MIT China PhD student develops quantum dot-enhanced terahertz camera with low cost and high sensitivity

Terahertz (THz) radiation, with wavelengths between microwave and visible light, can penetrate many non-metallic materials and detect the characteristics of certain molecules. Therefore, it shows good promise for many applications, such as airport security scanning, industrial quality control, astrophysical observations, non-destructive characterization of materials, and wireless communications with higher bandwidth than the current cellular frequency band. However, designing devices to detect and generate images from terahertz waves has been challenging, and most existing terahertz devices are expensive, slow, large, and require vacuum systems and extremely low temperatures.

Now, Professor Keith A. Nelson's team at the Massachusetts Institute of Technology (MIT) and Samsung have demonstrated a room-temperature complementary metal-oxide semiconductor terahertz camera and rotameter that can rapidly detect terahertz pulses at room temperature and pressure with high sensitivity through field-driven interdot charge transfer; more importantly, it can simultaneously capture real-time information about the direction or "polarization" of the wave. "polarization" information in real time, which is not possible with existing devices [1].

It is worth mentioning that the co-author of the paper, Jiaojian Shi, graduated from Peking University as an undergraduate, was sponsored by the Second Affiliated High School of East China Normal University to Peking University in 2011, and had an internship at Harvard University during his undergraduate years. He is currently a postdoctoral fellow in Aaron Lindenberg's group at Stanford University. This work was completed during Jiaojian Shi's PhD at MIT.

Dr. Jiaojian Shih

01Terahertz camera and polarimeter: low intensity, high sensitivity, high resolution

The new system uses particles called "quantum dots" that emit visible light when stimulated by terahertz waves; the visible light can then be recorded by a device similar to a standard electronic camera detector, and can even be seen with the naked eye. Based on this, the team has made two different devices that can operate at room temperature. One uses the ability of quantum dots to convert terahertz pulses into visible light, allowing the device to produce images of materials; the other produces images showing the polarized state of terahertz waves.

Schematic diagram of the qTV CMOS terahertz camera and polarimeter. The device consists of a FES (slit or coaxial) coated with a light emitter (in this case, CdSe/CdS QDs) and a visible light detector (CMOS camera). The illustration shows that terahertz photons are converted into visible photons by field-driven charge transfer and radiative recombination between the emitting subsystems.

The new "camera" consists of several layers and is fabricated using standard manufacturing techniques like those used for microchips. The substrate has rows of nanoscale parallel gold lines separated by narrow slits; above that is a layer of light-emitting quantum dot material; and above that is the CMOS chip used to form the image. The polarization detector, called a polarimeter, uses a similar structure but has nanoscale ring slits, which allows it to detect the polarization of the incoming beam.

Photons of terahertz radiation are extremely low in energy, which makes them difficult to detect, Nelson explained. "So what this device is doing is converting these tiny photon energies into something visible that can be easily detected with a normal camera." In the team's experiments, the device was able to detect terahertz pulses at low intensities that are beyond the capabilities of today's large, expensive systems, he said. Meanwhile, the researchers demonstrated the detector's capabilities by taking terahertz illumination photos of some of the structures used in their device, such as the nanometer-spaced gold wires and the ring slit used for the polarization detector, demonstrating the system's sensitivity and resolution.

CMOS cameras are used to capture the rotation of terahertz beams.

02Practical terahertz camera a major step forward with great commercial potential

Developing a practical terahertz camera requires a component that generates terahertz waves to illuminate an object, and another component that detects them. On this latter point, current terahertz detectors are either very slow because they rely on detecting the heat generated by the wave impinging on the material, which travels slowly, or they use relatively fast photodetectors but with very low sensitivity. In addition, until now, most methods have required entire arrays of terahertz detectors, each of which produces a single pixel of image. Each one is quite expensive, says Jiaojian Shih, "so once they start making cameras, the cost of the detectors starts to expand rapidly."

Nelson said conventional detectors of this wavelength operate at liquid helium temperatures (-233°C), which is necessary to pick out terahertz photons of very low energy from background noise. The new device's ability to detect and produce images of these wavelengths at room temperature with conventional visible light cameras was unexpected by scholars working in the terahertz field.

Even at the current level of detection, they say, the device could have a number of potential applications.

In terms of the commercialization potential of the new device, Nelson said quantum dots are now inexpensive and readily available and are currently used in consumer products, such as TV screens. The actual manufacture of camera devices is more complex, but is also based on existing microelectronics technology, he said. The fact that, unlike existing terahertz detectors, entire terahertz camera chips can be fabricated using today's standard microchip production systems means that eventual mass production of these devices should be possible and relatively inexpensive.

Sang-Hyun Oh, co-author of the paper and 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 low-cost nature of the CMOS camera used in the system makes it "a big step toward building a practical terahertz camera. The huge commercialization potential prompted Samsung, which makes CMOS camera chips and quantum dot devices, to invest in the research in collaboration with MIT.

This work was supported by the U.S. Army Research Office through the MIT Soldier Institute of Nanotechnology, the Samsung Global Research Outreach Program and the Center for Energy Efficiency Research Science.

03Next step, improved precision

While the researchers say their new work has cracked the terahertz pulse detection problem, the lack of a good source remains: and many research groups around the world are working on it. the terahertz source used in the new study is a large and cumbersome array of lasers and optical devices that cannot easily be scaled up for practical applications, Nelson said, but new sources based on microelectronics are well under development. "I think it's really a speed-limiting step." Can you make [terahertz] signals in a simple way that's not expensive?" he says. But there's no question that this is going to be the way of the future."

There are a number of ways to further improve the sensitivity of this new camera, the researchers say, including further miniaturization of components and ways to protect quantum dots.

While terahertz waves could in principle be used to detect some astrophysical phenomena, these sources would be extremely weak, and the new device would not be able to capture such weak signals, Nelson said [2], although the team is working to improve its sensitivity. The next generation lies in making everything smaller, so it will be more sensitive.

Reference link：

[1]https://www.nature.com/articles/s41565-022-01243-9

[2]https://news.mit.edu/2022/terahertz-camera-low-cost-1104