Authors of the article - FTMC researchers Dr. Ignas Grigelionis, PhD student Vladislovas Čižas, Dr. Mindaugas Karaliūnas, Dr. Vytautas Jakštas, Dr. Kȩstutis Ikamas, Dr. Andrzej Urbanowicz, Dr. Marius Treideris, Dr. Andrius Bičiūnas, Dr. Domas Jokubauskis, Dr. Renata Butkutė and Dr. Linas Minkevičius.

 

The field of research in this case is terahertz waves and their application in illumination of various objects. This light, invisible to our eyes, can be used in fields such as airport security systems, medicine, food quality inspection or even the observation of distant galaxies.

 

A team from FTMC Department of Optoelectronics is working on terahertz imaging: for example, let's imagine a sealed cardboard box with a key inside. We can't see the key, and we don't even think it might be there. However, if we radiate a terahertz light through the box, we can see an image on the computer screen with a clearly visible silhouette of the key.

 

Terahertz waves come from every living (or warm) thing in the Universe and are perfectly safe - so scientists are looking for solutions to replace the health-threatening X-rays.

 

Lithuanians are the world's true experts on terahertz, and FTMC team has recently developed a metalens that greatly facilitates and improves this research. Read more about it (and about terahertz waves themselves) here.

 

And what's the innovation this time?

 

 

 

To help answer this question, let's first take a quick look at what a terahertz imaging system is and what it consists of.

 

In very simple terms, it all starts with a source that emits terahertz (in this case, scientifically known as a "Thermal Terahertz Emitter"). There is a metal box containing a heater (something very similar to a cooktop, or hob in the kitchen), and on top of this is a tiny specially created element, which is called a thermal source. When heated to around 400 °C (twice as hot as a switched-on iron), it emits terahertz waves.

 

This radiation goes everywhere - but more freely through a hole in the box. It looks like this:

 

 

 

These rays are then "captured" by a device called the imaging system (mirrors and other elements are used to do this), and the terahertz rays are "scanned" by a special lens (or a metalens developed by FTMC) that can "steer" the rays in the direction you want them to go.

 

The terahertz continues its journey by "bouncing" off the object it wants to scan - for example, the aforementioned box with the key inside. Finally, once the beams have cleared all the obstacles, they are "caught" by a detector - a device that allows us to see an altered image of the box (or other object) on our computer.

 

In FTMC Deparment of Optoelectronics, the whole system looks like this:

 

 

 

Everything works, and works well. But it could be even better. That is why Dr. Ignas Grigelionis and PhD student Vladislovas Čižas once had the idea to improve the radiation source - the crystal that emits terahertz.

 

 

"We wanted the thermal terahertz emitters to be more compact and to emit stronger radiation at the frequencies we wanted," says the study's lead author, I. Grigelionis, a Senior Researcher in the Department of Optoelectronics at FTMC. According to him, making a thin thermal source requires the help of colleagues from different fields.

 

"These sources are usually made on the 'sandwich' principle: in the laboratory, an electrically non-conductive layer is placed on top of a metal (usually copper), and then the metal structure is placed back on top.

 

Our idea is to replace the original copper with a doped gallium arsenide crystal. "Doped" means that it contains impurities that "give" the crystal free carriers (electrons), thereby increasing the electrical conductivity of the material.

 

We asked our colleague Dr. Renata Butkutė to grow gallium arsenide in the laboratory - both doped and undoped, without impurities. So you have a doped crystal underneath, and on top of that you grow an undoped crystal, which is no longer electrically conductive and acts as an insulator."

 

Once the two-part 'sandwich' was ready, laser lithography was used to create certain 'patterns' on the top - gold squares just 40 microns wide, or just 4% of a millimetre. This structure is called a metasurface: it helps scientists control terahertz light (see the article mentioned before for more on promising metamaterials).

 

This part of the process was taken care of by Dr. Marius Treideris and Dr. Vytautas Jakštas, scientists of FTMC Department of Physical Technologies. The metasurface is made from gold because this material conducts electricity well.

The final three-storey "house" - doped gallium arsenide, undoped gallium arsenide and gold metasurface - is illustrated as follows:

 

 

 

In reality, examples of the thermal source are:

 

 

(Tiny terahertz thermal sources thus seen by the naked eye. Photo: FTMC)

 

What are the benefits of the new "sandwich" instead of the old one? According to Grigelionis, the gallium arsenide-based item emits terahertz, which is stronger and more "abundant" and concentrated in the desired frequency range. And this strength subsequently improves the quality of imaging.

 

Imagine that you are sprinkling spices on a dish - you do it widely, but you sprinkle them more generously in one precise spot. Similarly, with terahertz radiation, which scientists amplify at the right "spot".

 

"Think of a simple incandescent light bulb. It is also a thermal source, but a slightly different one. Only two per cent of the light it emits is visible light. It also emits infrared waves (which is why we feel heat when we touch the bulb) and some of the radiation falls into the terahertz region. All this means that the bulb shines a very wide spectrum of electromagnetic waves over a wide range of frequencies. But we need a specific, narrow spectrum for our research.

 

The same is true for terahertz sources - we want to improve the radiative properties in a specific narrow band of the spectrum. To do this, we need to excite a resonance in the thermal source at selected frequencies, to amplify the oscillations of the light electromagnetic field. The metallic metasurface helps to select the resonance frequency, while the underlying undoped and doped gallium arsenide "sandwich" helps to maintain that resonance. In other words, we can control the strength of the light over the right frequency range," says Ignas.

 

After making a few samples of the crystal, the researchers tested how it works. This was done with the help of Dr. Andrzej Urbanowicz, who measured the reflectivity of the terahertz waves emitted.

 

 

 

What's next?

 

According to Ignas Grigelionis, the experiments are promising further improvements to the terahertz imaging system, not only in terms of higher quality images, but also in terms of making the device itself smaller and more user-friendly.

"Gallium arsenide can be integrated with other optoelectronic elements designed for the terahertz region. In other words, it is possible to put everything on one chip. But with copper, this would be difficult.

 

In the future, we want to go further with the external heater and simplify the system by making the thermal source heat itself, simply by connecting wires to it and generating an electric current. This is possible because its "foundation" is doped gallium arsenide, which is electrically conductive. 

 

We will also try to make our system not only simpler but also more compact. We have a small thermal source, a metalens developed by our colleagues PhD student Rusnė Ivaškevičiūtė-Povilauskienė and Dr. Linas Minkevičius is also thin. Now a small terahertz detector should be made. This would be much more compact, and the imaging system would take up the size of a credit card instead of a whole table, or even less space," hopes FTMC scientist.

 

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