By measuring the polarization changes in terahertz, infrared, and visible radiation over an extended energy range (3–2330 meV), we observe symmetry breaking in cuprate high-temperature superconductors over wide energy, doping, and temperature ranges. We measure the polarization rotation $\left(\mathrm{Re}\left[{\theta }_{\mathrm{F}}\right]\right)$ and ellipticity $\left(\mathrm{Im}\left[{\theta }_{\mathrm{F}}\right]\right)$ of transmitted radiation through thin films as the sample is rotated. We observe a twofold rotational symmetry in ${\theta }_{\mathrm{F}}$, which is associated with linear dichroism (LD) and occurs when electromagnetic radiation polarized along one direction is absorbed more strongly than radiation polarized in the perpendicular direction. Such polarization anisotropies can be generally associated with symmetry breakings. We measure the amplitude of the LD signal and study its temperature, energy, and doping dependence. The LD signal shows a resonant behavior with a peak in the few hundred meV range, which is coincident with the midinfrared optical feature that has been associated with the formation of the pseudogap state. The strongest LD signal is found in underdoped films, although it is also observed in optimally and overdoped samples. The LD signal is consistent with an electronic nematic order which is decoupled from the crystallographic axes as well as novel magnetoelectric effects.

• Received 18 July 2019
• Revised 30 March 2020
• Accepted 15 July 2020
• Corrected 25 September 2020

DOI:https://doi.org/10.1103/PhysRevB.102.054520