arXiv subjects

Amir Hamed Majedi

Publications and source records attributed to Amir Hamed Majedi.

3 recordsLinked to original sources

arXiv2019

Anomalous optical saturation of low-energy Dirac states in graphene and its implication for nonlinear optics

We reveal that optical saturation of the low-energy states takes place in graphene for arbitrarily weak electromagnetic fields. This effect originates from the diverging field-induced interband coupling at the Dirac point. Using semiconductor Bloch equations to model the electronic dynamics of graphene, we argue that the charge carriers undergo ultrafast Rabi oscillations leading to the anomalous saturation effect. The theory is complemented by a many-body study of the carrier relaxations dynamics in graphene. It will be demonstrated that the carrier relaxation dynamics is slow around the Dirac point, which in turn leads to a more pronounced saturation. The implications of this effect to the nonlinear optics of graphene is then discussed. Our analysis show that the conventional perturbative treatment of the nonlinear optics, i.e., expanding the polarization field in a Taylor series of the electric field, is problematic for graphene, in particular at small Fermi levels and large field amplitudes.

cond-mat.mes-hall↗

arXiv2019

Experimental Characterization of Ultrafast, Tunable and Broadband Optical Kerr Nonlinearity in Graphene

In this study we systematically measure the near-infrared spectral dependence, the sub-picosecond temporal evolution and pulse-width dependence of the effective Kerr coefficient ($n_{2,eff}$) of graphene in hundreds of femtosecond regime. The spectral dependence measured using the Z-scan technique is corroborated by quantum theory to extract a $n_{2,eff} \propto λ^2$ dependence. The temporal evolution extracted using the time-resolved Z-scan measurement shows the nonlinear response peaking at zero delay time and relaxing on a time scale of carrier relaxation. Since the Kerr-type response originated from the optically induced carrier population difference, the time-scale of the evolution of the nonlinear response is apt. The $n_{2,eff}$ shows a dependence on the pulse-width attributed to the relative heating and cooling times of the carriers. This dependence is strong when the pulse-duration is on the same time-scale as the decay constant. Throughout our study the $n_{2,eff}$ remains positive.

physics.optics↗

arXiv2015

Nonlinear quantum optical properties of graphene: the role of chirality and symmetry

We present a semiclassical theory of linear and nonlinear optical response of graphene. The emphasis is placed on the nonlinear optical response of graphene from the standpoint of the underlying chiral symmetry. The Bloch quasiparticles in low energy limit, around the degeneracy points are dominantly chiral. It is shown for the first time that this chiral behavior in conjunction with scale invariance in graphene around the Dirac points results in the strong nonlinear optical response. Explicit expressions for the linear and nonlinear conductivity tensors are derived based on Semiconductor Bloch Equations (SBEs). The linear terms agree with the result of Kubo formulation. The three main additive mechanisms contribute in the nonlinear optical response of graphene: pure intraband, pure interband and the interplay between them. For each contribution, an explicit response function is derived. The Kerr-type nonlinearity of graphene is then studied and it is demonstrated that its Kerr nonlinear coefficient is several orders of magnitude higher than that of many other known semiconductors. In addition, the nonlinear refractive index of graphene can also be tuned and enhanced by applying a gate voltage.

cond-mat.mes-hall↗