It is well established that causal linear response functions can be found by computing the much simpler imaginary-time-ordered Matsubara functions and performing an analytic continuation. This principle is the basis for much of our understanding of linear response for interacting and disordered systems, via diagrammatic perturbation theory. Similar imaginary-time approaches have recently been introduced for computing nonlinear response functions as well,  as in [Ann. Phys. (Berlin) 536, 2300504 (2024); Phys. Rev. X 11, 041006 (2021)], where the authors analytically continue the Matsubara functions to obtain the Keldysh response functions. In this work, we provide a proof of this connection to all orders in perturbation theory using an equation of motion based approach. We show by induction that causal nonlinear response functions at every order can be obtained from the analytic continuation of an appropriate time-ordered Matsubara function. We demonstrate this connection explicitly for second-order response functions in the Lehmann representation. As a byproduct of our approach, we derive an explicit expression for the Lehmann representation of the $n\mathrm{th}$-order response functions by solving the equations of motion. We also use our result to find an analytic spectral density representation for both causal response functions and Matsubara functions. As an example, we apply our method to derive the nonlinear $A^3$ term in the $SU\left(2\right)$ spin Hall response of an insulator with spin rotation symmetry. Finally, we show how our results lead to a family of generalized sum rules, focusing explicitly on the asymptotic expression for the $n\mathrm{th}$-harmonic-generation rate. Our work opens the door to using imaginary time approaches to study nonlinear response functions in general condensed matter systems.

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• Sum rules
• Green's function methods
• Linear response theory
• Numerical techniques