Meet the world’s smallest optical spectrometer. Crafted from a single nanowire less than 100 µm long, the device can measure the intensity of colored light across the visible spectrum and may eventually lead to miniature spectrometers that could be installed in a cell phone (Science 2019, DOI: 10.1126/science.aax8814).
“Our spectrometer is about one thousand times smaller than any previous spectrometer that people have demonstrated,” says Tawfique Hasan at the University of Cambridge who is part of the team that built the device.
Scientists routinely use optical spectrometers to study the composition of materials, including the atmospheres of exoplanets and human tissue samples. Advances in electronics and optics have already delivered handheld spectrometers. But building spectrometers that are even smaller—less than a few millimeters wide—is a huge challenge, Hasan says, not least because of the difficulty of miniaturizing optical components like interference gratings that split white light into its component colors.
His team’s spectrometer doesn’t use any optical components, instead it relies on a semiconducting nanowire grown from a vapor of cadmium sulfide and cadmium selenide. At one end, the nanowire is mostly cadmium selenide, which absorbs redder light. The composition gradually changes along the length of the wire, so that the opposite end is mostly cadmium sulfide, which absorbs bluer light.
In the new spectrometer, the researchers mount one of these nanowires on a silicon substrate and then overlay it with a comb of indium-gold electrodes, forming up to 38 distinct regions along the nanowire. Each region responds to a different wavelength of visible light, producing a current between the pair of electrodes around it. Software then analyzes these signals and turns them into a spectrum. Hasan credits his former PhD student Zongyin Yang for coming up with the idea for the spectrometer, and current student Tom Albrow-Owen for getting it to work reliably.
“It’s excellent research,” says Chennupati Jagadish of the Australian National University. “Being able to use a nanowire as a spectrometer is something unique.”
To demonstrate the nanowire’s capabilities, Hasan’s team scanned it over a picture of a heraldic lion, combining a series of measurements to reconstruct a color image. They also used it to visualize a red onion cell, showing that the spectrometer has a spatial resolution of about 1 µm.
Hasan suggests that a portable nanowire spectrometer might be used to detect counterfeit medicines, assess the freshness of fruit, or enable a drone to monitor environmental pollutants. Such a spectrometer could also be a boon for space-based missions that require compact, low-mass instruments.
“But a lot of work needs to be done to develop the technology to the stage where you’d be able to manufacture this,” Jagadish cautions. “Demonstrating an idea in the lab is one thing, taking it to a manufacturing process is an entirely different ball game.”
Hasan agrees there is plenty of room for improvement. It took 900 individual measurements to reproduce the lion picture, so the researchers hope to create an array of many nanowires that could speed up the process. They also plan to investigate whether other semiconductor materials could be used to create nanowire spectrometers that respond to infrared and ultraviolet light, which could extend the range of potential applications.
This story was updated on Sept. 8, 2019, to correct the units for the length of the nanowire and the spectrometer's spatial resolution.