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, however, 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, my colleagues and I at IBM Research reported that 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 we produced in our lab. We found that when we 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 we precisely controlled the filtration process, the nanotubes organized into beautiful two-dimensional hexagonal crystals (Fig. 1).
After fabricating these carbon-nanotube films, we then etched the crystallized nanotube films into nanoribbons (Fig. 2). 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.” We 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.
There are several reasons why light-matter coupling in crystallized carbon-nanotube films is so strong:
- Excitons in carbon nanotubes are particularly strong optical emitters.
- The small size of the nanotubes concentrates the optical field and increases its propensity to interact with matter.
- The crystallization of the nanotube films leads to a very high emitter density, thereby increasing the overall rate at which excitons in the system can absorb or emit light.
- The nanotubes’ dual role as emitters and cavities leads to an optimal spatial overlap between the emitters and the optical field.
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.
What comes after ultrastrong coupling? When a system’s light-matter interaction frequencies are truly stronger than any other frequency scale of the system, the deep-strong coupling regime is reached. Experimentally, our nanotube films ought to reach this regime if they are modestly improved. Another tactic would be to apply the concept of intrinsic strong coupling – a single material functioning as both cavity and emitter – to other systems, like two-dimensional materials.
The generation of crystallized carbon-nanotube films is a milestone in the larger endeavor of assembling nanostructures into macroscopic functional materials. Technologically, it could drive the next generation of tunable infrared devices, including nanoscale light sources, multispectral detectors, and wavefront-shaping chips. Crystallized nanotube films could thus serve as a bridge between fundamental concepts in cavity QED and practical devices.