Terahertz radiation, whose wavelengths are between microwaves and visible light, may penetrate many nonmetallic materials and identify molecular fingerprints. These useful properties might be used for a broad range of applications, including airport security scanning, industrial quality control, astronomical observations, nondestructive material characterisation, and wireless communications with larger bandwidths than existing cellular bands. However, building devices to detect and generate pictures from terahertz waves has proven difficult, with most contemporary terahertz devices being costly, sluggish, and massive, requiring vacuum systems and very low temperatures.
MIT, the University of Minnesota, and Samsung researchers have invented a new kind of the camera that can detect terahertz pulses quickly, with great sensitivity, and at ambient temperature and pressure. Furthermore, unlike previous devices, it can record information regarding the orientation, or “polarization,” of the waves in real time. This information may be utilised to describe asymmetrical molecules in materials or to identify the surface topography of materials.
The novel device employs quantum dots, which have recently been shown to generate visible light when activated by terahertz vibrations. The visible light is then captured by a mechanism comparable to the detector of a normal electronic camera and can even be seen with the naked eye. The gadget is reported in a study published in the journal Nature Nanotechnology on November 4 by MIT PhD student Jiaojian Shi, chemistry professor Keith Nelson, and 12 others. The researchers created two separate room-temperature devices: one that leverages the quantum dot’s capacity to convert terahertz pulses to visible light, allowing the device to take pictures of materials, and the other that creates images of the polarisation state of the terahertz waves.
The new “camera” is composed of many layers that were created using normal manufacturing procedures similar to those used for microchips. On the substrate is an array of nanoscale parallel lines of gold separated by microscopic slits; above that is a layer of the light-emitting quantum dot material, and above that is a CMOS chip used to produce an image. A polarimeter, or polarisation detector, has a similar construction but with nanoscale ring-shaped slits that enable it to detect the polarisation of incoming beams.
Terahertz photons have incredibly little energy, which makes them difficult to detect, according to Nelson. “So, what this technology does is transform that very small photon energy into something observable that a standard camera can detect,” he explains. In the team’s tests, the gadget detected terahertz pulses at low-intensity levels that outperformed today’s huge and costly systems. The researchers confirmed the detector’s capabilities by shooting terahertz-illuminated photographs of some of the structures utilised in their devices, such as the nano-spaced gold lines and the ring-shaped slits used for the polarisation detector, demonstrating the system’s sensitivity and resolution.
Creating a workable terahertz camera necessitates the development of two components: one that generates terahertz waves to light a subject and another that detects them. Current terahertz detectors are either extremely slow since they depend on sensing heat created by waves contacting a substance, and heat propagates slowly, or they employ photodetectors, which are somewhat quick but have very limited sensitivity. Furthermore, most techniques needed a whole array of terahertz detectors, each creating one pixel of the picture. “Each one is pretty costly,” Shi adds, adding that “once they start making a camera, the cost of the detectors begins to scale up very, very rapidly.”
While the researchers claim that their new work has solved the terahertz pulse detection issue, the shortage of excellent sources persists—and is being addressed by several research organisations across the globe. According to Nelson, the terahertz source employed in the current work is a massive and bulky array of lasers and optical devices that cannot be readily scaled to practical uses, but other sources-based microelectronic approaches are already under development.
“I believe that’s really the rate-limiting step: Can you create the [terahertz] signals easily and cheaply?” he adds. “However, there’s no doubt about that.”
According to Sang-Hyun Oh, a co-author of the paper and the McKnight Professor of Electrical and Computer Engineering at the University of Minnesota, while current versions of terahertz cameras cost tens of thousands of dollars, the low cost of the CMOS cameras used in this system makes it “a big step forward toward building a practical terahertz camera.” The commercialization potential prompted Samsung, which manufactures CMOS camera chips and quantum dot devices, to cooperate with this study.
According to Nelson, traditional detectors for such wavelengths work at liquid helium temperatures (-452 degrees Fahrenheit), which is required to separate the very low energy of terahertz photons from background noise. The fact that this new technology can detect and create pictures of these wavelengths at room temperature using a standard visible-light camera has surprised scientists working in the terahertz area. “‘What?’ say the onlookers. It’s almost unheard of, and people are taken aback “Oh says
According to the researchers, there are several ways to improve the sensitivity of the new camera, including further downsizing of the components and methods of safeguarding the quantum dots. Even at the current detection levels, they believe the gadget has some potential uses.
Nelson claims that quantum dots, which are now employed in consumer items such as television screens, have a high commercialization potential for the new technology. He claims that the actual manufacture of the camera devices is more complicated, but that it is still based on current microelectronics technology. In fact, unlike current terahertz detectors, the full terahertz camera chip may be created using today’s regular microchip manufacturing techniques, implying that mass production of the devices might be achievable and reasonably affordable in the future.
Even though the camera system is still under development, MIT researchers have started utilising it in the lab when they need a rapid technique to detect terahertz radiation. “Nelson explains, “We don’t have one of those pricey cameras, but we have a bunch of these tiny gadgets.” People will simply place one of these in the beam and observe the visible light output to determine when the terahertz beam is active. People found it quite useful.”
While terahertz waves may theoretically be used to detect certain astrophysical occurrences, the sources would be incredibly faint, and the current gadget, according to Nelson, is not capable of capturing such weak signals, but the team is attempting to improve its sensitivity. “The next generation will be a lot more sensitive because everything will be much smaller,” he argues.
