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Light-Matter Coupling Offers Implications for Quantum Optics

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An international research team led by the University of Minnesota Twin Cities has produced a quantum state that is part light and part matter. The research has implications for the next generation of quantum-based optical and electronic devices, and may also contribute to increasing the efficiency of nanoscale chemical reactions.

The researchers achieved ultrastrong coupling between infrared light and matter by trapping light in tiny annular holes in a thin layer of gold. The holes were as small as 2 nm, or 25,000× smaller than the width of a human hair.
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. Courtesy of the Oh Group, University of Minnesota.
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. Courtesy of the Oh Group, University of Minnesota.

The nanocavities, the researchers said, can be thought of as greatly scaled-down versions of coaxial cables, and were filled with silicon dioxide, similar to the blend of glass used in common windows. Fabrication methods based on those used in the computer chip industry made it possible to produce millions of these cavities simultaneously, all of them exhibiting the ultrastrong photon-vibration coupling.

“Others have studied strong coupling of light and matter, but with this new process to engineer nanometer-sized versions 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 San-Hyun Oh, professor of electrical and computer engineering and the senior author of the study. “This ultrastrong coupling of light and atomic vibrations opens up all kinds of possibilities for developing new quantum-based devices or modifying chemical reactions.”

Infrared light interacts with the vibration of atoms in materials. For example, when an object is heated, the atoms composing the object vibrate faster, giving off more infrared radiation, which enables thermal imaging and night-vision cameras.

The wavelengths of infrared radiation absorbed by materials depends on the particular atoms that make up the materials and how they are arranged, which enables chemists to use infrared absorption to identify different chemicals.

Increasing how strongly infrared light interacts with atomic vibrations in materials can improve these applications, among others. This can be accomplished by trapping the light into a small volume containing the materials. Trapping light can be as simple as making it reflect back and forth between a pair of mirrors, but stronger interactions can be realized if nanometer-scale metallic structures, or “nanocavities,” are used to confine the light on ultrasmall length scales.

Under these conditions, interactions can be strong enough that the quantum-mechanical nature of the light and the vibrations comes into play. In that situation, the absorbed energy is transferred back and forth between the light in the nanocavities and the atomic vibrations in the material at a rate fast enough that the light photon and matter phonon can no longer be distinguished. Those strongly coupled modes result in new quantum mechanical objects that are part light and part vibration at the same time, known as polaritons.

The stronger the interaction, the stranger the quantum mechanical effects that can occur. If the interaction becomes strong enough, it could be possible to create photons out of the vacuum or to make chemical reactions proceed in ways that are otherwise impossible.

In the current coupling regime, the vacuum is not empty, instead containing photons with wavelengths determined by the molecular vibrations. The photons are confined, and a minute number of molecules “shares” them, said Luis Martin Moreno, a professor at the Instituto de Nanociencia y Materiales de Aragón (INMA) in Spain, another author of the paper.

“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 so-called zero energy fluctuation to do something useful.”

The research was published in Nature Photonics (www.doi.org/10.1038/s41566-020-00731-5).

Photonics Spectra
Feb 2021
GLOSSARY
quantum optics
The area of optics in which quantum theory is used to describe light in discrete units or "quanta" of energy known as photons. First observed by Albert Einstein's photoelectric effect, this particle description of light is the foundation for describing the transfer of energy (i.e. absorption and emission) in light matter interaction.
Research & Technologyquantum photonicsquantum opticslight-matter couplinglight-matter interactionlight-matter interactionsinfrared lightinfraredIRgold filmUniversity of MinnesotaUniversity of Minnesota-Twin CitiesINMAInstituto de Nanociencia y Materiales de Aragónatomic vibrationsTech Pulse

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