Terahertz radiation, whose wavelengths lie between those of microwaves and visible light, can penetrate many non-metallic materials and detect the signatures of certain molecules. These practical qualities could lend themselves to a wide range of applications, including airport security digitization, industrial quality control, astrophysical observations, non-destructive material characterization, and wireless communications with higher bandwidth. higher than current mobile phone bands.
However, designing devices to detect and image terahertz waves has been difficult, and most existing terahertz devices are expensive, slow, bulky, and require vacuum systems and extremely low temperatures.
Now, researchers from MIT, the University of Minnesota, and Samsung have developed a new type of camera that can detect terahertz pulses quickly, with high sensitivity, at room temperature and pressure. Additionally, it can simultaneously capture information about the orientation, or “polarization,” of waves in real time, which existing devices cannot. This information can be used to characterize materials that have asymmetric molecules or to determine the surface topography of materials.
The new system uses particles called quantum dots which, it was recently discovered, can emit visible light when stimulated by terahertz waves. The visible light can then be registered by a device similar to the detector of a standard electronic camera and can even be seen with the naked eye. The device is described in an article published today in the journal Nature’s nanotechnologyby MIT PhD student Jiaojian Shi, chemistry professor Keith Nelson and 12 others.
The team produced two different devices that can operate at room temperature: one uses the ability of the quantum dot to convert terahertz pulses into visible light, allowing the device to produce images of materials; the other produces images showing the state of polarization of terahertz waves.
The new “camera” consists of several layers, made with standard manufacturing techniques like those used for microchips. A network of parallel nanoscale lines of gold, separated by narrow slits, rests on the substrate; above is a layer of the light-emitting quantum dot material; and above is a CMOS chip used to form an image. The polarization detector, called a polarimeter, uses a similar structure, but with nanoscale ring-shaped slits, which allows it to detect the polarization of incoming beams.
Photons from terahertz radiation have extremely low energy, Nelson explains, which makes them difficult to detect. “So what this device does is convert that tiny little photon energy into something visible that’s easy to detect with a regular camera,” he says. In the team’s experiments, the device was able to detect terahertz pulses at low intensity levels that exceeded the capability of today’s large, expensive systems.
The researchers demonstrated the capabilities of the detector by taking terahertz-illuminated images of some of the structures used in their devices, such as the nano-spaced gold lines and ring-shaped slits used for the polarized detector, proving the sensitivity and resolution of the system.
Developing a practical terahertz camera requires a component that produces terahertz waves to illuminate a subject, and another that detects them. On this last point, the current terahertz detectors are either very slow, because they rely on the detection of heat generated by the waves striking a material, and the heat propagates slowly, or they use relatively fast photodetectors, but of very low sensitivity. Moreover, until now, most approaches required an entire array of terahertz detectors, each producing one pixel of the image. “Each one is quite expensive,” Shi says, so “once they start making a camera, the cost of the detectors starts going up very, very quickly.”
While the researchers say they have solved the problem of detecting terahertz pulses with their new work, the lack of good sources remains – and is what many research groups around the world are working on. The terahertz source used in the new study is a large and bulky array of lasers and optical devices that cannot be easily adapted to practical applications, Nelson says, but new source-based microelectronic techniques are being developed.
“I think that’s really the rate-limiting step: can you do the [terahertz] signals in a simple way that does not cost much? ” he says. “But there is no doubt that it will come.”
Sang-Hyun Oh, co-author of the paper and McKnight Professor of Electrical and Computer Engineering at the University of Minnesota, adds that while current versions of terahertz cameras cost tens of thousands of dollars, the inexpensive nature of the cameras CMOS used for this system makes it “a big step forward in building a practical terahertz camera”. Commercialization potential led Samsung, which makes CMOS camera chips and quantum dot devices, to collaborate on this research.
Traditional detectors for such wavelengths operate at liquid helium temperatures (-452 degrees Fahrenheit), Nelson says, which is necessary to detect the extremely low energy of background terahertz photons. The fact that this new device could detect and image these wavelengths with a conventional visible light camera at room temperature was unexpected for those working in the terahertz range. “People are like, ‘What?’ It’s a bit unheard of and people are very surprised,” says Oh.
According to the researchers, there are many avenues to further improve the sensitivity of the new camera, including further miniaturization of components and ways to protect the quantum dots. Even at current detection levels, the device could have potential applications, they say.
In terms of the new device’s market potential, Nelson says quantum dots are now inexpensive and readily available, currently being used in consumer products such as television screens. The actual manufacturing of the cameras is more complex, he says, but it is also based on existing microelectronic technology. In fact, unlike existing terahertz detectors, the entire terahertz camera chip can be fabricated using today’s standard microchip production systems, which means that ultimately mass production of devices should be possible and relatively inexpensive.
Already, even though the camera system is still far from commercial, MIT researchers are using the new lab device when they need a quick way to detect terahertz radiation. “We don’t have one of those expensive cameras,” Nelson says, “but we have a lot of those little devices. People will simply stick one in the beam and watch the visible light emission so they know when the terahertz beam is activated. …People found it really convenient.
While terahertz waves could in principle be used to detect certain astrophysical phenomena, these sources would be extremely weak and the new device is not capable of capturing such weak signals, Nelson says, although the team is working to improve its sensitivity. . “Next-gen is all about making everything smaller, so it’s going to be a lot more responsive,” he says.
The research team included Daehan Yoo of the University of Minnesota; Ferran Vidal-Codina, Ngoc-Cuong Nguyen, Hendrik Utzat, Jinchi Han, Vladimir Bulović, Moungi Bawendi, and Jaime Peraire at MIT; Chan-Wook Baik and Kyung-Sang Cho of Samsung Advanced Institute of Technology; and Aaron Lindenberg at Stanford University. The work was supported by the US Army Research Office through the MIT Institute for Soldier Nanotechnology, Samsung’s Global Research Outreach Program and the Center for Energy Efficient Research Science.