Mystery of Liquid Light Interactions

Experiment to analyze the mechanisms of blueshifts in organic polariton condensates. (Source: Skoltech)

A team of scientists from the Hybrid Photonics Labora­tory at the Skolkovo Institute of Science and Technology (Skoltech) and the Uni­versity of Sheffield made a breakthrough in under­standing nonlinear physics of the strong inter­action of organic molecules with light. Principles of strong light matter inter­action open new horizons of ultra-fast and low energy all-optical data processing.

Perhaps everybody knows organic matter as the essential building blocks for living nature. Indeed, the inter­actions between organic molecules and light is a pivotal process for photo­synthesis, light-induced biochemical regulation and many other mechanisms in nature making our life on Earth feasible. Beyond that side, there are dozens of appli­cations for a variety of light-matter interactions in organic systems. Nowadays, organic materials represent a wide class of materials actively used in light-emitting devices industry, in flexible electronics and solar cells fabri­cation, as photosensitive sensors and bio labels of cancer, etc. The rapidly growing organic LED market is a good example showing the great commercial potential of organic materials in real-life tech­nologies.

Skoltech’s Hybrid Photonics Laboratory, led by Pavlos Lagoudakis, focuses on developing a new paradigm of opto­electronics based on strong inter­action between organic materials and light. The key difference from conven­tional approaches is that light in such systems is strongly correlated with collective electronic exci­tations on a molecule, which gives rise to polaritons. These light-matter entangled particles inherit ultra-fast propa­gation of light and electronic pro­perties of materials resulting in a very exotic hybrid form of light and matter called liquid light.

“Does this make a world of difference? Sure it does, because the strong light-matter coupling can slow down the photo­degradation of molecules, extending their lifetime, change the course of photo­chemical reactions, and provide photons with an ability to interact with each other; the latter feature allows us to develop efficient optical signal pro­cessing devices,” says Lagoudakis. Currently, fiber optic networks handle huge amounts of data, but if one wants to process optical signals then light has to be converted into electrical signals and back. In contrast, strong-coupling principles offer unique oppor­tunities for all-optical data processing tech­nologies with record speeds and better energy conversion effi­ciencies. The past decade has witnessed remarkable achievements in the field of polari­tonics, running the gamut from the first organic polariton laser to room-temperature super­fluidityand the invention of the first organic polariton transistor.

However, despite remarkable progress, the mechanisms of polariton inter­actions in organic systems have remained poorly understood, fueling debates in the scientific community. The mystery of polariton inter­actions has finally been resolved: Skoltech’s research gives a decisive answer to this controversial question. The scientists carried out an in-depth experimental study that revealed a clear origin of nonlinear phenomena related to polariton conden­sates – the state consisting of hundreds and even thousands of polaritons sharing the same properties.

“Our experiments indicate an abrupt shift in spectral properties of polariton conden­sates when established, which always drives the frequency of polaritons toward higher values. We find this specific to nonlinear processes occurring in the system. For example, as we can measure the temperature of metal through its change in color as it is heated, similarly, we extract non­linearity of organics by means of in-depth analysis of the frequency shifts,” explains junior research scientist at the Hybrid Photonics Labs, Timur Yagafarov.

The compre­hensive experimental study, accompanied by a thorough data analysis, allowed for the ascer­tainment of important dependencies of the polariton nonlinear properties on the key parameters of the inter­­action between organic molecules and light. The scientists were the first to discover the strong impact of the energy transfer between neigh­boring molecules on the nonlinear properties of organic polaritons and now understand underlying mechanisms driving polaritons in organics. With the proposed theory, one can find the experi­mental parameters required to couple several polariton condensates into a single circuit and build a polariton all-optical signal processor.

From a fundamental point of view, the new knowledge may help to explain the phenomenon of polariton superfluidity in organic matter. “These findings are of high interest not only to our research area but can be helpful in other fields as well. I believe the non­linearity mechanisms discovered are quite general among the organic materials, therefore it might prove to be universal for strongly-coupled organic systems,” comments senior research scientist at the Hybrid Photonics Laboratory, Anton Zase­datelev. (Source: Skoltech)

Reference: T. Yagafarov et al.: Mechanisms of blueshifts in organic polariton condensates, Commun. Phys. 3, 18 (2020); DOI: 10.1038/s42005-019-0278-6

Link: Center of Photonics and Quantum Materials, Skolkovo Institute of Science and Technology, Moscow, Russia

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