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Center for Physical Sciences and Technology (FTMC) is world-famous for its research in the terahertz range. Terahertz are electromagnetic waves invisible to the naked eye, occupying the spectrum region between microwaves and infrared waves, and ranging from 3 millimeters to 30 micrometers (thousandths of a millimeter).

Terahertz is being tested in security systems, packaging inspection, medicine, and a variety of other fields. These electromagnetic waves are also used in waveguides - and what has happened at the FTMC a few weeks ago is laying the foundations for faster communications and easier determination of material composition.

On the very last day of training that was late Thursday evening, physicist Dr. Irmantas Kašalynas walked into the FTMC Terahertz photonics laboratory under his supervision and sensed that something important was happening: "I saw the shiny faces of my colleagues. They showed me that what we had been planning for four weeks had finally come to a breakthrough."

Ashish Kumar, a PhD student at Charles III University of Madrid, Spain who is here at FTMC for his one-month secondment under the guidance of Dr. Irmantas Kašalynas head of Terahertz photonics laboratory at FTMC. The purpose of his visit is to explore the coupling mechanism of free-space terahertz (THz) radiation into a dielectric slot waveguide. And on the very last day of his training at FTMC astonishingly, he with the help of Surya Revanth Ayyagari, a PhD student at FTMC, achieved a breakthrough by successfully demonstrating a free-space THz radiation coupled to a waveguide capable of transmitting a signal with a bandwidth of over 100 gigahertz. "This is a new way and, I would say, a new record for such type of communications," says I. Kašalynas.

Bandwidth is the difference between the highest and lowest frequencies at which information is transmitted.

What is a Waveguide?

A waveguide is a component designed to transmit electromagnetic waves from one place to another virtually without loss. The analogue for transmitting visible spectrum waves (light) is a fiber optic cable - but terahertz waveguides are usually metal, in various shapes. In simple terms, it is like a tube in which electromagnetic waves (radio waves, microwaves, etc.) travel. It all works on the principle of "mirror" reflection: when waves enter the waveguide, they propagate through it by bouncing off the inner walls, zigzagging until they reach the exit point and another device such as a slot, an antenna, a splitter, a receiver, or a detector.

Waveguides are designed for a specific frequency range in order to direct and transmit specific types of electromagnetic waves as efficiently as possible (and with the lowest possible losses). One of the most common examples is the microwave oven, where a magnetron device generates microwaves which are directed through a waveguide to the inside of the oven to heat food.

The FTMC Department of Optoelectronics is involved in the study of terahertz, one type of electromagnetic wave. For example, here they are applied in spectroscopy, which is a method that studies the chemical composition and structure of materials by radiating (in this case) terahertz waves at them.

Ashish and his colleague Surya Revanth Ayyagari, have even managed to carry out spectroscopy of certain low refractive index materials using such sophisticated waveguide system.

Identify materials and communicate safely

As the young physicists say, this is just the beginning. There are many more experiments to come, and the terahertz waveguide itself is still only two centimeters long. But the results are encouraging.

What is Ashish's big innovation? As definition, each waveguide is designed to operate in a specific frequency range. For example, if the so-called center frequency (for which the waveguide is designed to operate optimally) is 100 gigahertz (GHz). This would mean that it will perform best with an operating range of 10 % (either side of the "factory" center frequency), i.e.at frequencies between 90 and 110 GHz only. If you deviate from this, the waveguide will no longer work.

"Electromagnetic waves of all frequencies propagate in free space, which would be an infinitely wide range of spectrum. The problem is how to design a waveguide that works in the widest possible range, because waveguides by default are very "narrow" in this respect," explains Dr. Kašalynas.

This led to the idea of developing a waveguide with a bandwidth (signal transmission range) of 100 GHz or even wider. "This means that we go well beyond the usual 10% limit and aim to have a bandwidth 2-3 times wider. We are going from 10% to 100-200%! Waveguides still use the principles of electromagnetic wave (light) transmission, but now we can transmit signals over a much wider spectrum," says FTMC physicist.

This is exactly what A. Kumar was able to demonstrate on the last evening at the FTMC. "On a single dielectric (does not conduct a dc-current) rod, we have achieved a very wide multi-octave bandwidth, so this is really some kind of milestone that will be useful in THz spectroscopy, next-generation communications, and sensor design. It opens up a wide range of applications," says Ashish.

How will this help in spectroscopy, chemical detection? According to the researchers, this is where the so-called evanescent field comes into play. It occurs when an electromagnetic wave propagates across the surface of a material (the outer surface of a terahertz waveguide) and the strength of the field rapidly decays as it moves away from the surface in a perpendicular direction. This happens when part of the electromagnetic wave travelling in the waveguide starts to propagate outwards. Importantly, the evanescent field is very sensitive to the environment.

"The evanescent field is formed at the boundary between air and material. If any other material enters the field, an interaction occurs and the material leaves its 'fingerprints' on the field. This is useful in spectroscopy, but we could also consider sensors or rapid tests," states A. Kumar.

He adds that the technology could also lead to breakthrough in communications, with much faster data transmission:

"The losses in these terahertz waveguides are very low, so if we use them to transmit electromagnetic waves, not only will most of the information reach the target, but also the absorption from the atmosphere, which is quite significant for terahertz in open space, will be reduced.

In addition, this waveguide system would allow higher frequencies to be transmitted, e.g. 500 GHz, which would allow high-speed data transmission. In the future, when we need terabytes of bandwidth (e.g. one or ten terabytes per second), these waveguides will be essential. In the future, we will need higher speeds to support our current systems and applications, as user interfaces evolve rapidly. So new waveguides and antennas can really help in this area."

Dr. Kašalynas talks about the idea of using waveguides for secure communication over long distances:

"Similar to fiber optics, which are used to connect one point to another, waveguides also transmit information and route it to specific users, enabling them to communicate with each other. Only in this case, no one would be able to hear what the users are talking about. 

Unlike antennas, which need to be "visible" to each other to transmit signals efficiently, waveguides can do so safely, even when placed around obstacles such as walls or the ground.

In addition, unlike fiber optics, waveguides could transmit more information. For example, if we have a 100 GHz  of bandwidth, this means that we can transmit signals to a billion users at once. For this reason, we need as much spectral range as possible for communications."

Helping young scientists

Ashish Kumar grew up in India and was interested in electronics and optics from an early age. After completing his Master's degree in China, he became interested in spectroscopy and is now studying for his PhD in Spain.

The young physicist's internship at the FTMC is part of the EU-funded Marie Sklodowska-Curie Actions (MSCA) project TERAOPTICS. This network for training young scientists brings together 11 academic and non-academic beneficiaries (universities, research institutions, SMEs, and industry) and 13 partners not only from Europe, but also from the USA and Japan. TERAOPTICS promotes research on terahertz photonics in the fields of communications, space, security, radio-astronomy, and material science.

The program has fifteen PhD students and young researchers, who travel to other countries to work with partners, carry out internships, and gain the knowledge to accomplish their PhD theses. While A. Kumar return to Spain,   Surya Revanth Ayyagari continue work on the TERAOPTICS topics. He is a physics talent from India, who is working on the development of diffractive optical elements such as THz lenses, waveplates  at the FTMC and is a PhD student of Dr. Irmantas Kašalynas.

FTMC is inviting applications for PhD studies now, with more than 50 dissertation topics in physics, chemistry, and materials engineering on offer for young talents from Lithuania and abroad.

Written by Simonas Bendžius