This research was initiated by PhD student E. Bužavaitė-Vertelienė and prof. dr. Z. Balevičius of the Plasmonics and Nanophotonics Laboratory, Department of Laser Technologies, FTMC.
The fundamental results obtained by FTMC researchers published in “Nanophotonics” journal proves presence of strong coupling effect between two hybrid plasmonic excitations – Tamm plasmon and surface plasmon. According to the authors of the study, a plasmonic-nanophotonic structure consisting of a 1D photonic crystal with a thin (40 nm) gold layer was used.
The phenomenon studied by the FTMC group of scientists has unique properties that allow the plasmonic system to efficiently exchange energy, so this effect in nanophotonic systems is aimed at application in nanolasers, next-generation optical biosensors and integrated nanophotonic elements. Ernesta Bužavaitė-Vertelienė, a third-year FTMC doctoral
student in the field of physics, told more about the relevant fundamental research, the goals pursued and her interesting journey through the science.
How would you describe the object, topic and goals of the research published in “Nanophotonics”?
In this research, the interaction between two different surface plasmonic excitations was investigated. These plasmonic excitations are two surface electromagnetic waves that occur under certain conditions on thin (about 40 nanometers thick) layers of metal that can interact with each other if there is a short distance between them. In this work we used a photonic-plasmonic structure. It is a multilayer periodic structure, also called a 1D photonic crystal (PC) or a Bragg mirror, covered with a thin layer of gold.
By illuminating such structure with polarized light, we can generate two plasmonic excitations that occur on different sides of the metal: between metal and air – the surface plasmon polariton (SPP), and between the periodic structure and metal – the Tamm plasmon polariton (TPP). Such a system of two plasmonic excitations can be in strong coupling, which means that the excitations can exchange energy with each other faster than the energy losses in the thin metal layer. By measuring the reflectance spectrum of such excitations, which are in strong coupling, we can see that due to efficient energy exchange, the resonance half-width of a surface plasmon is much narrower than that of such single surface plasmon.
In the article, which was published in the journal “Nanophotonics”, we used a total internal reflection ellipsometry method with optical filters embedded in the optical scheme. These filters do not transmit a certain part of the light spectrum, so if properly selected, we can filter the spectrum so that only one component of plasmonic excitation is generated in hybrid mode. If the system is in strong coupling, then the plasmonic excitation will be unchanged, at the same point in the spectrum as without filtering the spectrum. Exactly in this work experimental studies using such a method have shown that the system is in strong coupling.
The effect of strong coupling can be applied in the production of sensors, optical circuits and, most importantly, in the new generation of plasmonic nanolasers, also called SPACER. These are the next generation laser radiation sources that are currently being developed and tested in scientific laboratories.
Ernesta, what led you to go deep into this research field: what was the original idea for this scientific study and why did you and your colleagues call it a "miracle"?
It was like a joke, me and my supervisor prof. dr. Zigmas Balevičius started to call in such a word the very first our idea, that between two plasmonic excitations in strong coupling regime, both plasmonic excitations would exist, regardless of which component we generate. Although by the definition of strong coupling this is obvious.
This effect is found in various systems, from mechanical to quantum mechanical. One of the simpler examples is the pendulums connected by a joint: if we move one pendulum, after a while the second will start to move and the first will stop. However, this process will be repeated when one pendulum transfers energy to another. Due to the connection between the pendulums, they are in strong coupling. It is also the case with photons - light particles that interact with matter and under certain conditions create hybrid states of a photon and an electron of a material that are called plasmon polaritons.
It turns out that it is possible to create a nanophotonic structure in which two such plasmonic excitations interact with each other in a very short time (femtoseconds), transmitting energy to each other without losses, similarly as in a two pendulum system. There are a number of studies analyzing the strong coupling between plasmon and exciton. However, in our study experimental evidence of a strong coupling between two plasmonic excitations, that create a new hybrid plasmonic mode, was demonstrates for the first time. To generate such a plasmonic mode, it is sufficient to excite only one of its components, and if the system is in strong coupling, both components of the hybrid mode are always excited.
What important results have been achieved in this research?
We have demonstrated that strong coupling can be investigated by a relatively simple spectral methods using optical filters. We have also shown how changing the excitation conditions of surface plasmons one can control the strength of the interaction between them and how the properties of the excitations themselves change as a result.
This is a great way to show how such excitations can be handled. Which means that by applying them in the areas mentioned above, we can control chemical reactions, the sensitivity of optical sensors, and apply plasmonic excitations in photonic devices. In other words, it is possible to create devices that are more efficient than current devices. We are very pleased that we have been able to publish these results in the prestigious scientific journal “Nanophotonics”.
You said that the research you are doing is fundamental, but very relevant to society because of the wide range of applications in various fields. Could you explain more how these discoveries, when applied in real-life, would improve various technological processes, improve medical diagnostics, monitor and manage chemical processes, communicate information and more?
As I mentioned, plasmonics, strong coupling regime in nanophotonic-plasmonic structures can be applied in various fields: to develop more sensitive biosensors, for faster optical information transmission systems - faster computers and a new generation of lasers, as well as to control chemical reactions.
We have already conducted studies in which such hybrid plasmonic TPP-SPP excitations and strong coupling effect have been used in optical biosensors, where such sensors are more sensitive than standard single plasmon sensors and allow the detection of lower protein concentrations.
Thus, for medicine, immunology, the results of our research by the FTMC team are particularly relevant, especially at this time of the coronavirus pandemic. Using the strong coupling between plasmonic excitations, we can detect lower concentrations of virus or antibodies and investigate their interactions. The results we obtained may facilitate the study of various viruses and the human immune response. From antibody-antigen interactions (immune response to the virus), kinetic constants can be determined that describes how quickly immune system responds to the virus. By applying the effect of strong coupling, we could detect extremely low concentrations of virus or antibodies. We can also study the effectiveness of various drugs in the initial stages of their development. Such innovative plasmon-nanophotonic biosensors could be a significantly faster and reusable method of virus detection.
In what other areas could your scientific discoveries be applied to make complex processes easier and more efficient: the new generation drug development, research of biological systems, etc.?
Strong coupling can be used to control various chemical reactions. For example, in the study of various biological systems, some of the methods used in the measurements cause certain chemical reactions that change the properties of a biological object, for example a cell. However, by applying strong coupling, we can inhibit those adverse photochemical reactions. One such adverse chemical reaction is photobleaching. By applying strong coupling, one can control the unwanted photobleaching effect. When studying cells, e.g. cancerous, luminescent markers are used to label such cells. These markers are molecules that bind to certain parts of the cell, and when the cell is illuminated with UV light, they begin to shine a certain color. However, UV light can induce the chemical reactions of molecules, so the image we see will be different from what the original view of the cell would look like. We could overcome this problem with the help of the strong coupling effect which allows us to control these chemical reactions. Then "unnecessary" reactions would no longer take place and we would see the real picture.
These excitations and structures can also be used to detect various gases, toxic fumes, such as detection of hazardous mercury vapor or even real-time monitoring of the formation and release of various chemical compounds.
How would modern technologies and information flow change? This is very relevant to today’s society.
Yes, another potential application of strong coupling is optical circuits: information transmission systems (fiber optic internet) and computer technology in the development of quantum computers. This would allow information to be transmitted faster and with smaller energy losses.
As I have mentioned, these excitations and strong coupling can be applied to the production of nanolasers - very small-sized plasmonic lasers. These are a new generation laser sources that should have significantly higher performance than the currently used lasers and could be integrated into electronic circuits.
Probably one of the most interesting applications for us is plasmonic lasers (SPACER). The resonators of such lasers are very small in size: several hundred nanometers or even smaller. Lasing with such devices is extremely efficient, as the application of plasmonic effects can bypass the diffraction limit, which has not been possible so far with traditional lasers. Due to their small size, plasmonic lasers could be integrated into everyday devices (telephones, computers) and sensors. Plasmon lasers would take up less space, so the devices in which they would be used would also be small. For example, the thickness of the plasmonic-photonic structure used in our study is about 850 nanometers, which is 100 times less thick than human hair.
Why is this investigation performed by FTMC researchers is so important, especially in the international context?
We want to emphasize that our team’s research and results are unique in that they demonstrate that excitation of one component of plasmonic excitation another always exists too if the system is in strong coupling. To date, numerous studies have been conducted to analyze the strong coupling between a single plasmon and an exciton. However, such plasmonic excitations, where two different plasmonic waves are generated (the Tamm-surface plasmon polariton hybrid system) are not widely studied. There are several larger groups of researchers in the world in the United Kingdom, Finland, France, and the United States conducting research on strong coupling. In Lithuania, research on surface electromagnetic waves was started at the former Institute of Physics. These works are now continued in the Plasmonics and Nanophotonics Laboratory of the Department of Laser Technologies of the FTMC, where we are conducting research in the field of plasmonics.
How difficult was the whole scientific path to successful results for your team and for you, the main author of this study? Have you set a final goal for what you are aiming for in your research?
Firstly, the scientific question of whether both plasmonic excitations would exist if only one were excited, was previously raised by prof. dr. Z. Balevičius and dr. A. Paulauskas. Later, we started developing this theme together. Eventually, this idea evolved into such a method of "checking" for strong coupling using filters.
Together with FTMC prof. dr. Z. Balevičius we performed measurements and analyzed the obtained data. A fellow PhD student V. Vertelis performed calculations to estimate the magnitudes of the strong coupling obtained from experiment reflection spectra. These studies are extremely important to us because the application of such interactions in lasers has great potential. I would like to fulfill the dream I have at the moment: to create a plasmonic laser that uses such a plasmonic-photonic structure and strong coupling. We plan to expand our research and we want to apply the achieved results and acquired knowledge in the development processes of nano-laser devices and biosensors.
How did you become a researcher in this field of science, why did you choose to study plasmonic processes?
During my bachelor’s and master’s studies, I worked with organic light emitting devices (OLEDs). I spent a lot of time in the laboratory: growing OLED structures, researching materials, and performing optical measurements. Even then, I really enjoyed working in the laboratory because I had to overcome a number of new, exciting challenges. During our master's studies, we had a "Photonics" course, which was led by dr. Rolandas Tomašiūnas. During the studies from time to time we heard about plasmons, which turned out to be an extremely interesting phenomenon for me. Fate continued in this direction: when doctoral topics were published, one of them was presented in FTMC by prof. dr. Z. Balevičius. The studies were related to photonics and plasmons. After talking to the professor, I was really interested in this topic and now I can say that I am very happy to stay in science. The desire to learn, to figure out the processes, is a good drive in life.
Prof. dr. Zigmas Balevičius
Why did you choose PhD studies at FTMC? How do you assess the environment and opportunities to create and work right here? What encouraged the continuation of the scientific path in Lithuania, not abroad?
I chose my doctoral studies at FTMC
because the scientific topic I was interested in was published here. My choice was also greatly influenced by the opportunities for the development of science provided by the center itself. While still studying for a master's degree, we had a course with the director of FTMC prof. habil. dr. Gintaras Valušis, who taught us a lot about the possibilities of scientific research at the Center of Physical Sciences and Technology (FTMC), we visited many laboratories that are here. Perhaps the fact that I had the opportunity to see for myself, to understand how many opportunities this center offers in science, led me to come to study right here. It is difficult to compare studies here with foreign countries, I didn’t study there, but some colleagues who have studied both in Lithuania and abroad (Sweden, France, Germany) have repeatedly said that Lithuania does not lag behind other countries. Currently I’m in my third doctoral course, studying the excitation processes of Tamm’s plasmon-polariton optical states using photonic crystals, my supervisor is prof. dr. Z. Balevičius.
PhD students of FTMC are provided with really good conditions to study, do research, an inspiring environment and opportunities to create and work for a young person. It is good to study because there is a wide variety of scientific directions here. Another, in my opinion, very important feature is the interdisciplinary of the research in the center (physics, chemistry, biology, electronics, textiles, etc.), concentrated, modern laboratories, the latest, high-quality equipment. Here, the scientists have all the possibilities to perform all the necessary measurements in one place. It is gratifying that this center is developing not only science but also technology: various sensors are produced and innovative materials are grown. For me, FTMC is a really great place to develop ones’ scientific research and satisfy curiosity.