During the past decade, graphene has attracted immense interest, mainly due to its excellent transport and optical properties, which make it an attractive candidate for possible applications in nanoscale electronics and optoelectronics.Using the Kubo linear response formalism, we study the effects of intrinsic graphene optical and surface polar phonons (SPPs) on the optical conductivity of doped graphene, both at zero and finite magnetic fields B.
We find that inelastic electron-phonon scattering contributes significantly to the phonon-assisted absorption in the optical gap at B=0. At room temperature, this midgap absorption can be as large as about 20-25% of the universal ac conductivity for graphene on polar substrates (such as Al2O3 or HfO2) due to strong electron-SPP coupling. The midgap absorption, moreover, strongly depends on the substrates and doping levels used. We predict that with increasing temperature, the midgap absorption increases, while the Drude weight decreases. These predictions can serve as an experimental signature for the role of SPPs on transport and optical properties of graphene, which has important implications for the performance of graphene-based electronic devices and broadband modulators.
At finite magnetic fields, our calculations suggest that polaronic shifts of the intra- and interband absorption peaks can be significantly larger for substrates with strong electron-SPP coupling than those in graphene on nonpolar substrates, where only intrinsic graphene optical phonons with much higher energies contribute. Electron-phonon scattering and phonon-assisted transitions are, moreover, found to result in a loss of spectral weight at the absorption peaks. The strength of these processes is strongly temperature dependent and with increasing temperatures the magneto-optical conductivity becomes increasingly affected by polar substrates, most noticeably in polar substrates with small SPP energies such as HfO2.