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We discuss the optical conductivity of several non-interactingtwo-dimensional (2D) semiconducting systems focusing on gapped Dirac andSchr\"odinger fermions as well as on a system mixing these two types. Close tothe band-gap, we can define a universal optical conductivity quantum of$\sigma_0=\frac{1}{16}\frac{e^2}{\hbar}$ for the pure systems. The effectiveoptical conductivity then depends on the degeneracy factors $g_s$ (spin) and$g_v$ (valley) and on the curvature around the band-gap $\nu$, i.e., itgenerally reads $\sigma=g_sg_v\nu\sigma_0$. For a system composed of both typesof carriers, the optical conductivity becomes non-universal.

Universal absorption of two-dimensional systems