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Congratulations to our colleagues, we are very proud of you!

 

The paper is entitled "Light Engineering and Silicon Diffractive Optics Assisted Nonparaxial Terahertz Imaging". Dr. Sergejus Orlovas, a researcher in the FTMC Department of Fundamental Research, is the main author of the article.

 

The cover illustration of the journal was designed by Dr. Rusnė Ivaškevičiūtė-Povilauskienė, a physicist at the FTMC Department of Optoelectronics. She is also one of the co-authors of the paper.

 

 

"My colleagues and I joke that our achievement could be compared to being on the cover of Cosmopolitan," smiles Rusnė, "Laser & Photonics Reviews is a prestigious journal with a very high impact factor, which is very nice.

 

The editorial team of the journal itself offered us the opportunity to create a cover based on our article. Several groups of researchers send in their own versions, and the editors choose the winner."

 

"Usually, the cover is given to work that the editors of those journals consider to be outstanding," Sergejus notes, "In our case, we could see from the very beginning that this paper was one of the editors' favourites. When we sent in the revised version, we got a response in record time - within a few hours - that the article had been accepted into the journal."

 

 

 

What's on the cover? It's four optical elements, lenses made of silicon (and laser-etched), each of which uniquely shapes the emission of terahertz light invisible to the naked eye. The research work was designed to compare the performance of these elements: physicists from the Coherent Optics Laboratory of the Fundamental Research Division carried out the calculations and produced the lenses, which were then experimented on by specialists from the Department of Optoelectronics.

 

Let's examine R. Ivaškevičiūtė-Povilauskienė's illustration more closely.

 

At the very top is the so-called Airy lens, which is characterised by the ability of the terahertz light that passes through it to 'recover' itself, even if it hits an obstacle. In other words, this curved beam of light makes it possible to study and observe samples 'around the corner'.

 

By the way, this was one of the topics of Rusnė's recently defended doctoral thesis. Airy lens research has already attracted international attention: she and her colleagues published a paper in the prestigious Nature group journal Light: Science & Applications. "And the fun of telling people about it is that they immediately get hooked: how is it possible to see things 'around the corner'?", recalls the physicist.

 

What do we see below? First on the left is the Fresnel lens, which focuses the terahertz image to a single point; in the middle is the Bessel lens: it forms a straight, long, needle-shaped beam of light, so that you can see a fairly bright image in the deeper layers of the sample; while on the right is the Fibonacci lens, which is capable of focusing the light to two different points simultaneously!

 

All of these optical elements, as clearly illustrated on the cover, may be of some use to scientists testing the possibilities of terahertz imaging.

 

 

 

 

The main author of the paper, Dr. Sergejus Orlovas, explains the background to this study in more detail. To get a better understanding of terahertz light experiments, let's recall a much more common pastime for all of us: photography:

 

"People who are professionally involved in photography use cameras. These use lenses or lens systems that project an image of millions of pixels onto a dedicated matrix. This is called single-shot imaging: you aim, you 'fire', and you get the picture."

 

According to the scientist, there are certain natural forces that govern how far the subject should be from the lens of the camera, and also how far the camera's pixel matrix should be from the lens. Sometimes you have to move the lens - zoom in or out - to get a sharp and contrasty image.

 

"The problem with terahertz imaging is that it is difficult and expensive to build something like a camera in this lightwave band. So another concept is used - single pixel imaging. Various tricks are used to make a merged image out of many single pixel images," says S. Orlovas.

 

In the FTMC Department of Optoelectronics, objects illuminated by terahertz light are imaged using so-called raster scanning. What is it? A small sensor moves slowly across the sample and records the image on a computer by "reacting" to the terahertz waves reflected back. The sensor collects information from each pixel that is scanned, and all these individual pixels are finally combined into a single image of the object. It's like using a flashlight in a dark room and moving it around to see different parts of the room.

 

 

 

"In principle, our paper is about such a methodology, but we have raised the question of whether there are, as my colleague Prof. Gintaras Valušis joked, "Orlovas' forces" in such imaging, which help to determine the positions where the pixel, the sensor, the object should be, and the light with which that object should be illuminated.

 

We have already seen that it is not necessary to use standard optical lenses alone for single pixel imaging. It turns out that exotic elements that people don't use in single-shot imaging can also be useful here - non-standard Airy, Fresnel, Bessel and Fibonacci lenses," says Sergejus.

 

The authors of the paper have thus raised the question of whether it is possible to use these elements in a high-quality way for experiments with terahertz light.

 

"To our surprise, it turns out we can do it. We were able to numerically evaluate and experimentally confirm which combination of illuminating and light-collecting elements gives the best image contrast, resolution and depth of field," says the physicist.

 

The FTMC team's research paper can be accessed by clicking on this link.