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Terahertz (THz) represents the portion of the electromagnetic radiation between the microwave and the infrared region. It is within the frequency range of 0.1–10 THz, corresponding to wavelengths of radiation from 3000 to 30 µm. The advantages of the terahertz light are that they are non-invasive, intrinsically safe, and non-ionizing, along with being non-destructive.
This radiation does have some uniquely attractive qualities: For example, it can yield extremely high-resolution images and move vast amounts of data quickly. And yet it is nonionizing, meaning its photons are not energetic enough to knock electrons off atoms and molecules in human tissue, which could trigger harmful chemical reactions. Terahertz may revolutionize medical imaging, security screening and manufacturing quality control because of THz’s non-ionizing property and capability to penetrating dielectrics, fabrics and body tissue.
Terahertz antennas are vital components for THz wireless communication devices to radiate and detect terahertz devices. The terahertz (THz) antennas, have features of small size, wide frequency bandwidth and high data rate, are vital devices for transmitting and receiving THz electromagnetic waves in the emerging THz systems. However, most of THz antennas suffer from relatively high loss and low fabrication precision due to their small sizes in high-frequency bands of THz waves.
THz antennas are metallic antennas, dielectric antennas, and new material antennas. The variety of terahertz antennas is growing through advances in fabrication and computational techniques.
Carbon NanoTube-based THz antenna
materials. CNTs have several forms of structures derived from an original graphene sheet and are classified into single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube (MWCNT) based on their structures.
Terahertz Photoconductive Antennas (PCA)
Photoconductive antennas (PCAs) have been extensively utilized for the generation and detection of both pulsed broadband and single frequency continuous wave terahertz (THz) band radiation. These devices form the basis of many THz imaging and spectroscopy systems, which have demonstrated promising applications in various industries and research fields.
Here, an example of a femtosecond optical pulse with a pulse duration of < 1 ps is incident on a PCA. The PCA consists of a DC biased metal dipole antenna patterned on a photoconductive substrate. The optical pulse is incident on the antenna gap (G), propagates into the photoconductor, and begins to generate photocarriers inside the photoconductor as it is absorbed. The generated photocarriers are accelerated in the DC bias field, producing a transient photocurrent, which drives the dipole antenna and ultimately re-emits as a THz frequency pulse
To measure the emitted THz pulse, another PCA is utilized as the receiver. Unlike the emitter, the receiver PCA does not have an external DC bias. Instead, the emitted THz beam is focused on the dipole antenna, so the beam polarization is aligned across the antenna gap. As the THz pulse propagates into the antenna, it induces a transient bias voltage across the gap. To measure this transient voltage, a portion of the femtosecond optical pulse is split from the source beam, propagates through an adjustable optical delay line, and focuses in the gap of the receiver PCA. This provides a narrow impulse of photocarriers at a time that can be controlled by the optical delay line. When the photocarrier impulse and THz field induces transient voltage overlap in time, a measurable photocurrent proportional to the instantaneous antenna voltage is induced across the antenna.
By sweeping the optical delay line, the photocarrier impulse signal is convoluted with the THz field induced transient voltage signal. Collecting and correlating both the optical delay position and induced photocurrent data allows the temporal profile of the THz pulse to be measured. The coherent nature of this detection method provides a high signal-to-noise ratio (SNR), since it greatly reduces the effects of blackbody radiation and other sources of THz radiation on the receiver
Electronically steerable, terahertz antenna array
The Terahertz Integrated Electronics Group at MIT, led by Associate Professor Ruonan Han, seeks to bridge this so-called terahertz gap. These researchers have now demonstrated the most precise, electronically steerable, terahertz antenna array, which contains the largest number of antennas. The antenna array, called a “reflectarray,” operates like a controllable mirror with its direction of reflection guided by a computer.
The reflectarray, which packs nearly 10,000 antennas onto a device the size of a credit card, can precisely focus a beam of terahertz energy on a tiny area and control it rapidly with no moving parts. Built using semiconductor chips and innovative fabrication techniques, the reflectarray 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 reflectarray uses terahertz waves instead of light, it can operate effectively in rain, fog, or snow. This small reflectarray was also able to generate radar images with twice the angular resolution of those produced by a radar on Cape Cod, which is a building so large it is visible from space. While the Cape Code radar is able to cover a much larger area, the new reflectarray is the first to bring military-grade resolution to a device for commercial intelligent machines.
“Antenna arrays are very interesting because, just by changing the time delays that are fed to each antenna, you can change what direction the energy is being focused, and it is completely electronic,” says Nathan Monroe ’13, MNG ’17, first author of the paper who recently completed his PhD in MIT’s Department of Electrical Engineering and Computer Science (EECS). “So, it stands as an alternative to those big radar dishes you see at the airport that move around with motors. We can do the same thing, but we don’t need any moving parts because we are just changing some bits in a computer.”
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 challenges which previously prevented such arrays from scaling to the number of antennas required. Instead, the researchers built a reflectarray that uses one main source of energy to fire terahertz waves at the antennas, which then reflect the energy in a direction that the researchers control (similar to a roof-top satellite dish). After receiving the energy, each antenna performs a time delay before reflecting it, which focuses the beam in a specific direction.
The phase shifters that control that time delay typically consume a lot of the radio wave’s energy, sometimes as much as 90 percent of it, Monroe says. They designed a new phase shifter that is made from only two transistors, so it consumes about half as much power. In addition, typical phase shifters require an external power source such as a power supply or battery for their operation, which creates problems with power consumption and heating. The new phase shifter design consumes no power at all.
Steering the beam of energy is another problem — computing and communicating enough bits to control 10,000 antennas at once would dramatically slow the reflectarray’s performance. The researchers avoided this problem by integrating the antenna array directly onto computer chips. Because the phase shifters are so small, just two transistors, they were able to reserve about 99 percent 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 the 10,000 antennas needs to steer a beam in a certain direction, you just need to tell it once and then it remembers. Then you just dial that up and essentially it pulls the page out of its library. We found out later on that this allows us to think about using this memory to implement algorithms, too, which could further enhance the performance of the antenna array,” Monroe says.
To achieve their desired performance, the researchers needed about 10,000 antennas (more antennas lets them more precisely steer the energy), but building a computer chip big enough to hold all those antennas is a huge challenge in itself. So they took a scalable approach, building a single, small chip with 49 antennas that is designed to talk to copies of itself. Then they tiled the chips into a 14 x 14 array and stitched them together 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 assist with the assembly of the array.
“Intel’s high-reliability advanced assembly capabilities combined with the state-of-art, high-frequency transistors of the Intel 16 silicon process enabled our team to innovate and deliver a compact, efficient, and scalable imaging platform at sub-terahertz frequencies. Such compelling results further strengthen the Intel-MIT research collaboration,” says Dogiamis.
“Before this research, people really did not combine terahertz technologies and semiconductor chip technologies to realize this ultra-sharp and electronically-controlled beam forming,” Han says. “We saw this opportunity and, also with some unique circuit techniques, came up with some very compact but also efficient circuits on the chip so we can effectively control the behavior of the wave at these locations. By leveraging the integrated circuit technology, now we can enable some in-element memory and digital behaviors, which is definitely something that didn’t exist in the past. We strongly feel that using semiconductors, you can really enable something amazing.”
They demonstrated the reflectarray by taking measurements called radiation patterns, which describe the angular direction in which an antenna is radiating its energy. They were able to focus the energy very precisely, so the beam was only one degree wide, and were able to steer that beam in steps of one degree.
When used as an imager, the one-degree-wide beam moves in a zigzag pattern over each point in a scene and creates a 3D depth image. Unlike other terahertz arrays, which can take hours or even days to create an image, theirs works in real-time.
Because this reflectarray 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 because it is lightweight and has no moving parts. In addition, 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 bring the technology to market through a startup.
