ECE researchers report ultrastrong coupling of light and phonons within nanoscale coaxial cable

Opens up possibilities for developing new quantum-based devices

A multi-institutional team of scientists led by ECE’s Professor Sang-Hyun Oh have reported vibrational ultrastrong coupling of light and matter within nanocavities at mid-infrared frequencies (MIR). The finding is particularly significant because it throws open a new frontier in cavity quantum electrodynamics (QED) that could enable quantum-based devices and even modify chemical reactions. The results are published in their paper titled, “Ultrastrong plasmon–phonon coupling via epsilon-near-zero nanocavities,” in Nature Photonics, a premier journal in the field of photonics. The report is the result of scientific collaboration among researchers at the University of Minnesota and other domestic and foreign institutions (names of collaborating institutions and authors included at the end).

Commenting on the significance of the study, corresponding author and University of Minnesota professor Sang-Hyun Oh says, “Researchers have studied coupling, but with this process we are pushing the frontiers of ultrastrong coupling. We are discovering new quantum states where matter and light can have very different properties and unusual things start to happen. This ultrastrong coupling of light and atomic vibrations opens up all kinds of possibilities for developing new quantum-based devices or modifying chemical reactions.”

A PRIMER ON LIGHT MATTER INTERACTIONS

The interaction between light and matter is central to life on earth. It allows for such fundamental phenomena as plants converting sunlight into energy, and allows us to see objects around us. At a more complex and less obvious level, infrared light, with wavelengths longer than that of visible light, 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, a phenomenon harnessed for 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. Chemists can use such infrared absorption spectrum as a “fingerprint” to identify different chemicals.

These and other applications can be improved by manipulating the strength with which infrared light interacts with atomic vibrations in materials. This 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 metallic structures, or “nanocavities,” are used to confine the light on ultra-small length scales.

Until recently, light-matter interaction in the field of cavity QED had been confined to weak coupling and strong coupling. The strong coupling phenomenon can enable quantum information processing, and energy states with properties different from the original matter which can in turn change the chemical characteristics of the participant matter.

When nanocavities are used to trap light, the interactions can be strong enough that the quantum-mechanical nature of the light and the vibrations comes into play. Ultrastrong coupling (USC), a recent entrant to light-matter interactions, is a regime in which far more exotic phenomena can occur because of the strength of the light-matter coupling. In this mode, the absorbed energy is transferred back and forth between the light (photons) in the nanocavities and the atomic vibrations (phonons) in the material at a rate fast enough that the photon and phonon can no longer be distinguished. These strongly coupled modes make up new quantum-mechanical objects known as polaritons. The stronger the interaction, the more exotic 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 make chemical reactions proceed in ways that are otherwise impossible. This state has the potential to enable novel ultrafast optoelectronic devices, modify chemical reactions, and one can even extract light from the modified ground state.

Commenting on the unusual idea of creating something out of a vacuum, co-corresponding author of the paper, Professor Luis Martin-Moreno at the Instituto de Nanociencia y Materiales de Aragón (INMA) in Spain says, “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.”

Professor Oh adds, “Normally we think of vacuum as basically nothing, but it turns out that this vacuum fluctuation always exists. This is an important step to actually harness quantum vacuum fluctuation to do something useful.”

USC has previously been demonstrated by means such as photochromic molecules, superconducting circuit QED systems, two-dimensional electron gases, and others. Strong coupling at mid-infrared frequencies have also been demonstrated in various systems showing their use for applications such as thermal emission and signature control, and modified heat transfer. However, the challenge has been to attain ultrastrong coupling at MIR frequencies, particularly in solid-state systems. Previous demonstrations have involved extended microcavity structures which limit the possibilities for novel nonlinear effects.

SIGNIFICANCE OF THE STUDY

Commenting on the significance of the study, corresponding author and University of Minnesota professor Sang-Hyun Oh says, “Researchers have studied coupling, but with this process we are pushing the frontiers of ultrastrong coupling. We are discovering new quantum states where matter and light can have very different properties and unusual things start to happen. This ultrastrong coupling of light and atomic vibrations opens up all kinds of possibilities for developing new quantum-based devices or modifying chemical reactions.”

Emphasizing the profound implications that USC can have for chemistry, professor Joshua Caldwell of Vanderbilt University says: “A specific chemical may typically react with another through the thermodynamically favorable pathway. Thermodynamics drives that reaction. However, by strongly coupling a photonic cavity to the vibration of a competing bond in the initial molecule, the process could induce that bond to break first, giving it the preferential position to react and thus changing the resultant product chemical.”

In other words, the same two reactants would produce different products.

An electron micrograph of annular holes in a gold film with a 10 nm gap and 200 nm diameter. The holes filled with silicon dioxide enable ultrastrong coupling between light and atomic vibrations. Image credit: Oh Group, University of Minnesota

The current study has broken through barriers that have constrained previous demonstrations of light-matter interaction. The team demonstrated vibrational USC in nanocavities at the technologically important MIR frequencies, which is a significant breakthrough for multiple reasons. The size of the cavity is a critical first: at 2 nm, it is a nanoscale version of a coaxial cable (approximately 25,000 times thinner than a strand of human hair), which is filled with silicon dioxide. This drastically reduces the size of the system.

“The tiny coaxial holes that enabled our experiments were manufactured using a new technique called atomic layer lithography. This method is compatible with standard processes used in the microelectronics industry and makes it possible to produce millions of nanocavities simultaneously, with all of them exhibiting this ultrastrong photon-vibration coupling. We are excited about finding new ways to contribute to the field of cavity QED using our technique,” says lead author of the paper, Daehan Yoo.

To sum it up, the study demonstrates significant improvement on 3 fronts: performance, efficiency, and scalability. These factors are critical to open and/or extend new pathways for research and development in several areas: quantum nonlinear optics, multiphoton effects, and single-photon excitation are some of them. Although the study is currently at the level of fundamental research, it holds the potential for the development of novel applications in the future such as new light sources, optoelectronic devices, and setting up chemical reactions in ways that were previously not possible.

Prof. Sang-Hyun Oh is a Distinguished McKnight University Professor, and holds the Sanford P. Bordeau Chair in Electrical and Computer Engineering. Learn more about research in his Laboratory of Nanostructures and Biosensing.

In addition to Oh and Martin-Moreno, the research team included Daehan Yoo, In-Ho Lee, and Daniel A. Mohr, University of Minnesota; Fernando de León-Pérez, Centro Universitario de la Defensa de Zaragoza and Instituto de Nanociencia y Materiales de Aragón (INMA) in Spain; Matthew Pelton, University of Maryland at Baltimore County; Markus B. Raschke, University of Colorado Boulder; and Joshua D. Caldwell, Vanderbilt University.

Daehan Yoo earned his PhD from the University of Minnesota in 2016 and is a postdoctoral associate in Prof. Sang-Hyun Oh’s laboratory.

The research was funded by the U.S. National Science Foundation and the Samsung Global Research Outreach Program. Additional support was provided by the Spanish Ministry of Economy and Competitivity, Aragón Government Project, U.S. Office of Naval Research, and the Sanford P. Bordeau Chair in Electrical Engineering at the University of Minnesota.

*The cover illustration shows annular holes in a gold film with a 10 nm gap and 200 nm diameter. The holes filled with silicon dioxide enable ultrastrong coupling between light and atomic vibrations. Image credit: Oh Group, University of Minnesota