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We theoretically study a topological nanospaser, which consists of a silver nanospheroid and MoS 2 monolayer flake of a circular shape. The metal nanospheroid acts as a plasmonic nanoresonator that supports two rotating modes, which are coupled to the corresponding valleys of MoS 2 . We apply external circularly polarized light that selectively pumps only one of the valleys of MoS 2 . The generated spaser dynamics strongly depends on the size (radius) of the MoS 2 nanoflake. For a small radius, the system has only one spasing regime when only the chirally matched plasmon mode is generated, while at a larger size of MoS 2 , depending on the pump intensity, there are two regimes. In one regime, only the chirally matched plasmon mode is generated, while in the other regime both chirally matched and chirally mismatched modes exist. Different regimes of spaser operation also have opposite handedness of the far-field radiated spaser system. Such a topological nanospaser has potential applications in different areas of infrared spectroscopy, sensing, probing, and biomedical treatment.

We study theoretically interaction of topological Weyl semimetals with an ultrafast optical pulse. The electron dynamics in such material is coherent and highly anisotropic. For some directions of pulse polarization, the electric dynamics is irreversible, which means that the residual electron conduction band population after the pulse is comparable to the maximum conduction band population during the pulse. For other directions of polarization, the electron dynamics is highly reversible and, after the pulse, the electron system returns to its initial state with almost zero conduction band population. Such high anisotropy in electron dynamics is related to anisotropy in interband dipole matrix elements. In the reciprocal space, the electron conduction band population density shows hot spots near the Weyl points. The optical pulse also causes net charge transfer through the system. The direction of transfer is the same as the direction of the field maximum. The transferred charge has highly anisotropic dependence on polarization direction with almost zero transferred charge for some directions.

We theoretically study the interaction of ultrashort optical pulses with gapped graphene. Such a strong pulse results in a finite conduction band population and a corresponding electric current, both during and after the pulse. Since gapped graphene has broken inversion symmetry, it has an axial symmetry about the y -axis but not about the x-axis. We show that, in this case, if the linear pulse is polarized along the x-axis, the rectified electric current is generated in the y  direction. At the same time, the conduction band population distribution in the reciprocal space is symmetric about the x-axis. Thus, the rectified current in gapped graphene has an inter-band origin, while the intra-band contribution to the rectified current is zero.