Two-dimensional THz spectroscopy in electronic systems: a many-body diagrammatic approach

The term two-dimensional coherent spectroscopy (2DCS) usually refers to experimental setups where a coherently generated electric field in a sample is recorded over many runs as a function of two time variables: the delay $\tau$ between two consequent excitation pulses and the time $t$ over which the signal is emitted. While its implementation in the femtosecond time domain for studying vibrational molecular states has been developed for over two decades, its experimental application in the THz domain to interacting electronic systems remains in its infancy. This work provides a general theoretical framework for describing and interpreting 2DCS using a many-body language based on a perturbative diagrammatic expansion, as widely applied in linear spectroscopy. Focusing on centrosymmetric systems, we show that interpreting the 2D maps can be recast into two complementary problems. The first is the evaluation of a third-order response function to the gauge field. In the velocity gauge, this leads to semi-analytical expressions that both reduce computational complexity and assist in assigning spectral features to microscopic processes, as shown using a toy model of electrons undergoing a charge-density wave transition. The second is a careful treatment of multi-wave propagation effects, which, in bulk systems, can obscure the intrinsic nonlinear response, demonstrated here for soft superconducting Josephson plasmons. Our results provide a solid foundation for extending 2DCS to complex interacting systems and offer a flexible method to realistically model nonlinear responses across arbitrary spectral widths.