New terahertz camera device provides higher sensitivity and speed than previous versions, and can be used for industrial inspections, airport security and communications.
terahertz radiation , also known as submillimeter radiation, has a wavelength between microwave and visible light. It can penetrate many non-metallic materials and detect the characteristics of certain molecules. These convenient qualities can be used in a wide range of applications, including industrial quality control, airport security scanning, lossless characterization of materials, astrophysical observations, and wireless communication with bandwidth higher than the current mobile phone frequency band.
However, designing devices for detecting terahertz waves and making images has been challenging. Therefore, most existing terahertz equipment is expensive, slow, bulky, and requires a vacuum system and extremely low temperatures.
Now, researchers at MIT , University of Minnesota and Samsung have developed a new camera that can quickly and with high sensitivity at room temperature and pressure. More importantly, it can simultaneously capture information about the direction or "polarization" of the wave, which is something that existing devices cannot do. This information can be used to characterize materials with asymmetric molecules or to determine the surface morphology of materials.
The new system uses particles called quantum dot . Recently, they have been found to have the ability to emit visible light when stimulated by terahertz waves. The visible light can then be recorded through devices similar to standard electronic camera detectors, and can even be seen with the naked eye. The device was described in a paper published on November 3 in the journal , "Natural Nanotechnology ", described by MIT PhD student Jiaojian Shi, chemistry professor Keith Nelson and 12 others.
The team produced two different devices that could run at room temperature: one utilizing the ability of quantum points to convert terahertz pulses into visible light, allowing the device to produce images of the material; and the other to produce images showing the polarization state of the terahertz wave.
The new "camera" consists of several layers, made of standard manufacturing techniques, such as the technology used for microchip . A series of nanoscale parallel gold lines, separated by narrow slits, are located on the substrate; above is a layer of luminescent quantum dot material; above is a CMOS chip used to form images. The polarization detector, called the optical actuator , uses a similar structure but has a nanoscale annular slit, allowing it to detect the polarization of the incident beam.
Nelson explained that terahertz radiation photons are extremely low, which makes them difficult to detect. "So what this device is doing is converting tiny photon energy into something visible, which is easy to detect with a normal camera," he said. In the team's experiments, the device was able to detect terahertz pulses at low intensity levels, which exceeded the capabilities of today's large and expensive systems.
The researchers demonstrated the sensitivity and resolution of the system by taking terahertz illumination photos of some of the structures used in their devices, such as nano-spaced gold wires and annular slits used in polarization detectors.
CMOS Camera Used to Capture the Rotation of the Terahertz Beam
Developing a practical terahertz camera requires a component that generates terahertz waves to illuminate objects, and another component that detects them. At the latter point, current terahertz detectors are either very slow because they rely on the detection heat generated by wave impacting the material and the heat propagates slowly, or they use the relatively fast but very low sensitivity photodetector . Furthermore, so far, most methods require a complete set of terahertz detectors, each producing an image of one pixel. “Each one is very expensive,” Shi said, so “once they start to build cameras, the cost of the detector starts to expand very, very quickly.
Although the researchers say they have cracked the terahertz pulse detection problem with their new work, the lack of a good source remains - and many research groups around the world are studying it. Nelson said the terahertz light source used in the new study is a huge and bulky array of lasers and optical devices that is not easy to expand to practical applications, but new light sources based on microelectronics are under development.
Co-author of the paper, Sang-Hyun Oh, a McKnight professor of electrical and computer engineering at the University of Minnesota, added that while the current version of the terahertz camera costs tens of thousands of dollars, the cheap nature of the CMOS camera used in the system makes it a "big step towards building a practical terahertz camera." The potential of commercialization has prompted Samsung, which produces CMOS camera chips and quantum dot devices to collaborate on the research.
Nelson says that traditional detectors of this wavelength operate at liquid helium temperature (-452 degrees Fahrenheit), which is necessary to pick out the extremely low energy of terahertz photons from background noise. The fact that this new device can detect and generate images of these wavelengths with conventional visible light cameras at room temperature is unexpected and unheard of for those working in terahertz fields.
Researchers say there are many ways to further increase the sensitivity of new cameras, including further miniaturization of components and methods to protect quantum dots. They say the device may have some potential applications even at current detection levels.
In terms of the commercial potential of new devices, Nilsen says quantum dots are now inexpensive and easy to obtain and are currently used in consumer products such as TV screens. He said the actual manufacturing of camera devices is more complex, but is also based on existing microelectronics technologies. In fact, unlike existing terahertz detectors, the entire terahertz camera chip can be manufactured using today's standard microchip production systems, meaning that the ultimate mass production equipment should be possible and relatively inexpensive.
Although the camera system is far from commercialization, researchers at MIT have been using new laboratory equipment when they need a method to quickly detect terahertz radiation. "We don't have expensive cameras," Nelson said, "but we have a lot of small devices like this. People just put one of them in a beam and look at the visible light emission with their eyes, so they know when the terahertz beam is on. ... People find it very convenient.
Although terahertz waves can be used in principle to detect some astrophysical phenomena, these sources will be very weak, and new devices can't capture such a weak signal, and the team is working to improve its sensitivity. So it will be more sensitive.
