by the layered quantum dot photodetector.|
Researchers in the US and Korea are developing three-layered sensors based on quantum dots that promise low-cost, flexible, high-dynamic range sensors without the need for demosaicing.
Quantum dots are nano-scale semiconducting crystals whose size and composition can be used to ‘tune’ the frequency of light they absorb or emit. Researchers at Northwestern University, Illinois, and three South Korean universities have published their findings in the journal Advanced Materials.
The research focuses on the creation of a three-layered sensor that absorbs different frequencies of light at different depths. And, although the promise of QDs is that they can be tuned for specific frequencies, the ‘red’ quantum dots used by the team are actually sensitive to red, green and blue light. The ‘green’ dots, in turn, are sensitive to both green and blue, with only the blue dots being completely selective.
The research focuses on the creation of a three-layered sensor that absorbs different frequencies of light at different depths.
When light is received, its wavelength (color) can be detected based on the current generated by the sensor: the higher the current, the more of the layers of dots have been activated.
Interestingly, although it sounds conceptually similar to Foveon’s stacked CMOS sensors, it ends up being essentially the opposite. In both cases, the light isn’t neatly captured solely in a single layer, with some wavelengths being captured by multiple layers. But the specific layers at which each wavelength is captured differs.
|QD sensor||Foveon sensor|
|Top Layer||B’||B, G’, R’|
|Bottom Layer||B”’, G”, R||R”’|
However, because the quantum dot sensor reads out all three layers to the same place, you can either determine the color of the light hitting it or the intensity (brightness), not both. For color and intensity measurement, the team has developed a more complex circuit with dedicated red, green and blue sensors alongside the layered photodetector.
Although the research is early stage (don’t expect a flexible QD sensor in your next camera), it’s sufficiently advanced that many of its considerations are around the manufacturabilty of the devices.
The compound used to hold the quantum dots in place, and conduct charge from them to the circuitry can be fixed with UV light, and is not then affected when then next layer of dots is applied. This means the sensor can be built-up, layer by layer, using existing manufacturing techniques.
The device the researchers have made features 5,500 sensor elements per square cm, applied to a flexible film backing. This is around 3.5 times more elements than previous comparable devices. However, it would equate to a full-frame sensor with a little under 50,000 pixels, so there’s still plenty of miniaturization needed before it can compete with conventional CMOS.
With thanks to the paper’s first author Jaehyun Kim for additional information for this story.