The technology behind a new type of photodetector mimics the way squid likely sense colors.
Cephalopods like octopus and squid are masters of camouflage, but they are also color-blind. Scientists suspect that cephalopods may detect color directly through their skin.
Based on that hypothesis, Bob Zheng, a graduate student at Rice University, set out to design a photonic system that could detect colored light.
The photodetector, which sees colors in much the same way the human eye does, uses an aluminum grating that can be added to silicon photodetectors with the silicon microchip industry’s mainstay technology, “complementary metal-oxide semiconductor,” or CMOS.
The new device was created by researchers at Rice’s Laboratory for Nanophotonics (LANP) and is described in a new study in the journal Advanced Materials.
How photodetectors work
Conventional photodetectors convert light into electrical signals but have no inherent color-sensitivity. To capture color images, photodetector makers must add color filters that can separate a scene into red, green and blue color components.
This color filtering is commonly done using off-chip dielectric or dye color filters, which degrade under exposure to sunlight and can also be difficult to align with imaging sensors.
“Today’s color filtering mechanisms often involve materials that are not CMOS-compatible, but this new approach has advantages beyond on-chip integration,” says LANP Director Naomi Halas, the lead scientist of the study. “It’s also more compact and simple and more closely mimics the way living organisms ‘see’ colors.
The new technology
Zheng’s color photodetector uses a combination of band engineering and plasmonic gratings, comb-like aluminum structures with rows of parallel slits.
Using electron-beam evaporation, which is a common technique in CMOS processing, Zheng deposited a thin layer of aluminum onto a silicon photodetector topped with an ultrathin oxide coating.
Color selection is performed by utilizing interference effects between the plasmonic grating and the photodetector’s surface. By carefully tuning the oxide thickness and the width and spacing of the slits, Zheng was able to preferentially direct different colors into the silicon photodetector or reflect it back into free space.
The metallic nanostructures use surface plasmons—waves of electrons that flow like a fluid across metal surfaces. Light of a specific wavelength can excite a plasmon, and LANP researchers often create devices where plasmons interact, sometimes with dramatic effects.
“With plasmonic gratings, not only do you get color tunability, you can also enhance near fields,” Zheng says. “The near-field interaction increases the absorption cross section, which means that the grating sort of acts as its own lens. You get this funneling of light into a concentrated area.
The Office of Naval Research, the Department of Defense’s National Security Science and Engineering Faculty Fellowship Program, and the Robert A. Welch Foundation supported the research.
Scientists from the Marine Biological Laboratory in Woods Hole, Massachusetts, and the University of Maryland, Baltimore County, collaborated on the project.
Source: Rice University