Ashutosh Singh

Ashutosh Singh (McMaster University) Titre: Optical response in Dirac and Weyl semi-metals Résumé: This talk is divided into two parts. In the first part I will focus on the nonlinear and anisotropic optical response in Graphene. Optical response, in general, can be quantified in terms of optical susceptibility or optical conductivity which reveal the system’s behaviour to an external field in terms of Polarization or Current density respectively. In the presence of a continuous wave (cw) optical beam, the system attains quasi-equilibrium steady-state owing to the balance between continuous excitation of carriers and relaxation due to damping. This regime is well described by the steady-state solution of the Optical Bloch Equation with phenomenological damping rates. Using the steady-state solution, we study wave-mixing and other nonlinear effects in Graphene. In contrast to Graphene, Doped Weyl-semimetal optical conductivity varies linearly with respect to photon frequency, given that the frequency is greater that twice as that of doping level. Tilting the Weyl cones changes this feature dramatically. Tilting gives rise to regions in the spectrum where the optical conductivity is neither linear nor zero. We have calculated the absorptive part of the dynamic conductivity for both right and left handed circular polarized light in the Kubo formalism. Due to finite contribution from the imaginary part of Hall conductivity, we see dichroism in certain frequency regime. Including the effect of chiral pumping, which transfers charge from one node to the other, difference is created in the doping level of the two nodes. This leads to opening of new frequency regions where dichroism remains finite. Although the kinks in the spectrum at the boundaries of different frequency regimes are not expected to survive at finite temperature cases, still in some regions dichroism survives. (invité de R. Côté)