Understanding an exotic state of affairs

Physicists at the Adolphe Merkle Institute are investigating new methods to trap light, in order to control it precisely. Their findings may be useful to develop nano-lasers, sensors, and integrated circuits.

The researchers from the Soft Matter Physics group are focusing on bound states in the continuum (BICs), which are waves whose existence was first proposed in the context of quantum mechanics in the 1920s. However, only more recently physicists have begun to investigate these waves experimentally and to consider their potential applications more closely. In simple terms, BICs are waves that are completely trapped or confined in a medium. The partial trapping of waves is a common phenomenon that naturally leads to resonances.

For example, resonant sound waves bounce back and forth within musical instruments until they eventually find an exit. The same effect is used in optical fibers or lasers, in which light waves are reflected many times before they leave the device. BICs take such confinement to another level. In a perfect BIC structure, the waves could never escape, bouncing back and forth forever. This situation is in a way similar to an electron (which can be thought of as a quantum-mechanical wave) bound to a molecule. The bound electron, however, lacks the necessary energy to take flight, while BICs remain perfectly localized even though they have sufficient energy to “break out”.

This behavior defies conventional wisdom and makes BICs extremely interesting from both a fundamental and an application perspective. BICs have been studied in a wide variety of systems that are able to confine waves for very long periods. These include photonic crystals, optical fibers, quantum dots, and graphene. What is important is just how long, even if the trapping time is measured in nanoseconds! Nevertheless, this means that the wave bounces back and forth inside the material a million times before escaping, leading to a large number of potential applications, including coherent light generation, sensing, filtering, and integrated circuits.

The AMI researchers are investigating BICs in photonic structures based on metallic metamaterials. These materials are engineered to display unusual optical properties that are not found in nature. Theoretical evidence suggests that these innovative BICs can trap light more efficiently than previous designs. The metamaterials envisaged by the AMI researchers consist of two intertwined, spatially separated metallic networks, such that each network acts like an electromagnetic plasma – something like a soup of charged particles. Charge oscillations in the two plasmas lead to waves with vanishing macroscopic electric fields and perfect confinement. According to Plasmonic Networӿ group leader Dr. Matthias Saba, these double-net BICs therefore eliminate the need for an engineered solution and show an improved stability against experimental imperfections or other natural fluctuations when compared to existing designs. “This is exactly what you would want for example when designing a lasing cavity for a nano-laser,” Saba explains. “We generally want to use BICs to boost and study quantum emission inside metamaterials.” This involves placing fluorescent molecules or quantum dots into the metamaterial. According to Saba, exciting fluorescent species normally leads to the emission of incoherent light. If created within a BIC metamaterial, however, the waves would be highly uniform. They would all have the same frequency, and move in a preferred direction, with a particular polarization profile. Polarization coherence is important in that polarization itself contains information that is used for communication, molecule sensing, or even astrophysics. The scientists have so far developed a theory for microwave frequencies, which they are preparing to test in detail with an experimental setup. “For other applications, such as nano-lasers, we would have to progress to much higher frequencies,” explains Saba. “The challenge now is to demonstrate that our double-net BICs still exist at these higher frequencies, and fabricate the associated metamaterials. We know that our theory can be employed for terahertz applications and have preliminary results, which indicate that infrared frequencies used for telecommunications are feasible.”

Reference Wang, W.; Günzler, A.; Wilts, B. D.; Saba, M. Unconventional Bound States in the Continuum from Metamaterial Induced Electron-Acoustic Plasma Waves. arXiv 2022, 2112.13711