Seminar Talk of Dr. Hosein Cheraghchi (Damghan University)

Recent technological progresses in mid-infrared lasers have established a new route for engineering of the electronic band structure. An interplay between the periodicity of lattice and time originated from the oscillating field extends the Hilbert space by inducing the so-called Floquet-Bloch states and enables one to modify the band structure (opening the gaps) and to engineer phase transitions at will by using photon-assisted processes. Fortunately, such light-induced states have been experimentally observed on the surface of irradiated topological insulators (TI), optical lattices, graphene etc. Many striking light-induced phenomena are also predicted or observed in this field: quantum Hall conductivity and also quantum anomalous Hall insulator on TIs, etc. However, experimental challenge is still heat production during irradiation of the system by the laser beam. 

In this talk, I will present a short review on the Floquet theory including on- and off- resonant regimes, the gap opening and emergence of topological phases accompanied by anomalous edge states. Then I will focus on the special case of magnetically topological insulator thin film irradiated by circularly-polarized electromagnetic field in the off-resonant regime. Fascinating feature of distinct phases emerges in the phase diagram depending on the frequency and intensity of the light as well as the system parameters and also magnetic field. We also present the results of time-averaged optical conductivity of a driven two-dimensional TI at on-resonant regime. Depending on the type of considered occupation model and optical weights of Floquet states, optical conductivity is analysed by means of the time-averaged density of states and modified band structure.

Host: Götz S. Uhrig