A team of researchers from Center of Physical Sciences and Technology (FTMC) has published an article in the high-level international journal Frontiers in Physics on 5 May. The Lithuanians have developed an optical element metalens which is lightweight and thin. It is designed to improve and simplify terahertz imaging - the action of scanning an object with invisible terahertz radiation to produce an image that we can see.

 

The authors of this article are members of the Departments of Optoelectronics and Fundamental Research, PhD student. Rusnė Ivaškevičiūtė-Povilauskienė, PhD student Vladislovas Čižas, Ernestas Nacius, Dr. Ignas Grigelionis, Karolis Redeckas, Matas Bernatonis, Dr. Sergejus Orlovas, Prof. Dr. Gintaras Valušis and Dr. Linas Minkevičius.

 

You can read the original article here. And if you prefer a simpler language, continue reading this article and you find out what is metalens and what are its benefits.

 

 

 

 

Terahertz radiation is one of the "types" of electromagnetic waves found in nature. These are electromagnetic waves that are the same as visible radiation - but, because of their higher frequency, we cannot see them with the naked eye. In other words, terahertz is the same as light – but invisible for us. This faint radiation is emitted naturally by everything on Earth - and it happens in space too. To emit terahertz, you need to be warmer than -271 °C. So, as you can see, it is very easy.

 

A little bit more about space: although this thermal radiation is very weak, astronomers can observe it even from distant galaxies. For example, in Hawaii, on the Mauna Kea volcano, Caltech submillimeter observatory was operating until 2015 to "catch" distant terahertz radiation. This observatory was staffed for many years by a Lithuanian-American, Prof. Jonas Žmuidzinas (who is now working to set up another, giant, observatory – learn more here).

 

And what can "terrestrial" terahertz be useful for?

 

This radiation helps us see things "through" - similar to the widespread use of X-rays. However, the latter can be harmful to health and require special safety requirements and training. Meanwhile, terahertz imaging is a promising (and therefore of interest to researchers) option that is expected to be simpler, more convenient and safer than X-ray systems.

 

 

 

Where is it applicable? One area is security systems at airports: by scanning a terahertz light on a passenger, an employee can see on a screen whether the person is carrying weapons, drugs or other forbidden items. This is possible because terahertz radiation can penetrate clothing, paper, cardboard, ceramics, plastics, masonry and wood. It is true that these rays are blocked by metal and water - but this can also be useful for important tests, such as measuring small changes in water (detecting early skin cancer, etc.).

 

Terahertz also penetrates living tissues, so the technology is being developed in the medical field; as mentioned above, this radiation is completely harmless. It is also used to check the quality of food: when the packaging is illuminated by terahertz radiation, you can see whether the cheese inside is really as fresh and unique as it says it is.

 

Everything seems perfect here - just take it and use it. But there is one problem: terahertz imaging systems are still very expensive, complex and clumsy. That's why researchers at FTMC have been working for years to make the technology simpler, more compact and, of course, cheaper.

 

And this is helped by improvements in the optical elements - the lenses.

 

 

For these systems to work properly, the development of so-called terahertz diffractive optics is essential. Diffraction is the bending of light, sound or heat waves when they hit an obstacle. For example, a wave of light normally travels in a straight line but is deflected in one direction or the other when it encounters an obstacle.

 

Here are some classic examples of diffraction:

 

 

The effects of diffraction can often be seen in everyday life. If you are of a generation that still knows what CDs and DVDs are, you will remember how they shine like a rainbow - this is due to the tiny grooves on the surface of the disc, which reflect the light and create diffraction. The same properties sometimes make spider webs or even fresh meat which glow like a rainbow!

 

Have you ever seen a bright ring around the Sun or Moon? It's also caused by the diffraction of small particles in the atmosphere.

 

Diffraction doesn't just happen with light: sound can "bypass" obstacles, so you can hear the voice of a nearby friend even if you're standing behind an old, thick oak tree.

 

 

 

Dr. Linas Minkevičius, a senior researcher at FTMC Department of Optoelectronics and one of the authors of the paper, says he and his colleagues have been working on terahertz diffraction optics since 2012:

 

"In Lithuania, we are the strongest in this field, and in the world we are competing. One of our goals now was to develop a lens that could improve terahertz imaging systems. To be cheap, lightweight, compact and with better properties than a conventional lens," he says.

 

And they made it. First a computer model is created and then the lens is made by laser cut or ultraviolet lithography (where a laser cuts a blueprint of the lens in a photoresist (a light-sensitive material) and then chemically etches it out and vaporises the metal).

 

A stainless steel lens is different from the glass lenses we are used to see: it is flexible and thin. Let's compare: a standard lens is normally about 1-2 cm thick. The lens developed by FTMC team is 25 microns thick, i.e. 0.025 millimetres (the average thickness of a human hair is 70 microns). This thinness is helping to improve terahertz imaging systems, as the thickness of the lens is thinner than the wavelength of the terahertz radiation used in this case, which is 3 mm.

 

 

 

 

That's not all. As you may have noticed at the beginning of the text, the elements developed by our scientists are not just lenses, but metalenses! What does this mean?

 

"It's a combination between a lens and metamaterials - repeating elements that, like a lens, can also affect light in a certain way," says another author of the paper, Dr. Rusnė Ivaškevičiūtė-Povilauskienė.

 

A metamaterial is any man-made material that has properties not found in nature. It can block, absorb, amplify or bend video and sound waves, as well as thermal radiation. Metamaterials can be metal, plastic or other particles arranged in special repeating patterns - it is their shape, structure, size and arrangement that give them their incredible smart properties.

 

"Imagine a spoon dipped into a glass of water. Due to the refractive index, the part of the spoon that is in the water is seen to be slightly 'shifted' to the side compared to the dry part. But because of the metamaterials, the spoon in the water appears to be facing the opposite direction," says the FTMC scientist.

 

Such a phenomenon is not possible in nature, but diffractive (i.e. light-scattering) metamaterials would make it possible.

 

 

 

Thanks to such properties, the development of metamaterials is now on the rise worldwide. Harry Potter fans should be happy: serious scientists around the world are developing an Invisibility Cloak! The aim is to use metamaterials on the surface of the cloak to make it so that visible light "passes by" it from the side - so that the wearer can blend in with their surroundings.

 

When such an invention becomes available to all, we can safely go to Hogsmeade!

 

And back at the FTMC, thanks to the "connected" metamaterials, the lenses developed by the Lithuanians not only focus (concentrate) the terahertz rays, but also reverse the polarisation of the rays. How to understand this? Imagine a caterpillar moving forward, lifting its body up and down. This would be vertical polarisation. After passing through a metalens, the caterpillar would fall on its side, but would continue to crawl forward with the same motion (this would be "horizontal" polarisation).

 

In other words, a metalens can 'lay the wave on its side'. Not only that, but scientists change the polarisation as needed. And for FTMC in particular, these advantages in light management allow terahertz to better see a wide range of objects. For example, one polarisation method will help you see some layers in a material and another will help you see other layers.

 

 

 

The authors of the paper published in Frontiers in Physics would like to thank the Research Council of Lithuania for funding the Young Scientists Project No. S-MIP-22-76.

 

This is not the first time that the results of FTMC group's research have been published in prestigious scientific journals. In November 2022, Light: Science & Applications, one of the world's most influential optics journals (the third most ranked in its field) and a member of the Nature group, published a collaborative work on terahertz imaging by researchers from the Departments of Optoelectronics and Fundamental Research.