Nematic-driven anisotropic electronic properties of underdoped detwinned Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ revealed by optical spectroscopy*

We collect optical reflectivity data as a function of temperature across the structural tetragonal-to-orthorhombic phase transition at T$_s$ on Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ for x = 0, 2.5\% and 4.5\%, with uniaxial and in-situ tunable applied pressure in order to detwin the sample and to exert on it an external symmetry breaking field. At $T < T_s$, we discover a remarkable optical anisotropy as a function of the applied pressure at energies far away from the Fermi level and very much reminiscent of a hysteretic-like behavior. Such an anisotropy turns into a reversible linear pressure dependence at $T\ge T_s$. Moreover, the optical anisotropy gets progressively depleted with increasing Co-content in the underdoped regime, consistent with the doping dependence of the orthorhombicity but contrary to the non-monotonic behavior observed for the dc anisotropy. Our findings bear testimony for an important anisotropy of the electronic structure and thus underscore an electronic polarization upon (pressure) inducing and entering the nematic phase.