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John O | January 2019

Researchers develop method to control excitons and use them in energy-efficient electronics


By Josh Perry, Editor
jperry@coolingzone.com

 

Scientists from the École Polytechnique Fédérale de Lausanne (Switzerland) were the first to develop a method for controlling excitons, created when electrons absorb light and move to a higher energy level leaving a positively-charged hole in the previous energy band, and they have now demonstrated a way to control the properties of excitons and change the polarization of the light that they generate.

 


After developing a method to control exciton flows at room temperature, EPFL scientists have discovered new properties of these quasiparticles that can lead to more energy-efficient electronic devices. (EPFL)

 

According to a report from the EPFL, this could lead to next-generation electronics with more efficient transistors that run cooler.  

 

Excitons only occur in semiconducting or insulating materials and are most commonly accessed in 2-D materials. EPFL scientists used a combination of tungsten diselenide (WSe2) and molybdenum diselenide (MoSe2) in their experiments.

 

“By using a laser to generate light beams with circular polarization, and slightly shifting the positions of the two 2D materials so as to create a moiré pattern, they were able to use excitons to change and regulate the polarization, wavelength and intensity of light,” the report explained.

 

To achieve this control, scientists manipulated the excitons’ valley, which is the energy extremes of the electron and the hole. It is these valleys that scientists hope to link to code and process information at the nanoscale (valleytronics).

 

The research was recently published in Nature Photonics. The abstract stated:

 

“The long-lived interlayer excitons in van der Waals heterostructures based on transition-metal dichalcogenides, together with unique spin–valley physics, make them promising for next-generation photonic and valleytronic devices. Although the emission characteristics of interlayer excitons have been studied, efficient manipulation of their valley states, a necessary requirement for information encoding, is still lacking.

 

“Here, we demonstrate comprehensive electrical control of interlayer excitons in a MoSe2/WSe2 heterostructure. Encapsulation of our well-aligned stack with hexagonal boron nitride (h-BN) allows us to resolve two separate narrow interlayer transitions with opposite helicities under circularly polarized excitation, either preserving or reversing the polarization of incoming light.

 

“By electrically controlling their relative intensities, we realize a polarization switch with tunable emission intensity and wavelength. Finally, we observe large g-factors of these two transitions on application of an external magnetic field. These results are interpreted within the picture of moiré-induced brightening of forbidden optical transitions.

 

“The ability to control the polarization of interlayer excitons is a step towards the manipulation of the valley degree of freedom in realistic device applications.”

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