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Department of Energy Researchers Demonstrate Their 2D-Laser Breakthrough

22 October 2015

Researchers from the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) have taken a big leap toward next-generation ultra-compact photonic and optoelectronic devices. The researchers embedded a monolayer of tungsten disulfide into a special microdisk resonator to create a 2D excitonic laser that works at visible light wavelengths.

2-D excitonic laser made from a monolayer of tungsten disulfide and a microdisk resonator. Credit image: Xiang Zhang, Berkeley Lab2-D excitonic laser made from a monolayer of tungsten disulfide and a microdisk resonator. Credit image: Xiang Zhang, Berkeley Lab"Our observation of high-quality excitonic lasing from a single molecular layer of tungsten disulfide marks a major step towards two-dimensional on-chip optoelectronics for high-performance optical communication and computing applications," says Xiang Zhang, director of Berkeley Lab's Materials Sciences Division and the leader of this study.

Two-dimensional transition metal dichalcogenides (TMDCs) are a very popular subject in the world of nanotechnology. These 2D semiconductors offer superior energy efficiency and conduct electrons much faster than silicon, and, unlike graphene (another 2D semiconductor), TMDCs have natural bandgaps that allow their electrical conductance to be switched "on and off.”

This characteristic makes them more device-ready than graphene.

Tungsten disulfide in a single molecular layer has been viewed as one of the most promising TMDCs for photonic and optoelectronic applications—until now—since coherent light emission, or lasing, which is considered essential for "on-chip" applications, had not been realized in this material.

"TMDCs have shown exceptionally strong light-matter interactions that result in extraordinary excitonic properties," says Zhang. "These properties arise from the quantum confinement and crystal symmetry effect on the electronic band structure as the material is thinned down to a monolayer. However, for 2D lasing, the design and fabrication of microcavities that provide a high optical mode confinement factor and high quality, or Q, factor is required."

Zhang and the team built upon previous research in which they had developed a "whispering gallery microcavity" for plasmons, electromagnetic waves that roll across the surfaces of metals based on the principle behind whispering galleries where soft-spoken words that could be heard clearly on the opposite side of a domed chamber.

The team was able to adapt this microcavity technology from plasmons to excitons—photoexcited electrons/hole pairs within a single layer of molecules.

In order to create the excitonic laser, the team dropped the metal coating and designed a microdisk resonator capable of supporting a dielectric whispering gallery mode instead of a plasmonic mode. When a monolayer of tungsten disulfide (which served as the gain medium) was sandwiched between the two dielectric layers of the resonator they created the potential for ultra-low-threshold lasing.

There are a plethora of additional applications for this 2D excitonic laser breakthrough aside from photonic and optoelectronic applications. The technology also has potential for valleytronic applications, in which digital information is encoded in the spin and momentum of an electron moving through a crystal lattice as a wave with energy peaks and valleys. Valleytronics is seen as an alternative to spintronics for quantum computing.

"TMDCs such as tungsten disulfide provide unique access to spin and valley degrees of freedom," says co-lead author Wong. "Selective excitation of the carrier population in one set of two distinct valleys can further lead to lasing in the confined valley, paving the way for easily-tunable circularly polarized lasers. The demand for circularly polarized coherent light sources is high, ranging from three-dimensional displays to effective spin sources in spintronics, and information carriers in quantum computation."

The paper detailing the research is titled "Monolayer Excitonic Laser” and is published in the journal Nature Photonics.

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