Strain effect on circularly polarized electroluminescence (EL) is theoretically analyzed for a p-in junction of transition metal dichalcogenides. The electrically controllable circularly polarized EL without the strain effect is understood by valley polarization combined with the electron-hole asymmetry due to the trigonal warping effect in the valence band. The strain enhances circularly polarized EL more efficiently than the trigonal warping effect since the asymmetry occurs in the same direction as the direction of the applied electric field. The calculated result shows that the circular polarization in the strained MoS 2 becomes 100% without losing the EL intensity.

We experimentally demonstrated significant enhancement of terahertz-wave absorption in monolayer graphene by simply sandwiching monolayer graphene between two dielectric media in a total internal reflection geometry. In going through this structure, the evanescent wave of the incident terahertz beam interacts with the sandwiched graphene layer multiple (up to four) times at varying incidence angles. We observed extremely large attenuation (up to ∼ 70% per reflection), especially for s-polarized radiation. The experimental results are quantitatively consistent with our calculations, where we modeled the experiment as an electromagnetic wave reflection process in monolayer graphene. We also derived analytical expressions for the absorptance, showing that the absorptance is proportional to the amount of Joule heating on the graphene surface induced by the terahertz radiation.

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We theoretically investigate intersubband plasmon excitations in doped single wall carbon nanotubes (SWNTs) by examining the dependence of plasmon frequency on the nanotube diameter, chirality, and Fermi energy. The intersubband plasmons can be excited by light with polarization perpendicular to the nanotube axis and thus the plasmon excitations corresponds to optical transitions between the two different subbands, which are sensitive to the Fermi energy. In every SWNT, this mechanism leads to the emergence of the optical absorption peak at the plasmon frequency for a given Fermi energy, EF . The plasmon frequencies calculated for many SWNTs with diameter dt < 2 nm exhibit a dependence on (1/dt) 0.7 and the frequencies are further affected by Fermi energy as E 0.25 F . With this knowledge, it is possible to develop a map of intersubband plasmon excitations in doped SWNTs that could be useful to quickly estimate the doping level and also be an alternative way to characterize nanotube chirality.

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