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New Pathway to Valleytronics

By Lawrence Berkeley National Laboratory

January 29, 2015

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The optical Stark effect can be used to control valley excitons in MX2 semiconductors, according to researchers with the Lawrence Berkeley National Laboratory.

The team has demonstrated the role the well-established phenomenon can play in coding information based on the wavelike motion of electrons moving through the two-dimensional (2D) semiconductors. "Our technique, which is based on the use of circularly polarized femtosecond light pulses to selectively control the valley degree of freedom, opens up the possibility of ultrafast manipulation of valley excitons for quantum information applications," says Berkeley Lab's Feng Wang.

The technique is analogous to spintronics, in which information is encoded in a quantum spin number. In valleytronics, electrons move through the lattice of a 2D semiconductor as a wave with two energy valleys, with each valley being characterized by a distinct momentum and quantum valley number. When the electrons are in a minimum energy valley, the quantum valley number can be used to encode information. The team recorded a charge transfer time of less than 50 femtoseconds.

"Using ultrafast pump-probe spectroscopy, we were able to observe a pure and valley-selective optical Stark effect in WSe2 monolayers from the non-resonant pump that resulted in an energy splitting of more than 10 milli–electron volts between the K and K' valley exciton transitions," Wang notes.

From Lawrence Berkeley National Laboratory
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