Ultrafast anisotropic valleytronics in SnSe.

Polarization-controlled photoexcitation in SnSe shows nearly unity, time-independent valley polarization when exciting valleys at global conduction minima, unlike rapid subpicosecond reversal in the other valley channel.

• Why it matters: Valleytronics needs control of valley-specific light–matter interactions and the non-equilibrium microscopic coupling after valley polarization is created, which has been less explored in anisotropic group-IV monochalcogenides.
• What they did: The study used time- and angle-resolved extreme-ultraviolet photoemission spectroscopy together with time-dependent Boltzmann equation simulations to track ultrafast valley polarization after polarization-controlled photoexcitation in SnSe.
• The result: Exciting the global conduction minima produces nearly unity, time-independent valley polarization, while exciting the other valley channel causes ultrafast decay and reversal on subpicosecond timescales via intervalley scattering mediated by strong electron-phonon coupling with an optical phonon mode.

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