Exciting breakthrough in 2D lasers
An important step towards next-generation ultra-compact photonic and optoelectronic devices has been taken with the realization of a two-dimensional excitonic laser. Scientists with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) embedded a monolayer of tungsten disulfide into a special microdisk resonator to achieve bright excitonic lasing at visible light wavelengths.
Among the most talked about class of materials in the world of nanotechnology today are two-dimensional (2D) transition metal dichalcogenides (TMDCs). These 2D semiconductors offer superior energy efficiency and conduct electrons much faster than silicon. Furthermore, unlike graphene, the other highly touted 2D semiconductor, TMDCs have natural bandgaps that allow their electrical conductance to be switched “on and off,” making them more device-ready than graphene.
Tungsten disulfide 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 realized in this material.
TMDCs have shown exceptionally strong light-matter interactions that result in extraordinary excitonic properties. 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—photoex
For the excitonic laser, the researchers dropped the metal coating and designed a microdisk resonator that supports a dielectric whispering gallery mode rather than a plasmonic mode, and gives a high Q factor with low power consumption. 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.