Sussex scientists develop ultra-thin terahertz source, paving the way to next generation of communication tech

 

Alice Ingall

http://www.sussex.ac.uk/broadcast/read/55078

Physicists from the University of Sussex have developed an extremely thin, large-area semiconductor surface source of terahertz, composed of just a few atomic layers and compatible with existing electronic platforms.

Terahertz sources emit brief light pulses oscillating at ‘trillion of times per second’. At this scale, they are too fast to be handled by standard electronics, and, until recently, too slow to be handled by optical technologies. This has great significance for the evolution of ultra-fast communication devices above the 300GHz limit – such as that required for 6G mobile phone technology – something that is still fundamentally beyond the limit of current electronics.

Researchers in the Emergent Photonics (EPic) Lab at Sussex, led by the Director of the Emergent Photonics (EPic) Lab Professor Marco Peccianti, are leaders in surface terahertz emission technology having achieved the brightest and thinnest surface semiconductor sources demonstrated so far. The emission region of their new development, a semiconductor source of terahertz, is 10 times thinner than previously achieved, with comparable or even better performances.

The thin layers can be placed on top of existing objects and devices, meaning they are able to place a terahertz source in places that would have been inconceivable otherwise, including everyday object such as a teapot or even a work of art – opening up huge potential for anti-counterfeiting and ‘the internet of things’ - as well as previously incompatible electronics, such as a next generation mobile phone.

Dr Juan S. Totero Gongora, Leverhulme Early Career Fellow at the University of Sussex, said: “From a physics perspective, our results provide a long-sought answer that dates back to the first demonstration of terahertz sources based on two-colour lasers. Semiconductors are widely used in electronic technologies but have remained mostly out of reach for this type of terahertz generation mechanism. Our findings therefore open up a wide range of exciting opportunities for terahertz technologies.”

Dr Luke Peters, Research Fellow of the European Research Council project TIMING at the University of Sussex, said: “The idea of placing terahertz sources in inaccessible places has great scientific appeal but in practice is very challenging. Terahertz radiation can have a superlative role in material science, life science and security. Nevertheless, it is still alien to most of the existing technology, including devices that talk to everyday objects as part of the rapidly expanding ‘internet of things’. This result is a milestone in our route to bring terahertz functions closer to our everyday lives.”

Lying between microwaves and infrared in the electromagnetic spectrum, terahertz waves are a form of radiation highly sought in research and industry. They have a natural ability to reveal the material composition of an object by easily penetrating common materials like paper, clothes and plastic in the same way X-rays do, but without being harmful. Terahertz imaging makes it possible to ‘see’ the molecular composition of objects and distinguish between different materials. Previous developments from Prof Peccianti’s team showcased the potential applications of terahertz cameras, which could be transformative in airport security, and medical scanners – such as those used to detect skin cancers.

One of the biggest challenges faced by scientists working in terahertz technology is that what is commonly accepted as an ‘intense terahertz source’ is faint and bulky when compared with, for example, a light bulb. In many cases, the need for very exotic materials, such as nonlinear crystals, makes them unwieldy and expensive. This requirement poses logistical challenges for integration with other technologies, such as sensors and ultrafast communications.

The Sussex team have overcome these limitations by developing terahertz sources from extremely thin materials (about 25 atomic layers). By illuminating an electronic-grade semiconductor with two different types of lasers light, each oscillating at different frequency or colour, they were able to elicit the emission of short bursts of Terahertz radiation.

This scientific breakthrough has been long-sought by scientists working in the field since the first demonstration of terahertz sources based on two-colour lasers in the early 2000s. Two-colour terahertz sources based on special mixtures of gas, such as nitrogen, argon or krypton, are among the best performing sources available today. Semiconductors, widely used in electronic technologies, have remained mostly out of reach for this type of terahertz generation mechanism.

The research was developed within the framework of the European Research Council project “TIMING”.

The full research paper, titled, ‘All-Optical Two-Color Terahertz Emission from Quasi-2D Nonlinear Surfaces’ is published in the four star journal, Physical Review Letters, and can be read in full here: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.263901   

Researchers combine lasers and terahertz waves in camera that sees 'unseen' detail

The time-resolved nonlinear ghost imaging camera uses a nonlinear crystal to convert standard laser light to terahertz patterns, allowing the reconstruction of complex samples using a single terahertz pixel. Credit: University of Sussex

https://phys.org/news/2020-02-combine-lasers-terahertz-camera-unseen.html


A team of physicists at the University of Sussex has successfully developed the first nonlinear camera capable of capturing high-resolution images of the interior of solid objects using terahertz (THz) radiation.

Led by Professor Marco Peccianti of the Emergent Photonics (EPic) Lab, Luana Olivieri, Dr. Juan S. Totero Gongora and a team of research students built a new type of THz camera capable of detecting THz electromagnetic waves with unprecedented accuracy.

Images produced using THz radiation are called 'hyperspectral' because the image consists of pixels, each one containing the electromagnetic signature of the object in that point.

Lying between microwaves and infrared in the electromagnetic spectrum, THz radiation easily penetrates materials like paper, clothes and plastic in the same way X-rays do, but without being harmful. It is safe to use with even the most delicate biological samples. THz imaging makes it possible to 'see' the molecular composition of objects and distinguish between different materials—such as sugar and cocaine, for example.

Explaining the significance of their achievement, Prof Peccianti said: "The core challenge in THz cameras is not about collecting an image, but it is about preserving the objects spectral fingerprint that can be easily corrupted by your technique. This is where the importance of our achievement lies. The fingerprint of all the details of the image is preserved in such a way that we can investigate the nature of the object in full detail. "

Artistic rendering of the terahertz field transmitted by an abstract object. Credit: University
of Sussex

 

Until now, cameras capable of capturing a hyperspectral image preserving all the fine details revealed by THz radiation had not been considered possible.

The EPic Lab team used a single-pixel camera to image sample objects with patterns of THz light. The prototype they built can detect how the object alters different patterns of THz light. By combining this information with the shape of each original pattern, the camera reveals the image of an object as well as its chemical composition.

Sources of THz radiation are very faint and hyperspectral imaging had, until now, limited fidelity. To overcome this, The Sussex team shone a standard laser onto a unique non-linear material capable of converting visible light to THz. The prototype camera creates THz electromagnetic waves very close to the sample, similar to how a microscope works. As THz waves can travel right through an object without affecting it, the resulting images reveal the shape and composition of objects in three dimensions.

Dr. Totero Gongora said: "This is a major step forward because we have demonstrated that all the possibilities explored in our previous theoretical research are not only feasible, but our camera works even better than we expected. While building our device, we discovered several ways to optimise the imaging process and now the technology is stable and works well. The next phase of our research will be in speeding up the image reconstruction process and taking us closer to applying THz cameras to real-world applications; like airport security, intelligent car sensors, quality control in manufacturing and even scanners to detect health problems like skin cancer."

Time-Resolved Technique Could Provide High Resolution for THz Imaging

Time-resolved nonlinear ghost imaging. Courtesy of University of Sussex/ACS Photonics.

https://www.photonics.com/Articles/Time-Resolved_Technique_Could_Provide_High/a63857

Using a single-pixel camera operating at THz frequencies, researchers at the University of Sussex have discovered a way to capture with a high degree of accuracy not just the shape of an object, but also its chemical composition. The technique uses time-resolved measurement to reconstruct the complexity of an object. 

The Sussex team’s approach was to illuminate an object with patterns of THz light containing a broad spectrum of colors. The team then used a single-pixel camera to capture the light that the object reflected for each pattern. The camera was able to detect how the pulse of light was altered in time by the object, even if the THz pulse was an extremely short event. Based on this information and what was known about the shape of the patterns, the researchers were able to derive the shape and composition of the object. 

The novel methodology, which the researchers describe as nonlinear ghost imaging, theoretically demonstrates potential advantages over state-of-the-art imaging systems for the THz frequency range. 

“Our approach produces a new type of image which is quite different from what you would get from a standard single-pixel camera as it provides much more information on the object,” said researcher Juan Sebastian Totero Gongora. “Compared to prior single-pixel images, we also demonstrated that our resolution is inherently higher.”

“Previous approaches to THz single-pixel cameras cannot preserve the complete information on an object, but we understood where the issue lay and identified a way to extract a more complete image,” said professor Marco Peccianti. “We hope that a similar system to ours could be used in real-life applications in biology, medicine, and security to determine the chemical composition of an object and its spatial distribution in just one step.”

In addition to its potential impact on THz cameras, the nonlinear ghost imaging technique could be used to design high-resolution cameras in other frequency ranges, which could be incorporated into technology for collision sensors, body scanners, or ultrafast radars for self-driving cars. 

The technology was published in ACS Photonics (doi:10.1021/acsphotonics.8b00653). 

Nonlinear ghost imaging: Research could lead to better security scanners

https://www.sciencedaily.com/releases/2018/08/180830113014.htm


Using a single pixel camera and terahertz electromagnetic waves, physicists have devised a novel imaging concept -- called nonlinear ghost imaging -- that could lead to the development of better airport scanners capable of detecting explosives.

Using a single pixel camera and terahertz electromagnetic waves, a team of physicists at the University of Sussex has devised a blueprint that could lead to the development of better airport scanners capable of detecting explosives.

Miss Luana Olivieri, PhD student and Dr Juan Sebastian Totero Gongora, a Research Fellow in Experimental Photonics of the Emergent Photonics Lab directed by Professor Marco Peccianti and Dr Alessia Pasquazi, have found an innovative way to capture with high accuracy, not just the shape of an object, but also its chemical composition using a special "single point" camera capable of operating at terahertz (THz) frequencies.

Although their work is mostly theoretical at this stage -- they introduced a novel imaging concept named nonlinear ghost imaging -- their ability to capture a more detailed image to previous studies has landed them on the front page of the scientific journal ACS Photonics.

Dr Juan Sebastian Totero Gongora said: "Our approach produces a new type of image which is quite different from what you would get from a standard single-pixel camera as it provides much more information on the object. Compared to prior single pixel images, we also demonstrated that our resolution is inherently higher."

Lying between microwaves and infrared in the electromagnetic spectrum, terahertz radiation has a much larger wavelength to visible light. It can easily penetrate several common materials like paper, clothes and plastics leading to the development of technology within security scanning and manufacture control which allows people to see inside objects and wrapping.

The radiation provokes a different response from biological samples though, allowing researchers to classify materials which are almost indistinguishable with visible light.

Scientists believe that THz waves could have enormous potential in developing critical applications such as explosives detection, medical diagnostics, quality control in manufacturing and food safety.

The challenge, however, lies in the development of reliable and cost-effective cameras as well as the ability to identify objects smaller than the wavelength.

But, by taking a different approach to previous studies in this field, the team of the Emergent Photonics Lab may have found a way to overcome these limitations.

While previous research has illuminated objects with many patterns of laser light in just one colour to extract an image, the researchers illuminated an object with patterns of THz light which contain a broad spectrum of colours.

A single pixel camera (rather than a standard one containing multiple pixels as sold on the high street) can capture the light reflected by the object for each pattern. In the team's study, they found that the camera can detect how the pulse of light is altered in time by the object (even if the THz pulse is an extremely short event). By combining this information with the known shape of the patterns, the shape of the object and its nature are revealed.

The technique may recall the way the brain develops understanding in the vision by focusing separately on different elements and then fusing the relevant information.

Professor Marco Peccianti added: "This is a really significant development and we're really happy that ACS Photonics decided to lead with our research on their front cover. Previous approaches to THz single-pixel cameras cannot preserve the complete information on an object but we understood where the issue lay and identified a way to extract a more complete image.

"We hope that a similar system to ours could be used in real-life applications in biology, medicine and security to determine the chemical composition of an object and its spatial distribution in just one step."

The team's findings are a considerable improvement on established technologies and could have a huge impact beyond the field of THz cameras.

For instance, their technique could be used to design high-resolution cameras in other frequency ranges which could then become part of technology for collision sensors, body scanner or ultra-rapid radars for self-driving cars.

The researchers are now following up on their research, which is largely based on simulations, to experimentally demonstrate their device.

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Story Source:

Materials provided by University of Sussex. Original written by Stephanie Allen. Note: Content may be edited for style and length.

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Journal Reference:

1. Luana Olivieri, Juan S. Totero Gongora, Alessia Pasquazi, Marco Peccianti. Time-Resolved Nonlinear Ghost Imaging. ACS Photonics, 2018; 5 (8): 3379 DOI: 10.1021/acsphotonics.8b00653

Physicists explore a safe alternative to x-ray security scanners

                                   Marco Peccianti and Luke Peters harness terahertz with the aid of lasers

http://www.sussex.ac.uk/broadcast/read/44372

A team of physicists at the University of Sussex are developing the science to create a safe and efficient ‘paint’ that can reveal, with terahertz (THz) radiation, the contents of luggage or objects hidden in clothing.

THz radiation could replace the use of harmful x-rays and ultraviolet light in security scanners. It cannot pass through water, which is why it does not pose a health risk to living tissue, but it can penetrate fabrics, plastics and wood to give internal images similar to an x-ray.

                    Body scanned by terahertz (This image or file is a work of a United States Department of Homeland Security employee, taken or made as part of that person's official duties.)

Lying between microwaves and infrared in the electromagnetic spectrum it, as with all other radiation, travels at the speed of light - but has a lower frequency than x-rays and ultraviolet.

Although its existence has been known for decades, the technology to harness it and use it in a meaningful capacity has not been readily available.

                                                          Terahertz on the radiation spectrum

However, a team at the Emergent Photonics Laboratory at the University of Sussex have come up with a simple demonstration that shows how the surfaces of objects up to any size can potentially be made into bright terahertz emitters.

Thin semiconductors, which are materials that have a conductivity between metals (as very good conductors) and glass (which has no conductivity), are bright emitters that could be “painted” onto inexpensive materials to do the trick.

Doctoral research student Luke Peters, the lead author of a paper published in Nano Energyjournal (https://doi.org/10.1016/j.nanoen.2018.01.027, says: “Our challenge is to make terahertz emanate from common objects such as walls or pieces of paper.

“We have found that it is practically possible to create terahertz in significant amounts by illuminating ultrathin electronic semiconductors, which could simply be placed upon any surface.”

In practical terms, this means that instead of travellers passing through electromagnetic scanners at airports, they could simply walk on a pathway or aside walls coated with terahertz-emitting materials.

The discovery could help to develop many other applications, from detecting fraud in art by illuminating beneath the layers of paint, to preventing counterfeiting in currency by incorporating ink that emits terahertz.

There are also potential uses in the medical world. Terahertz has been proposed in dentistry to identify cavities in teeth, and to help surgeons in detecting specific tumours, which have a higher water content than normal body tissue and are therefore visible under terahertz radiation. 

Professor Marco Peccianti, the team leader at the Emergent Photonics Lab, said:  “With Luke’s paper being published in such a prestigious journal, this is an exciting time for our team.

There is a consensus that terahertz is still very much in its youth. The first terahertz image was only created in 1993, and what we call intense terahertz is still a tiny amount compared to what is available from light sources. To us, placing terahertz on large surfaces is the challenge. In Sussex we aim to make this cheaper, effective and practical.”