Abstract
Table-top sources of intense multi-terahertz (THz) pulses have opened the door to studies of extreme nonlinearities in the previously elusive mid- to far-infrared spectral regime. We discuss two concepts of fully coherent coupling of phase-locked THz pulses with condensed matter. The first approach demonstrates two-dimensional multi-THz spectroscopy of the semiconductor material InSb. By phase- and amplitude-sensitive detection of the nonlinear optical response, we are able to separate incoherent pump–probe signals from coherent four-wave mixing and reveal extremely non-perturbative nonlinearities. While this class of interactions is mediated by the electric field component of the THz pulse, the second approach is complementary, as it demonstrates that, alternatively, the magnetic THz field may be exploited to selectively control the spin degree of freedom in antiferromagnetic NiO.
Highlights
In the first part of this article, we report on the coherent nonlinear THz response of an interband polarization in the narrowband semiconductor InSb, studied by means of fieldresolved four-wave mixing (FWM) spectroscopy [32]
Our experiment demonstrates that this condition is violated for peak electric fields in excess of 3 MV cm−1, as the FWM signal shows clear signatures of a non-perturbative response caused by coherent Rabi flopping
Our study of multi-THz two-dimensional spectroscopy confirms the onset of a non-perturbative response under even off-resonant excitation of interband transitions in InSb
Summary
In contrast to the FWM signal, the pump–probe signals observed in our experiment are temporally extended far beyond the duration of the interacting THz pulses themselves This observation indicates that the main contribution to the pump–probe response in InSb originates from a long-living electron–hole plasma produced by the intense THz field of the pump pulse. In the following discussion we demonstrate that this behavior is an unambiguous signature of a non-perturbative polarization response in InSb. In order to understand the character of the light–matter interaction in our experiment, we perform simulations of the FWM response assuming either a perturbative response due to a third-order nonlinearity χ (3) or a response of a quantum two-level system coherently driven by intense THz pulses.
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