Unique Process for Producing Light-Matter Mixture

Annular holes in a thin gold film filled with silicon dioxide enable ultrastrong coupling between light and atomic vibrations. This structure provides opportunities to probe molecules interacting with quantum vacuum fluctuations and develop novel optoelectronic devices. (Source: Oh et al., U. Minnesota)

An international team of researchers led by the Univer­sity of Minnesota Twin Cities has developed a unique process for producing a quantum state that is part light and part matter. The discovery provides funda­mental new insights for more effi­ciently developing the next generation of quantum-based optical and electronic devices. The research could also have an impact on increasing effi­ciency of nanoscale chemical reactions. In a new study, the researchers developed a unique process in which they achieved ultra­strong coupling between infrared light and matter by trapping light in tiny, annular holes in a thin layer of gold. These holes were as small as two nanometers.

These nano­cavities, similar to a highly scaled-down version of the coaxial cables that are used to send electrical signals were filled with silicon dioxide, which is essentially the same as window glass. Unique fabri­cation methods, based on techniques developed in the computer-chip industry, make it possible to produce millions of these cavities simul­taneously, with all of them simultaneously exhibiting this ultra­strong photon-vibration coupling. “Others have studied strong coupling of light and matter, but with this new process to engineer nanometer-sized version of coaxial cables, we are pushing the frontiers of ultra­strong coupling, which means we are disco­vering new quantum states where matter and light can have very different properties and unusual things start to happen,” said Sang-Hyun Oh, a University of Minnesota professor of electrical and computer engi­neering. “This ultrastrong coupling of light and atomic vibrations opens up all kinds of possi­bilities for developing new quantum-based devices or modifying chemical reactions.”

The inter­action between light and matter is central to life on earth – it allows plants to convert sunlight into energy and it allows us to see objects around us. Infrared light, with wavelengths much longer than what we can see with our eyes, interacts with the vibrations of atoms in materials. For example, when an object is heated, the atoms that make up the object start vibrating faster, giving off more infrared radiation, enabling thermal-imaging or night-vision cameras. Conversely, the wavelengths of infrared radiation that are absorbed by materials depend on what kinds of atoms make up the materials and how they are arranged, so that chemists can use infrared absorption as a finger­print to identify different chemicals.

These and other appli­cations can be improved by increasing how strongly infrared light interacts with atomic vibrations in materials. This, in turn, can be accomplished by trapping the light into a small volume that contains the materials. Trapping light can be as simple as making it reflect back and forth between a pair of mirrors, but much stronger interactions can be realized if nanometer-scale nanocavities are used to confine the light on ultra-small length scales. When this happens, the interactions can be strong enough that the quantum-mechanical nature of the light and the vibrations comes into play. Under such conditions, the absorbed energy is trans­ferred back and forth between the photons in the nano­cavities and the atomic vibrations in the material at a rate fast enough such that the light photon and matter phonon can no longer be distin­guished. Under such conditions, these strongly coupled modes result in new quantum-mechanical objects that are part light and part vibration at the same time, polaritons.

The stronger the inter­action becomes, the stranger the quantum-mechanical effects that can occur. If the inter­action becomes strong enough, it may be possible to create photons out of the vacuum, or to make chemical reactions proceed in ways that are otherwise impossible. “It is fascinating that, in this coupling regime, vacuum is not empty. Instead, it contains photons with wavelengths determined by the molecular vibrations. Moreover, these photons are extremely confined and are shared by a minute number of molecules,” said Luis Martin-Moreno at the Instituto de Nano­ciencia y Materiales de Aragón (INMA) in Spain. “Normally, we think of vacuum as basically nothing, but it turns out this vacuum fluctuation always exists,” Oh said. “This is an important step to actually harness this zero energy fluctuation to do something useful.” (Source: UMN)

Reference: D. Yoo et al.: Ultrastrong plasmon–phonon coupling via epsilon-near-zero nanocavities, Nat. Phot., online 7 December 2020; DOI: 10.1038/s41566-020-00731-5

Link:Laboratory of Nanostructures and Biosensing, Dept. of Electrical and Computer Engineering, University of Minnesota, Minneapolis, USA

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