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A mechanism of nonlinear dichroism of net gain in VCSELs, based on polarisation-dependent saturation of intersubband absorption in p-contact layers, is proposed and analysed. The mechanism is due to the dependence of the intersubband absorption coefficient on the mutual orientation of the light polarisation and the momentum of the holes involved in optical transition. Microscopic calculations are used to obtain an approximate analytical expression for the magnitude of the effect. Calculations (performed for typical VCSEL parameters) imply that the proposed mechanism can play a significant part in polarisation switching and bistability in VCSELs.