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Terahertz Imaging

April 11, 2022

Scientists are often released from laboratory confinement for an occasional field trip, or to attend some of the many annual and topical conferences hosted by scientific societies. On such outings, I've had occasion to meet some pivotal figures in the history of science. There was the half-minute trip in a hotel elevator in which the sole occupants, Nobel Physics Laureate, Phil Anderson (1923-2020), and I exchanged pleasantries. Naturally, I recognized him, but to him I was just another conference attendee. I was also excited to meet John Wheeler (1911-2008) at another conference at which he autographed one of his books for me.[1] I wrote about Wheeler in an earlier article (John Archibald Wheeler. March 25, 2019).

Portion of Geons dust jacket

A physicist's purposeful pose.

The culmination of John Archibald Wheeler's adventures in relativity is the 1973 publication of the book, Gravitation with coauthors Charles Misner (b. 1932) and Kip Thorne (b. 1940).[1]

Wheeler also published an autobiography entitled, "Geons, Black Holes, and Quantum Foam: A Life in Physics," in 1998.[2]

(Portion of Geons dust jacket, scan of my copy. Click for larger image.)

In the 1990s I was a member of a research team doing a project that involved microwave radiometry. We had decided on the purchase of a rather expensive instrument for this; and, as due diligence, we went on a field trip to the manufacturer. It was there that we met Harold "Doc" Ewen (1922-2015), a physicist best known for his 1951 discovery of the the 21-cm line of neutral hydrogen while a graduate student at Harvard University under Edward Purcell (1912-1997).[3] Purcell soon shared the 1952 Nobel Prize for Physics for the 1946 discovery of nuclear magnetic resonance in liquids and in solids.

This spectral line, also called the H I line, is associated with an electron spin-flip transition in which the spin of the electron of the hydrogen atom flips from parallel alignment with the proton spin to an inverse alignment. In this process, it emits electromagnetic radiation at the microwave frequency of 1.42 GHz. This transition was predicted by Hendrik van de Hulst (1918-2000) in 1944, and the detection by Ewen and Purcell led to a mapping of the spiral structure of the Milky Way, a significant milestone in radio astronomy

Harole 'Doc' Ewen

Harold "Doc" Ewen in 1951 with the microwave horn antenna he built (left), and posing with that same antenna on the grounds of the National Radio Astronomy Observatory, Green Bank, West Virginia, on May 22, 2001 (right). Ewen was a graduate student at Harvard University under Edward Purcell. Purcell's expertise in microwaves arose from his work on radar at the MIT Radiation Laboratory during World War II. Ewen's horn antenna was just outside a fourth floor window of the Lyman Laboratory of Physics at Harvard, and it connected to a receiver and chart recorder through a waveguide. The funding for the entire project was just $500 ($5,500 in today's money) with Ewen doing the work on weekends.[4] (From the right photograph, it appears that Ewen and I owned very similar Members Only jackets at the time. Left image, and right image, from the National Radio Astronomy Observatory Website.[4] Click for larger image.)

During our team visit, Ewen demonstrated an instrument he was perfecting. It was a radar imager operating at nearly 100 GHz, which is about a hundred times higher in frequency than the 21 cm hydrogen line. The imager was crude, since at that time the only way to scan was mechanically, but it did demonstrate the important fact that such radiation will penetrate thin walls and image a human within a building.

A step above these frequencies is terahertz (THz) radiation, 1012 hertz, about a thousand times higher in frequency than the 21 cm hydrogen line. However, by the strange way that the International Telecommunication Union designates terahertz frequency bands, the terahertz frequency band is the frequency range from 0.3 THz to 3 THz, and a wavelength range of 1 mm to 100 μm.

Our ability to generate and detect THz radiation has improved in the thirty years since that time, and the utility of such radiation in applications including those for security and communications has inspired continued research. A team of electrical engineers at MIT's Microsystems Technology Laboratories and Intel Corporation have created a device that electronically steers and focuses a beam of terahertz electromagnetic radiation.[5-6] Their array of 98 x 98 elements, which they've named a reflectarray operates at 265 GHz. Its 9,604 elements is about the size of a credit card (see figure).[5]

Terahertz radar on a test stand

The terahertz radar on a test stand.

fabricating the entire array of 98 x 98 elements would be too difficult, so the "reflectarray" is built from 14 x 14 chips that are interconnected with gold wires.[5]

The feed waveguide is shown in gold at the bottom, and it's coupled to a feed antenna at the central axis of the array.

(Still image from a YouTube video by MIT's Han Group.[6] Click for larger image.)

This research, led by MIT associate professor Ruonan Han, was presented at the 2022 International Solid-State Circuits Conference, February 20-28, which was an online-only conference this year. Coauthors include Nathan Monroe and Xibi Chen of MIT, and Georgios Dogiamis, Robert Stingel, and Preston Myers of Intel Corporation.[6]

Intel corporation fabricated the chips using the high-frequency transistors of the Intel "16" silicon process.[5] As Han explained,
"Before this research, people really did not combine terahertz technologies and semiconductor chip technologies to realize this ultra-sharp and electronically-controlled beam forming... 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."[5]

The device works using traditional phased-array antenna beamforming. Each array element delays reflection of the terahertz feed to achieve steering of the reflected beam.[5] Integrating the antenna array directly onto a device with memory allowed the array to store a library of different phases, and this increased scanning speed.[5] The resulting terahertz device images in a way similar to a LiDAR (light detection and ranging) device; but, unlike LiDAR, it can image in rain, fog, or snow at high resolution.[5] The radar beam is just a degree wide, the scan is in increments of one degree, and it images in real-time.[5]

Possible applications include imagers for autonomous vehicles and autonomous drones, the later application taking advantage of the potential low weight of the imager and the fact that it has no moving parts.[5] It can operate as a non-intrusive and rapid body scanner in security applications.[5] This research was funded by Intel Corporation and the MIT Center of Integrated Circuits and Systems.[5]

Performance of the THz radar imager

Performance of the terahertz (THz) radar imager.

In this case, the imaging is through a poster board that's opaque to light, but not the THz radiation.

Retroreflectors are used as targets, which enhances the radar return signal.

(Still image from a YouTube video by MIT's Han Group.[6] Click for larger image.)


  1. John Archibald Wheeler, Kip S. Thorne, and Charles W. Misner, Gravitation (2017 Revised Edition), Princeton University Press, 1279 pp. + xxvi, ISBN 0-7167-0344-0 (via Amazon).
  2. John Archibald Wheeler, Geons, Black Holes, and Quantum Foam: A Life in Physics, W. W. Norton & Company, Revised Paperback edition (February 17, 2000), ISBN: 978-0393319910, 416 pp. (via Amazon).
  3. H. I. Ewen and E. M. Purcell, "Observation of a Line in the Galactic Radio Spectrum: Radiation from Galactic Hydrogen at 1,420 Mc./sec," Nature, vol 168, no. 4270 (September 1, 1951), p. 356, https://doi.org/10.1038/168356a0.
  4. The Ewen-Purcell Horn at Green Bank, National Radio Astronomy Observatory Website.
  5. Adam Zewe, "Credit card-sized device focuses terahertz energy to generate high-resolution images," MIT Press Release, February 18, 2022. Also available here
  6. ISSCC 2022 Demo: Terahertz Imaging Radar via CMOS Reflectarray, YouTube Video by Nathan Monroe, February 3, 2022.

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