Floquet states are photon-dressed electronic states that emerge when driven by a time-periodic optical excitation. The presence of these states changes the equilibrium picture of electronic transitions into one where transitions between Floquet states are allowed. We have recently investigated the indirect exchange (RKKY) interaction mediated by itinerant electrons that are driven strongly out of equilibrium by circularly polarized light in two-dimensional electron systems with parabolic and linear dispersion (graphene). Our results indicate that emergence of non-equilibrium Floquet states leads to new and qualitatively different regimes of RKKY coupling, in which the sign (ferromagnetic or antiferromagnetic) and the period of the RKKY oscillation become tunable through the frequency and amplitude of the laser.
This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Early Career Award #DE-SC0019326.
Topological insulators are characterized by the quantum anomalous Hall effect on the topological surface states under time-reversal symmetry breaking. While this effect has been recently observed in a magneto-optical setup upon illumination of weak linearly polarized light, the influence of intense optical field remains largely unexplored. Using the Keldysh-Floquet Green's function formalism, we develop a theory for the dynamical Hall conductivity for arbitrary incident optical frequency in the intense optical field regime. We apply our general theory to the adiabatic, low-frequency regime, and study the breakdown of the one-half Hall quantization under intense optical field. Our results reveal a strong nonlinear dependence of the dynamical Hall conductivity on the incident optical field, which is triggered by the formation of Floquet subbands and the transitions between them.
In this contribution we study the magneto-optical Faraday effect of topological insulator (TI) films in the presence of an external magnetic field. In the first part, we give a short review of the essential results [Refs. 10-12] in the low-frequency regime. In strong magnetic fields, the low-frequency Faraday effect for TI thin films is found to be quantized at integer multiples of the fine structure constant. In the second part, we present results from our study on the influence of cyclotron and cavity resonance effects in thick TI films. For thick films, we find that the same quantization of the Faraday rotation re-emerges when cavity resonance occurs. At higher frequencies, the interplay of both cyclotron resonance and cavity resonance effects leads to interesting features in the resulting Faraday rotation.
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