Controlling valley polarisation in graphene via tailored light pulses

Analogous to charge and spin, electrons in solids endows an additional degree of freedom: the valley pseudospin. Two-dimensional hexagonal materials such as graphene exhibit two valleys, labelled as $\mathbf{K}$ and $\mathbf{K}^{\prime}$. These two valleys have the potential to realise logical operations in two-dimensional materials. Obtaining the desired control over valley polarisation between the two valleys is a prerequisite for the logical operations. Recently, it was shown that two counter-rotating circularly polarised laser pulses can induce a significant valley polarisation in graphene. The main focus of the present work is to optimise the valley polarisation in monolayer graphene by controlling different laser parameters, such as wavelength, intensity ratio, frequency ratio and sub-cycle phase in two counter-rotating circularly polarised laser setup. Moreover, an alternate approach, based on single or few-cycle linearly polarised laser pulse, is also explored to induce significant valley polarisation in graphene. Our work could help experimentalists to choose a suitable method with optimised parameter space to obtain the desired control over valley polarisation in monolayer graphene.

Optical Absorption and Reflectivity of Four 2D Materials: MoS2, MoP2, NbS2, and NbP2

We calculated the energy band structure and the optical absorption and reflectivity for each of the ultrathin 2D hexagonal materials MoS2, MoP2, NbS2, and NbP2. Our simulations included density functional theory, generalized gradient approximation (GGA), and the Quantum Espresso code. Other researchers already synthesized the first three materials. We obtained that NbP2 should be another hexagonal 2D material. In all cases in the infrared and visible ranges, the absorptions present much larger concerning graphene. However, the absorptions for MoP2, NbP2, and NbS2, are far more prominent concerning MoS2. In the ultraviolet region, the absorptions are like each other and differ from graphene. In all cases, the reflectivities are similar to each other and vary from graphene.