US team makes monolayer excitonic laser
Scientists with the US Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) have embedded a monolayer of WS2 into a special microdisk resonator to achieve bright excitonic lasing at visible light wavelengths.
"Our observation of high-quality excitonic lasing from a single molecular layer of WS2 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, the results of which are described in the journal Nature Photonics.
2D transition metal dichalcogenides (TMDCs) offer superior energy efficiency and conduct electrons much faster than silicon. And unlike graphene, TMDCs have natural bandgaps that allow their electrical conductance to be switched "˜on and off, making them more device-ready than graphene. WS2 in a single molecular layer is widely regarded as one of the most promising TMDCs for photonic and optoelectronic applications. However, until now, coherent light emission, or lasing, considered essential for on-chip applications, had not been realised in this material.
"TMDCs have shown exceptionally strong light-matter interactions that result in extraordinary excitonic properties," Zhang says. "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."
In a previous study, Zhang and his research group 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 words spoken softly beneath a domed ceiling can be clearly heard on the opposite side of the chamber - this micro-sized metallic cavity for plasmons strengthened and greatly enhanced the Q factor of light emissions. In this new study, Zhang and his group were able to adapt this microcavity technology from plasmons to excitons - photoexcited electrons/hole pairs within a single layer of molecules.
"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 with low power consumption," says co-lead author Ye. "When a monolayer of WS2 - serving as the gain medium - is sandwiched between the two dielectric layers of the resonator, we create the potential for ultralow-threshold lasing."
In addition to its photonic and optoelectronic applications, this 2D excitonic laser 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 WS2 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 polarised lasers. The demand for circularly polarised coherent light sources is high, ranging from three-dimensional displays to effective spin sources in spintronics, and information carriers in quantum computation."
The research was supported by the United States Air Force Office of Scientific Research and by the DOE Office of Science through the Light-Material Interaction in Energy Conversion Energy Frontier Research Center.
'Monolayer excitonic laser' by Yu Ye Zi and Jing Wong et al, Nature Photonics (2015).