Researchers have created a device that enables them to electronically steer and focus a beam
of terahertz electromagnetic energy with extreme precision.
This opens the door to high-resolution, real-time 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 classic electronics nor optical devices can
effectively manipulate their energy.
But these high-frequency radio waves have many unique properties, like the ability to
pass through certain solid materials without the health effects of X-rays.
They may also enable higher-speed communications, or vision systems that can see through foggy
or dusty environments.
The Terahertz Integrated Electronics Group at MIT, 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.
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.
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.
To achieve their desired performance, the researchers needed about 10,000 antennas,
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.
The team worked with Intel to fabricate the chips and assist with the assembly of the
array.
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.
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.