Two-dimensional spectroscopy of bosonic collective excitations in disordered many-body systems

Traditional probes in condensed matter physics typically rely on linear response techniques. While highly effective for measuring two-point correlation functions, these methods often struggle to differentiate between various sources of broadening, especially when similar spectral lineshapes arise from different mechanisms. Two-dimensional (2D) spectroscopy, a well-established technique in nuclear magnetic resonance and optical spectroscopy, is now emerging as a powerful complement to these linear-response methods in condensed matter systems. By extending measurements into the nonlinear regime, 2D spectroscopy offers access to higher-order correlations and serves, in particular, as a unique tool to disentangle distinct broadening sources. Recent advancements in terahertz technology have facilitated the development of 2D terahertz spectroscopy, enabling the study of low-energy excitations in systems such as superconductors, magnons, and topological materials. This talk will explore the application of 2D spectroscopy to probe collective bosonic excitations in quantum many-body systems, with a focus on its exceptional ability to distinguish and analyze the nature of the underlying broadening mechanisms.