Terahertz imaging using atomic vapor

By converting low-energy photons to optical frequencies, researchers can capture images at ultrahigh speeds.

R. Mark Wilson

https://physicstoday.scitation.org/do/10.1063/PT.6.1.20200228a/full/

For imaging through opaque materials, x rays are an obvious choice. But their high energies can ionize atoms. At frequencies between the far-IR and microwave regions of the electromagnetic spectrum, terahertz radiation can penetrate materials such as paper, cloth, and plastics without damaging them. That property makes the frequency band useful for such applications as nondestructive testing, security screening, and biomedicine. But the frequency band is an elusive part of the spectrum: Terahertz photons lie below the reach of most optical technologies and just above the reach of electronics.

Credit: Adapted from L. A. Downes et al., Phys. Rev. X 10, 011027 (2020)

The upshot is that terahertz sources tend to have low power and their detectors, low sensitivity. One promising approach to circumvent those limitations is to use an atomic gas that converts terahertz-frequency photons into easily detectable optical-frequency ones. Rydberg atoms—atoms excited to a high principal quantum number n—turn out to be excellent sensors for the job. Lucy Downes, a doctoral student at Durham University, her advisers Kevin Weatherill and Charles Adams, and their colleagues have now demonstrated a terahertz imaging system using cesium vapor to make the conversion. The researchers produced optical photons at frame rates two orders of magnitude higher than the current state of the art in terahertz imaging.

The transitions between Rydberg states of alkali atoms have large dipole moments. And with a high vapor pressure at room temperature and easily accessible transition wavelengths, Cs is an advantageous choice for the demonstration. In their experiment, illustrated in panels a and b of the first figure, Downes and her colleagues place Cs gas in a quartz cell. IR radiation (yellow) focused on the cell produces a two-dimensional sheet of excited Cs atoms. Onto that sheet the researchers project a terahertz field (red) that has passed through an object of interest. The subsequent interactions of terahertz photons with Cs atoms excite the atoms to the n = 13 D5/2 level and give rise to fluorescence (green), whose emission is captured with an optical camera.

Credit: Adapted from L. A. Downes et al., Phys. Rev. X 10, 011027 (2020)

For their demonstration of high-speed terahertz imaging, the researchers captured subsequent video of dynamical processes—including a rotating optical chopper—at frame rates up to 3 kHz, as shown in the second figure. The white arrow highlights the movement of one spoke of the chopper between frames. (L. A. Downes et al., Phys. Rev. X 10, 011027, 2020.)

Focus: Green Light for Terahertz Movies

Terahertz vision. Terahertz radiation can be used for many kinds of imaging, such as in airports or in nondestructive materials testing. A new technique uses it for high-speed video

https://physics.aps.org/articles/v13/15

Researchers generated video at 3000 frames per second using so-called terahertz radiation, waves at the long-wavelength end of the infrared region of the electromagnetic spectrum. To capture images rapidly, they first passed the radiation through a cloud of excited cesium atoms to convert it into green light, which allowed them to use a standard optical camera. The work opens up a new avenue for noninvasive imaging, potentially allowing engineers to peer inside products as they move through a production line.

In theory, terahertz signals—which have wavelengths between roughly 30 micrometers and 3 mm—are ideal for nondestructive examination of products from production lines. These waves can pass through fabrics, plastics, and paper, and at the same time are nonionizing, which means that, unlike x rays, they don’t carry enough energy to remove electrons from atoms and molecules. But in practice, this range of wavelengths is difficult to detect, compared with shorter and longer wavelengths. Additionally, sources of terahertz waves tend to operate at lower power than those in other parts of the electromagnetic spectrum, and terahertz detectors are less sensitive. While terahertz imaging technologies exist, they can’t capture images rapidly and at high resolution.

Instead of trying to address the limitations of current technology, Kevin Weatherill and his colleagues at Durham University in the UK tried a different approach, explains team member and graduate student Lucy Downes. “We convert the terahertz photons into visible photons, [so] we can make use of already very well-developed optical high-speed camera technologies.”

L. A. Downes et al., Phys. Rev. X (2020)

This terahertz video shows a small wheel containing many slots spinning at 700 rpm, captured at an unprecedented speed of 3000 frames per second.

She and her colleagues used cesium atoms to perform the conversion. First they excited a cesium vapor in a quartz cell by hitting it with three continuous laser beams that constantly reexcited the atoms as they spontaneously decayed to the ground state. The team chose laser frequencies that produced excited atoms in a so-called Rydberg state, in which the most excited electron is only barely attached to the atom. Then, to create images, they shined a separate terahertz beam through a moving object and allowed the “shadow image” to hit the excited cesium vapor.

“The [Rydberg] atoms are very sensitive to the incident terahertz field,” Downes says. When one of these atoms absorbs a terahertz photon, it is transferred to a different Rydberg state that has a high probability of emitting a green photon during its decay process. For every terahertz photon absorbed, there is a 52% chance that a green photon will be emitted. With green light, the researchers could easily take a large number of relatively detailed snapshots in a short period of time.

The team created videos at 3000 frames per second of a small wheel spinning at 700 rpm, with the terahertz light peeking through slots between the wheel’s spokes. They also imaged a falling water droplet at a rate of 500 frames per second.

L. A. Downes et al., Phys. Rev. X (2020)

This terahertz video shows a falling water droplet captured at 500 frames per second.

Downes says that the team plans to improve the system’s video capabilities even further. “There are lots of small changes that we can make to optimize the system and push the frame rate even higher, such as improving the laser stabilization and using an optical camera that is optimized for green light.” However, the upper limit—constrained by the lifetime of the atomic state—is 1 million frames per second.

Peter Weightman, a biophysicist at the University of Liverpool in the UK, says the work has “enormous potential for applications in a variety of fields” and is a major step forward. “This is a remarkable advance in the development of rapid terahertz imaging systems that is made possible by the very clever exploitation of subtle features in the electronic structure of cesium atoms.”

This research is published in Physical Review X.

–Sarah Wild

Terahertz imaging reaches 3,000fps

https://www.imveurope.com/news/terahertz-imaging-reaches-3000fps

Researchers at the University of Durham have developed a method of terahertz imaging able to operate at 3,000fps.

Durham’s Lucy Downes said this was 100-times faster than any previous terahertz imager, speaking at the UK Knowledge Transfer Network’s Intelligent Imaging meeting in London on 29 April.

The technique uses laser-excited caesium atoms to perform terahertz-to-optical conversion, the light from which is captured with a standard camera.

Three infrared lasers are focused on a cell filled with caesium vapour. The lasers excite caesium atoms to a Rydberg state, at which point they can absorb a 0.55THz photon. The atoms then decay after around a microsecond and emit a green photon that can be detected with a visible camera.

The technique is only limited to how fast the atoms are excited and decay, which Downes said was potentially at frame rates of up to 1MHz. She said that this was the only method the group are aware of for imaging terahertz radiation in this way.

Downes noted a number of possible applications for the technology, such as beam profiling, mail sorting, and food inspection – for detecting plastic or foreign objects in food on fast production lines, for instance.

The terahertz imaging method is still at an experimental stage, but Downes noted that the laser technology has been miniaturised and it uses standard image sensors, so there is scope for packaging the components into a system.

High-speed terahertz imaging system uses Rydberg atoms

Green light: a psi-shaped object as revealed by the terahertz imaging system. The light emitted by the atomic vapour has been photographed using a Nikon DSLR camera. (Courtesy: Lucy Downes/University of Durham)

https://physicsworld.com/a/high-speed-terahertz-imaging-system-uses-rydberg-atoms/

A new imaging system that uses a laser-excited, room-temperature atomic vapour to convert terahertz radiation to visible light has been created by researchers at the University of Durham in the UK. The system can acquire terahertz images rapidly and efficiently using a conventional high-speed camera and the new technique could make it easier to develop practical technologies that use terahertz radiation.

Terahertz radiation lies in the region of the electromagnetic spectrum between infrared light and microwaves. In principle, it has great promise for a wide range of applications including security screening, medical imaging and industrial quality control. However, generating and detecting electromagnetic radiation at 0.1-10 THz remains an ongoing challenge. Several competing techniques are used for different applications, but they all have disadvantages – and this lack of practical technologies is often referred to as the “terahertz gap”.

So why bother with terahertz radiation when the remaining electromagnetic spectrum is available? Durham’s Kevin Weatherill explains, “It’s a region in which many everyday materials such as paper, plastics and cloth are transparent so, as with X-rays, you can image things that are optically opaque. But being low-energy, the radiation is non-ionizing and therefore safe for biological and medical applications – though it still has a sufficiently short wavelength for reasonably high-resolution imaging.”

Slow and noisy

Several techniques have already been developed for terahertz imaging. Some systems use a single-pixel detector and build-up images by scanning a terahertz beam across the object – which is a slow process. “There are a small band of focal plane arrays or full-field sensors, which can take a 2D image in one shot,” says Weatherill, “Probably the state of the art is an array of microbolometers [thermal sensors]. Their frame rate is limited to about 30 Hz because the sensitivity is low, so you need to collect photons for a long time to see an image above the background noise.”

Weatherill and colleagues created their terahertz imaging system by filling a cell with caesium vapour and focusing three infrared lasers on it. Each laser is precisely tuned to one of three successive atomic transitions in caesium. When excited by these three lasers in succession, caesium atoms end up in a highly excited “Rydberg state”. Such an atom can then absorb a 0.55 THz photon, which puts it in a different Rydberg state that decays after about a microsecond. This decay involves the emission of a green photon, which can be then detected by a standard optical camera.

The 0.55 THz absorption is a sharp resonance, and terahertz radiation at other frequencies will not be detected. Therefore, unlike other techniques for collecting terahertz photons, the technique can reliably pick out a narrowband signal from broad spectrum thermal noise.  The detection process is also about 100 times more sensitive than other techniques.

Two-colour imaging

The researchers acquired terahertz images at up to 3000 frames per second. They are now optimizing their equipment and believe that, in principle, it should be possible to collect data at frame rates up to 1 MHz. They are also keen to extend the research in other ways, such as detecting other terahertz frequencies and even two-colour terahertz imaging.

Durham’s Lucy Downes says, “I’m also very keen to try setting this up in reflection mode, so we can look for defects in the surfaces of bulk objects”.

Daniel Mittleman of Brown University in the US, says the most obvious applications of the imaging system are in the laboratory: “Things like explosions, shock wave tests, the fundamental physics of solids and fast, extreme phenomena are where you would need those kinds of frame rates, and any time the material is opaque to optics, terahertz could be an interesting alternative.”

For more commercial applications, he foresees challenges in creating practical devices. “Ultimately, it will be interesting to see how well they can package this for use outside a physics lab. If the applications are fundamental physics, that question becomes irrelevant.  If they’re thinking about applications outside the laboratory, that question is relevant and I’ve no idea how to answer it.”