Researchers have uncovered a new way of understanding how light interacts with atomically thin semiconductors.
They found that the process creates unique excitonic complex particles, multiple electrons, and holes strongly bound together, which enables the particles to possess a new quantum degree of freedom, called "valley spin".
The "valley spin" is similar to the spin of electrons, which has been extensively used in information storage such as hard drives and is also a promising candidate for quantum computing.
In a paper published in Nature Communications, Sufei Shi, assistant professor of chemical and biological engineering at Rensselaer, said results of this research could lead to novel applications in electronic and optoelectronic devices, such as solar energy harvesting, new types of lasers, and quantum sensing.
The research focuses on low dimensional quantum materials and their quantum effects, with a particular interest in materials with strong light-matter interactions. These materials include graphene, transitional metal dichacogenides (TMDs), such as tungsten diselenide (WSe2), and topological insulators.
It's these TMDs which represent a new class of atomically thin semiconductors with superior optical and optoelectronic properties. Optical excitation on the two-dimensional single-layer TMDs will generate a strongly bound electron-hole pair called an exciton, instead of freely moving electrons and holes as in traditional bulk semiconductors.
This is due to the giant binding energy in monolayer TMDs, which is orders of magnitude larger than that of conventional semiconductors. As a result, the exciton can survive at room temperature and can thus be used for application of excitonic devices.
As the density of the exciton increases, more electrons and holes pair together, forming four-particle and even five-particle excitonic complexes, Shi said.
"Now, for the first time, we have revealed the true biexciton state, a unique four-particle complex responding to light," explained Shi. "We also revealed the nature of the charged biexciton, a five-particle complex."
Shi's team has developed a way to build an extremely clean sample to reveal this unique light-matter interaction. The device was built by stacking multiple atomically thin materials together, including graphene, boron nitride, and WSe2, through van der Waals (vdW) interaction, representing the state-of-the-art fabrication technique of two-dimensional materials.
The results of this research could potentially lead to robust many-particle optical physics, and illustrate possible novel applications based on 2D semiconductors, Shi added.
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