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We address a long standing problem of pseudospin control in graphene from both experimental and theoretical point of view. The outcome of this work is twofold.
First, we provide a conclusive evidence of the anisotropic photocarrier occupation (i. e. pseudospin polarization) in graphene caused by interaction between pseudospin and linearly polarized light, see left figure. This peculiar phenomenon has long been discussed in numerous theoretical papers but its direct observation in a truly single layer graphene (rather than in multilayer stacks utilized before) was, up to now, missing.
Second, we clarify the mechanisms responsible for the photocarrier occupation isotropization (i. e. pseudospin randomization) using an entirely analytical model specially developed for our experimental settings. The main driving isotropization mechanism is discovered to be optical phonon emission rather than electron-electron scattering, further distinguishing graphene from conventional semiconductors. The predictive power of the model is demonstrated in the right figure, where the pseudospin occupation (i. e. the ratio between optical transmittance for opposite polarization configurations) is shown as a function of radiation fluence.

M. Trushin, A. Grupp, G. Soavi, A. Budweg, D. De Fazio, U. Sassi, A. Lombardo, A. C. Ferrari, W. Belzig, A. Leitenstorfer, D. Brida
Phys. Rev. B  92, 165429 (2015)