Higher-Order Multidimensional and Pump–Probe Spectroscopies

Abstract

Transient absorption and coherent two-dimensional spectroscopy are widely established methods for the investigation of ultrafast dynamics in quantum systems. Conventionally, they are interpreted in the framework of perturbation theory at the third order of interaction. Here, we discuss the potential of higher-(than-third-)order pump−probe and multidimensional spectroscopy to provide insight into excited multiparticle states and their dynamics. We focus on recent developments from our group. In particular, we demonstrate how phase cycling can be used in fluorescence-detected two-dimensional spectroscopy to isolate higher-order spectra that provide information about highly excited states such as the correlation of multiexciton states. We discuss coherently detected fifth-order 2D spectroscopy and its power to track exciton diffusion. Finally, we show how to extract higher-order signals even from ordinary pump−probe experiments, providing annihilation-free signals at high excitation densities and insight into multiexciton interactions.

An Exact Expression for Multidimensional Spectroscopy of a Spin‐Boson Hamiltonian

“…There are of course many systems that require a more sophisticated approach to model, for example those with multiple participating quantum states, [63][64][65][66] higher-order system-bath couplings, [67] or a bath with strong anharmonicities. [68] Excitation schemes that use pulses that break the impulsive approximation also will require numerical approaches.…”

Interpretations of High-Order Transient Absorption Spectroscopies

“…Linear absorption spectroscopy, in frequency ranges from THz to X-ray, gives information about the ground or thermal states of a material and its optically accessible excited states. Transient absorption (TA) spectroscopy measures the change in a probe pulse’s absorption spectrum when it arrives a time T after a pump pulse, revealing both the absorption and dynamics of excited states. − The recently developed high-order transient absorption (HOTA) spectroscopy extends TA spectroscopy by systematically separating higher orders of nonlinear response, which has not previously been possible using TA methods. − These higher orders of nonlinear response contain information about energies, transition dipoles, and dynamics of multiply excited states.…”

“…Many methods have been used or proposed to extract different high-order responses, frequently involving more than two pulses . References and discuss 3-pulse experiments that are high-order extensions of double-quantum (HODQ) spectroscopy .…”