Two-photon absorption in bilayer graphene.

Exciton-Polaritonics with Advanced Materials

Lasers and optical nonlinearities are highly desired for many technologies, such as information technology, medical imaging technology, and laser radar technology, named as a few examples. In these technologies, compact, fast, and efficient optical materials for lasers or nonlinear optical devices find an essential role. However, conventional semiconductor lasers require the pumping power for the lasing threshold has reached a bottleneck: a fundamental lower bound governed by the carrier density required to reach population inversion, or the transparency condition. This limitation can be overcome in a new type of laser, a polariton laser, which operates under a different mechanism, as called exciton-polaritonics. In this mechanism, an exciton in a material interacts with a photon mode in an microcavity, resulting in much lower lasing thresholds and/or greater optical nonlinearities. These prospects have propelled intense research effort on exciton-polaritons in a variety of materials including inorganic widegap semiconductors and organic crystals.

Here, we will first focus on metal-organic frameworks (MOFs), a newly emergent material platform, for their potential applications in polariton laser technology with an extreme-low lasing threshold. MOFs are a type of materials made up of a combination of metal ions coordinated to organic linkers. The linkers bind the whole framework together to keep it stable, and the metal ions are arranged in shapes to form one-, two-, or three-dimensional structures. Often, these materials have long pores and channels running through them, similar to porous pieces of rock, and small molecules can pass through these channels and interact with specific sites.

Prof. Ji Wei and his research team have been taking advantage of the excellent chemical binding capabilities to bind a dye molecule, Rhodamine B, into the MOF structure. These MOFs render the exciton-polariton properties in a two-dimensional crystalline platform. One of the key advantages is that these MOF crystals can be synthesized by an incredibly straightforward and cost-effective method, as compared to traditional approaches, which often involve trying to coat layers of the reflective material and chemical substrate to build up the structure that is required for exciton-polariton devices. In addition, the researchers found an extremely low lasing threshold with this novel exciton-polariton laser.