Optical manifestations of symmetry breaking in bilayer graphene

We propose a spectroscopic method of identifying broken symmetry states of bilayer graphene. We demonstrate theoretically that, in contrast to gapped states, a strained bilayer crystal or nematic phase of the electronic liquid are distinguishable by the dependence of the lineshape of absorption on the polarization of the light. This property is characteristic for both the infrared and far-infrared spectral ranges, which correspond to the absorption by transitions between low-energy bands and split bands, and transitions between the low-energy valence and conduction bands, respectively.