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

 

This technique allows you to "tag" specific molecules or cells, detecting very small amounts of substances - making fluorescence very important in nanotechnology, medicine, biology and other scientific fields.

 

Of course, as everywhere, there are many challenges - and physicists from the Plasmonics and Nanophotonics Laboratory of the FTMC Department of Laser Technologies are trying to solve them, having recently published a paper in the prestigious scientific journal Nanophotonics. The authors are FTMC scientists Justina Anulytė, Dr. Vytautas Žičkus, Dr. Ernesta Bužavaitė-Vertelienė and Prof. Dr. Zigmas Balevičius, together with Prof. Danielle Faccio, Head of the Extreme Light Group at the University of Glasgow.

 

The title of the article is not one of those that can be understood at a glance: "Strongly coupled plasmon-exciton polaritons for photobleaching suppression". Let's try to figure out what this means and how it matters.

 

 

 

 

Lithuanian study investigates how the strong coupling regimebetween surface plasmon polaritons (SPPs) and excitons affects the fluorescence lifetime of particles and the photobleaching effects. What is this?

 

Let's start with the strong coupling regime. In simple terms, this is a situation where two systems, such as light and matter (or two different quasiparticles), start to interact with each other very strongly. When this happens, they exchange energy with each other so rapidly that they start to behave practically as a single connected entity rather than as two separate systems. Something like young lovers who can't get away from each other.

 

Let's go further.

 

"SPPs are surface electromagnetic waves on thin metal layers. To understand this, we could imagine circles on the surface of water - a mechanical surface wave between water and air. In the case of the electromagnetic waves we are studying, SPPs are like 'circles' between metal and air," explains lead author of the paper Justina Anulytė, a PhD student at the FTMC Department of Laser Technologies.

 

And what is an exciton? It's a kind of tiny "pair" of elements, made up of a photon, a particle of light, and a material to be studied in the laboratory. Thanks to the strong coupling regime we recently discussed, these two particles are bound together in a certain way - and stay close together.

 

So, an SPP is a special wave between a metal and air, an exciton is a tiny "compound" where the photon and the material under investigation have merged.

 

 

 

What happens next? "SPP and exciton, the two quasiparticles, merge into a plasmon-exciton polariton state. This means that in this new quasi-state, the properties of part plasmon (SPP) and part exciton are manifested - and we can't separate them, they both act together. And in a very short time (femto or picoseconds), there is a lossless exchange of mutual energy. For such a 'duo' to exist, special conditions must be achieved in which a strong coupling regime is present," says J. Anulytė.

 

Exiton can emit light (fluoresce) if excited by a laser beam or just plain white light. But this is often where trouble happens: when exciton enters an energetic state with oxygen excited by the environment, so-called photobleaching occurs: the intensity of the light decreases over time. Why? Because photobleaching causes the laser-excited particles to "switch off" one by one, i.e. to stop glowing.

 

Not tired yet? We've come to the most important point: excitons need their fluorescence for scientific experiments - so it's important to keep them glowing as long as possible. This is where our scientists have perfected the "third" plasmon-exciton polariton state: when excitons are combined with SPPs (surface electromagnetic waves, let's remember) in the strong coupling regime, the desired fluorescence intensity of the particles is maintained for several to even tens of times longer.

 

To summarise very simply: a team from the FTMC and the University of Glasgow has developed a method that allows the fluorescence of the observed particles to be maintained for longer using a special dye, a laser and a strong coupling regime.

 

 

 

 

Now let the lead author of the study, Dr. Justina Anulytė, speak:

 

"For our experiment, we formed a structure consisting of nanometre (millionth of a millimetre) layers of silver and gold, as well as a layer of glow-inducing Rhodamine 6G dye.

 

Our structure shows a clear shift of the plasmon resonance and R6G absorption lines as the angle of incidence of light is changed, which in turn indicates a strong coupling between the plasmon and the exciton, with a measured interaction strength of around 90 millielectronvolts.

 

Fluorescence lifetime imaging microscopy (FLIM) has shown that using polaritonic nanostructures where plasmons and excitons interact strongly, the fluorescence intensity remains about 25% stronger and the organic molecules in the Rhodamine dye are about six times less photobleached."

 

According to the young scientist, the findings demonstrate the essential role of the strong coupling regime between light and organic matter in reducing the photobleaching effect and stabilising the fluorescence intensity (the glow of particles).

After all this, you might naturally ask: what's in it for you? The answer is very intriguing: this approach offers promising ways to develop and improve quantum devices - such as quantum biosensors, quantum nano-lasers - or to develop quantum information processing, which would take our computing and other technologies to a whole new level.

 

 

 

 

The importance of the work done by the Lithuanian and Italian researchers is also demonstrated by the fact that they have managed to publish their findings in the international scientific journal Nanophotonics.

 

The journal's editors focus on the interaction of photons (particles of light) with nanostructures such as carbon nanotubes, metal nanoparticles, nanocrystals, photonic crystals, biological tissues and DNA. A large number of research results have been published on plasmonics, nano-lasers, light interactions with matter, and light control at the nanometric level.

 

The journal's citation index is 6.5. What does this mean? According to statistics, only 5% of scientific journals worldwide have a citation score of 6 or higher. And that's saying a lot.

 

"Publishing your scientific results in this journal is not easy. Nanophotonics invites high-ranking scientists to review the manuscripts they send in, so the reviews are really deep and skilled. On the other hand, this is exactly what the scientific community values this journal for.

 

In our case, we received very detailed reviews and even went through several stages. We faced a lot of challenges in answering the reviewers' questions. We even had to add new measurements to the paper. The whole process was not easy - but it was a pleasure to discuss with top scientists, to have our results positively evaluated and, I think, to have their advice make our work more relevant and attractive to the reader," says physicist J. Anulytė.

 

 

 

 

These are far from the only experiments carried out by the FTMC Plasmonics and Nanophotonics Laboratory. It carries out fundamental and applied research on the control of light in infinitesimal nanoscale structures, with the aim of applying such light control to nano-lasers, optical biosensors, nano-conductors and a variety of optical circuits (systems in which information is transmitted in the form of light).

 

"To steer light at nanometre scales, it needs to be focused to dimensions below the diffraction limit (which is a physical constraint that defines the smallest size objects can be clearly seen using optical instruments such as microscopes or telescopes).

 

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

 

These optical properties are subsequently used in a variety of applications such as plasmonic biosensors. Our laboratory has developed a measurement technique - total internal reflection ellipsometry - which is used to determine the kinetic rates of antibody-antigen interactions (the basis of immunosensors), which is then used by our biochemist colleagues to develop a variety of pharmaceutical formulations," says Prof. Dr. Zigmas Balevičius, Head of the laboratory.

 

His team aims to make these sensors as reliable as possible, using a variety of physical phenomena.