Researchers have created a device that allows them to electronically steer and focus a beam of terahertz electromagnetic energy with pinpoint precision. This opens the door to real-time, high-resolution imaging devices that are hundredths the size of other radar systems and more robust than other optical systems.
Terahertz waves, located on the electromagnetic spectrum between microwaves and infrared light, exist in a “no man’s land” where neither conventional electronics nor optical devices can effectively manipulate their energy. But these high-frequency radio waves have many unique properties, such as the ability to pass through certain solid materials without the health effects of X-rays. They can also enable higher-speed communications or vision systems capable of seeing through foggy or dusty environments.
MIT’s Terahertz Integrated Electronics Group, led by Associate Professor Ruonan Han, seeks to bridge this so-called terahertz gap. These researchers have now demonstrated the most accurate and electronically steerable terahertz antenna array, which contains the largest number of antennas. The antenna array, called the “reflector array”, works like a controllable mirror with its direction of reflection guided by a computer.
The reflector array, which packs nearly 10,000 antennas into a device the size of a credit card, can precisely focus a beam of terahertz energy onto a tiny area and quickly control it with no moving parts. Built using semiconductor chips and innovative manufacturing techniques, the reflector array is also scalable.
The researchers demonstrated the device by generating 3D depth images of scenes. The images are similar to those generated by a LiDAR (light detection and ranging) device, but because the reflector array uses terahertz waves instead of light, it can work effectively in rain, fog or snow. This small reflector array was also capable of generating radar images with twice the angular resolution of those produced by radar at Cape Cod, which is a building so large it is visible from space. While Cape Code radar is capable of covering a much larger area, the new reflector array is the first to bring military-grade resolution to a device for commercial smart machines.
“Antenna arrays are very interesting because just by changing the delays that feed each antenna, you can change the direction in which the energy is focused, and it’s completely electronic,” says Nathan Monroe ’13, MNG’ 17, first author of the paper who recently completed his doctorate in the Department of Electrical Engineering and Computer Science (EECS) at MIT. “So it comes as an alternative to those big radar dishes you see at the airport that move with motors. We can do the same thing, but we don’t need moving parts because we just change a few bits in a computer.
Co-authors include EECS graduate student Xibi Chen; Georgios Dogiamis, Robert Stingel and Preston Myers of Intel Corporation; and Han, lead author of the article. The research is presented at the International Solid-State Circuit Conference.
Inventive manufacturing techniques
With typical antenna arrays, each antenna generates its own radio wave power internally, which not only wastes a lot of energy, but also creates complexity and signal distribution problems that previously prevented these arrays from s adapt to the number of antennas required. Instead, the researchers built a reflective array that uses a primary power source to send terahertz waves to the antennas, which then reflect the energy in a direction the researchers control (similar to a satellite dish on the roof ). After receiving the energy, each antenna performs a delay before reflecting it, which focuses the beam in a specific direction.
Phase shifters that control this delay typically consume a large portion of the radio wave’s energy, sometimes as much as 90%, Monroe explains. They designed a new phase shifter that consists of only two transistors, so it consumes about half the power. Additionally, typical phase shifters require an external power source such as a power supply or battery for operation, which creates power consumption and heating issues. The new design of the phase shifter does not consume any energy.
Directing the energy beam is another problem – calculating and communicating enough bits to control 10,000 antennas at once would significantly slow down the performance of the reflector array. The researchers avoided this problem by integrating the antenna array directly on the computer chips. Because the phase shifters are so small, just two transistors, they were able to reserve about 99% of the space on the chip. They use this extra space for memory, so each antenna can store a library of different phases.
“Rather than telling this antenna array in real time which of 10,000 antennas should point a beam in a certain direction, you just tell it once and then it remembers. Then you just dial that and, essentially, it fetches the page from its library. We later discovered that this allows us to think about using that memory to also implement algorithms, which could further improve the performance of the antenna array,” says Monroe.
To achieve the desired performance, the researchers needed about 10,000 antennas (more antennas allow them to direct energy more precisely), but building a computer chip large enough to hold all those antennas is a huge challenge in itself. . So they took an evolutionary approach, building a single, small chip with 49 antennae that is designed to communicate with copies of itself. Then they tiled the chips into a 14 x 14 array and assembled them with microscopic gold wires that can communicate signals and power the array of chips, Monroe explains.
The team worked with Intel to fabricate the chips and help with die assembly.
“Intel’s advanced high-reliability assembly capabilities combined with state-of-the-art high-frequency Intel 16 silicon process transistors have enabled our team to innovate and deliver a compact, efficient, and scalable imaging platform to frequencies below the terahertz. These compelling results further strengthen the Intel-MIT research collaboration,” says Dogiamis.
“Prior to this research, people really didn’t combine terahertz technologies and semiconductor chip technologies to achieve this ultra-sharp, electronically controlled beamforming,” Han says. “We saw this opportunity and, also with unique circuit techniques, came up with very compact but also efficient on-chip circuits so that we could effectively control the behavior of the wave at these locations. By taking advantage of IC technology, we can now enable certain built-in memory and digital behaviors that certainly did not exist in the past. We believe that by using semiconductors, you can truly enable something amazing.
A range of applications
They demonstrated the reflective array by taking measurements called radiation patterns, which describe the angular direction in which an antenna radiates its energy. They were able to focus the energy very precisely, so that the beam was only one degree wide, and were able to steer that beam in one degree steps.
When used as an imager, the one degree wide beam travels in a zigzag pattern over each point in a scene and creates a 3D depth image. Unlike other terahertz networks, which can take hours or even days to create an image, theirs operates in real time.
Because this reflective array works quickly and is compact, it could be useful as an imager for a self-driving car, especially since terahertz waves can see through bad weather, Monroe says. The device could also be well suited for autonomous drones as it is lightweight and has no moving parts. Additionally, the technology could be applied in security settings, enabling a non-intrusive body scanner that could work in seconds instead of minutes, he says.
Monroe is currently working with the MIT Technology Licensing Market to commercialize the technology through a startup.
In the lab, Han and his collaborators hope to continue advancing this technology by using new advances in semiconductors to reduce the cost and improve the performance of chip assembly.
The research is funded by Intel Corporation and the MIT Center of Integrated Circuits and Systems.