Lithuanian research on luminescent particles and the strong interaction – in the prestigious physics journal “Nanophotonics” | Business

Fluorescence, or glow, occurs when certain desired particles in a material are painted with a special dye – and then in a solution, powder, gas, thin layer of material or crystal, when illuminated by a laser, they begin to glow and become visible to scientists.

This method allows you to “label” specific molecules or cells, detect very small amounts of substances – that’s why fluorescence is very important in nanotechnology, medicine, biology and other fields of science.

FTMC photo/Staff of the Plasmonics and Nanophotonics Laboratory of the Laser Research Department of the FTMC, doc. Justina Anulytė, prof. Dr. Zigmas Balevičius and dr. Ernesta Bužavaitė-Vertelienė

Of course, as everywhere, there are a number of challenges here – and they are being solved by the physicists of the Plasmonics and Nanophotonics Laboratory of the FTMC Laser Technology Department, who recently published an article in the prestigious scientific journal Nanophotonics. Authors – FTMC scientists Justina Anulytė, dr. Vytautas Žičkus, dr. Ernesta Bužavaitė-Vertelienė and prof. Dr. Zigmas Balevičius together with the head of the Extreme Light Group of the University of Glasgow prof. Danielle Faccio.

The title of the article is not an easy one: “Influence of the strong interaction between exciton and plasmon on inhibition of photobleaching”. Let’s try to figure out what it means and why it is important.

How to preserve “decaying” particles?

First, let’s remember what the strong interaction is. It is one of the fundamental forces of nature that govern (and explain) practically all physical processes in nature. There are four fundamental forces: gravitational, electromagnetic, weak and strong.

The strong interaction (or strong force) is… the strongest of them all. It is “responsible” for keeping protons and neutrons together in atomic nuclei. If one bad day the strong interaction suddenly stopped, all the atoms would simply disintegrate and the stars would no longer shine. In other words, neither would we.

We go further.

The Lithuanian study we discuss examines the influence of the strong interaction between surface plasmon polaritons (SPPs) and excitons on the fluorescence lifetime and photobleaching effects. What is it?

“SPPs are surface electromagnetic waves on thin metal layers. To understand this, we can imagine circles on the surface of water – a mechanical surface wave between water and air. In the case of the electromagnetic waves we study, SPPs are similar “circles” between metal and air”, explains the main author of the article, PhD candidate of the Laser Research Department of FTMC, Justina Anulytė.

And what is an exciton? It is a kind of “pair” of tiny elements, consisting of a light particle, a photon, and a substance being studied in a laboratory. Due to the strong interaction, these two particles are bound together in some way – and remain close to each other.

Thus, an SPP is a special wave between metal and air, an exciton is a tiny “formation” in which a photon and the material under study have combined.

What happens next? “The SPP and the exciton, these two quasiparticles, combine into a plasmon-exciton polaritonic state. This means that this new quasi-state exhibits part plasmon (SPP) and part exciton properties – and we cannot separate them, they both work together. And in a very short moment (femto or picoseconds), mutual energy is exchanged without loss. In order for such a “duo” to exist, it is necessary to achieve special conditions in which a strong interaction occurs.

An exciton can emit light (fluoresce) if we excite it with a laser beam or just plain white light. However, a problem often occurs here: when the exciton enters an energetic state with oxygen excited by the environment, the so-called photobleaching occurs: the light intensity decreases over time. Why? Because due to photobleaching, the laser-excited particles “turn off” one by one, that is, they no longer shine.

Not tired yet? We have come to the most important thing: the glow of excitons is necessary for scientific experiments – so it is very important to keep the light emitted by them as long as possible. And this is where the “third” plasmon-exciton polaritonic state perfected by our scientists is useful: when excitons are combined with SPPs (remember, these are surface electromagnetic waves) in a strong interaction, the desired fluorescence light intensity of the particles lasts several to even tens of times longer.

FTMC photo/FTMC Laser Research Department physicist dr. Ernesta Bužavaitė-Vertelienė

To sum it up very simply: a team from FMTC and the University of Glasgow has developed a method that makes it possible to keep the observed particles glowing longer with the help of special dyes, lasers and strong interactions.

Significant achievements

And now let the main author of the study, Dr. Justina Anulytė:

“For our experiment, we formed a structure consisting of nanometer (millionth of a millimeter) silver and gold layers, and also used a layer of Rhodamine 6G, a luminescent dye.

Our structure showed a clear shift of the plasmon resonance and R6G absorption lines by changing the light incidence angle, which in turn demonstrates the strong interaction between the plasmon and the exciton, with a measured interaction strength of about 90 millielectron volts.

Fluorescence lifetime imaging microscopy (FLIM) showed that using polaritonic nanostructures in which plasmons and excitons interact strongly, the fluorescence intensity is maintained by about 25 percent. stronger, and the organic molecules of Rhodamine dye are approximately six times less photobleachable.”

According to the young scientist, her findings and those of her colleagues prove the essential role of the strong interaction between light and organic matter in reducing the photobleaching effect and stabilizing the intensity of fluorescence (the glow of particles).

After all this, you probably have a natural question: so what? The answer is very intriguing: this method offers promising ways to create and improve quantum devices such as quantum biosensors, quantum nanolasers – or to develop quantum information processing that would take our computing and other technologies to a whole new level.

Publishing in a publication is a serious job

The importance of the work done by Lithuanians and Italians is also proven by the fact that they managed to publish their discoveries in the above-mentioned international scientific journal “Nanophotonics” – just any articles do not go there.

The editors of the journal focus on the interaction of photons (particles of light) with nanostructures such as carbon nanotubes, metal nanoparticles, nanocrystals, photonic crystals, biological tissues and DNA. Many research results are published on the topics of plasmonics, nanolasers, interaction of light with matter, control of light at the nanometer level.

The citation rate of the journal is 6.5. What does this mean? According to statistics, only 5 percent of scientific journals worldwide have reached the level of 6 or more such scores. And that says a lot.

“Publishing your scientific results in this journal is not easy. “Nanophotonics” invites highly classified scientists to review the drafts of the articles sent to them, so the reviews are really deep and qualified. On the other hand, this journal is appreciated in the scientific community.

In our case, we received very detailed reviews, we even went through several stages. We faced a number of challenges in answering reviewers’ questions. I even had to supplement the article with new measurements and simulations. This whole process was not easy – but it was nice to discuss with top-level scientists, to receive a positive assessment of our results, and I think their advice made our work more important and attractive to the reader,” says physicist J. Anulytė.

Towards more sensitive and reliable sensors

These are far from the only experiments that the FTMC Plasmonic and Nanophotonics Laboratory is engaged in. Fundamental and applied scientific research related to the control of light in infinitesimal – nanometer – structures is carried out here; aims to apply such light control to nanolasers, optical biosensors, nanowaveguides and various optical circuits (systems in which information is transmitted in the form of light).

“In order to manipulate light in nanometer dimensions, it must be focused to dimensions smaller than the diffraction limit (the diffraction limit is a physical limitation that determines the smallest size of objects that can be seen clearly using optical instruments such as microscopes or telescopes).

This is achieved by laser light excitation of various resonant phenomena of surface electromagnetic waves, such as surface plasmon resonance, Bloch surface waves or the like. This is done in both metallic and dielectric (electrically non-conductive) nanostructures.

Subsequently, these optical properties are used in various applications, such as plasmonic biosensors. The measurement methodology developed in our laboratory is total internal reflection ellipsometry, which is used to determine the kinetic rates of antibody-antigen interaction (this is the basis of immunosensors), which our fellow biochemists later use to create various pharmaceutical preparations”, says the head of the laboratory, prof. Dr. Zigma Balevičius.

His team aims to make these sensors as sensitive and reliable as possible, using various physics phenomena such as the strong interaction.