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Exciting electrons in black phosphorus – A route towards ultrafast functions and novel states of matter –

University of Hiroshima
The University of Tokyo
University of Hyogo

Overview:

Black phosphorus is a potential successor to silicon. Thanks to the highly mobile electrons therein as well as the ability to absorb light directly, black phosphorus is a promising platform to realize ultrafast optical functions where silicons cannot meet. Besides, black phosphorus can be downsized to an atomically-thin sheet; and within the sheet, the mobility of the electrons depends on the direction. Theories suggest that these unique characteristics of black phosphorus are favorable in realizing a novel light-induced state of matter, in which excitons generated by light show Bose-Einstein condensation. It thus becomes of paramount importance to reveal how electrons behave in black phosphorus exposed to light.

The joint research group of University of Hiroshima, the University of Tokyo and University of Hyogo investigated a black phosphorous hit by a femtosecond light pulse. The group succeeded in visualizing the electrons that were pumped up and stayed in the excited state for more than 400 ps, or near nanoseconds. The results contrast the cases often seen in metals, wherein light pulse is mostly reflected, and the excited electrons last for ~1 ps at most. The results were brought about by a pump-and-probe apparatus developed in ISSP, the University of Tokyo, by which snapshots of excited electrons can be taken in an energy-and-momentum-resolved manner at the highest energy resolution in the world. The direct observation of the excited electrons in black phosphorus provides a firm basis to pursue the novel functions and states of black phosphorus under illumination.

The research article was published in Scientific Reports, the online scientific journal of the UK Nature Publishing Group on June 13, 2018 (10 a.m. UK time).

Figure 1. (a) Crystal structure of black phosphorus. (b) Snapshots of the electron distribution after black phosphorus was hit by a femtosecond pump pulse.
Figure 1. (a) Crystal structure of black phosphorus. (b) Snapshots of the electron distribution after black phosphorus was hit by a femtosecond pump pulse.

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(Published on: Friday June 22nd, 2018)