Light and matter are normally thought of as the two distinct building blocks of the physical world. However, in cavity quantum electrodynamics (QED), light and matter lose their distinct characteristics. When optical emitters (objects like atoms or quantum dots that can emit or absorb light) are placed inside a small optical cavity, the emitters and the cavity’s light field can participate in rapid energy exchanges, known as vacuum Rabi oscillations. These oscillations hybridize light and matter into new states that have both matter-like and light-like attributes.
Cavity QED, a key frontier in physics research, is an important avenue for improving telecommunications technologies and could lead to nanolasers and extremely low-power optical switches. It was also the focus of the 2012 Nobel Prize in Physics.
To date, all existing instances of engineered strong light-matter coupling could be classified as “hybrid systems” in which the optical emitters and cavities are separate objects.
In the December issue of the Proceedings of the National Academy of Sciences USA, IBM Research reported strong light-matter interactions can happen in a single material that both emits and confines light. This material is a crystallized film of carbon nanotubes, a new material that has been produced in the lab.
Researchers found that when they very slowly filtered carbon nanotubes, each a cylindrical nanocrystal of carbon atoms, from an aqueous suspension onto a polycarbonate membrane, the nanotubes would self-assemble into aligned, monolithic films. When the filtration process was controlled precisely, the nanotubes organized into beautiful two-dimensional hexagonal crystals.
After fabricating these carbon-nanotube films, the researchers etched the crystallized nanotube films into nanoribbons. The etched ends of the nanotubes reflect light, creating an optical cavity. Meanwhile, excitons, which are matter excitations in the nanotubes comprising electron-hole pairs, can either be excited by light or annihilate each other and emit light.
The interaction strength of strongly coupled light-matter systems is typically characterized by the frequency of the vacuum Rabi oscillations. In our crystallized nanotube films, this frequency is so high that it approaches that of the infrared light that the excitons emit – the so called “ultrastrong regime.”
Researchers found that the light-matter coupling rate in crystallized nanotube films can be up to 75 percent of the exciton emission frequency, a near record for light-matter interactions in any room temperature system.
Crystallized nanotube films could play an important role in infrared optics. Through simple electrostatic control, the nanotubes’ excitations can now be tuned from being either “matter-like” to “light-like.” Thus, our nanotube films function as strongly tunable infrared antennas. In the future, arrays of such tunable antennas could be a means of routing infrared light in free space without moving parts, for applications like 3D sensing for autonomous vehicles.
Outside of optics, the applicability of crystallized nanotube films could extend to battery anodes, high-ampacity conductors, and microelectromechanical systems.
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