Theoretical seminar | 29 April 2020

Electromagnetic field interacting with a topologically protected one-dimensional electron helical state is shown to support a one-dimensional plasmon-polariton, collective electron excitation dressed with the electromagnetic field. The electronic helical state arises at the surface of three-dimensional topological insulator in the proximity of the ferromagnet and is localized at the magnetization domain wall. This opens the possibilities to manipulate quantum optical states by altering magnetic domain configurations. An exact spectral equation for such topological plasmon-polariton is derived. Also, we show that the second harmonic generation (SHG) is enhanced in the chiral one-dimensional electron currents in a broad frequency range. The origin of the enhancement is two-fold: first, the linear dispersion of the current and the associated plasmonic mode as well as the quasi-linear dispersion of plasmon-polariton result in the lift of the phase matching condition. Moreover, the strong field localization leads to the further increase of the SHG in the structure. The results suggest that the chiral currents localized at the domain walls of magnetic topological insulators can be an efficient source of second harmonic signal in the terahertz frequency range.

Increasing the light-matter interaction is of crucial importance for many fields of nanophotonics such as cavity quantum electrodynamics, non-linear nanophotonics, optomechanics, etc. In our work we study how the optical quasi-bound states in the continuum, recently predicted and observed in dielectric nanoantennae can be utilized to substantially enhance quality factor to mode volume ratio in pillar microcavities. In order to additionally boost the quality factor of mode we suggest the using distributed Bragg reflectors which can suppress the radiation in some direction thereby increasing the cavity mode lifetime without a significant increase of its volume.