A team from the US Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) has created a 2D laser just one molecule thick that promises to make significant advances in ultra-compact photonic components for the likes of quantum computers and the next generation of optoelectronic devices.

"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," said Xiang Zhang, director of Berkeley Lab’s Materials Sciences Division and the leader of this study.

Working on previous research where the Berkeley Lab team created a "whispering gallery microcavity" for plasmons (a "whispering gallery" is usually a circular or hemispherical structure where whispered acoustic communication is possible from any part of the internal circumference to any other; this same effect can be replicated in an analogous way with light), they developed a modified version for excitions.

In other words, the researchers altered the microcavity that they used to amplify plasmons (a quantity of collective electron oscillations), into an amplifying device for excitons (excited electrons bound to the hole produced by their excitation) to create a 2D excitonic laser that operates at visible light wavelengths by implanting a single-molecule thick layer of tungsten disulfide between the two existing disks of the microcavity.

"For our excitonic laser, we dropped the metal coating and designed a microdisk resonator that supports a dielectric whispering gallery mode rather than a plasmonic mode, and gives us a high Q factor (resonant quality) with low power consumption," explained Berkeley Lab researcher Yu Ye. "When a monolayer of tungsten disulfide – serving as the gain medium – is sandwiched between the two dielectric layers of the resonator, we create the potential for ultralow-threshold lasing."

A single molecular layer of tungsten and sulfide is becoming a highly regarded and promising structure for 2D semiconductors for photo-electronic applications. Among a collective range of devices known as two-dimensional transition metal dichalcogenides (TMDCs), these new semiconductors offer greater energy efficiencies and increased electron conduction speed than other semiconductor materials, such as silicon.

And, unlike that other much-vaunted semiconductor material, graphene, TMDCs have superior electrical conductance for rapid "on/off" switching, making single-molecule layer tungsten disulfide a promising TMDC for photonic and optoelectronic applications. Prior to the Berkely Lab research, however, laser capabilities essential for use in such things as "Lab-on-a-chip" devices, had not been realized using this material.

"TMDCs have shown exceptionally strong light-matter interactions that result in extraordinary excitonic properties," said 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."

According to the researchers, 2D excitonic laser technology also has potential for applications in valleytronics. This is where the energy of electrons is plotted relative to their momentum on a graph, the resultant curve shows two valleys and, by manipulating these valleys to have unequal depth, electrons will take a preference to populate one of the two valleys. The two different charge states then created can be used to represent zeroes and ones. In this way, valleytronics is proffered as an alternative to spintronics (which uses the intrinsic spin of the electron and its associated magnetic moment to emulate binary encoding) for quantum computing.

"TMDCs such as tungsten disulfide provide unique access to spin and valley degrees of freedom," said Berkeley Lab researcher Jing 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 details of this research were recently published in the journal Nature Photonics.

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