Speaker
Description
Optical spectroscopy is a powerful tool for characterizing excitonic systems, ranging from solid-state photovoltaic materials to molecular light-harvesting complexes. Beyond the linear light–matter interaction, techniques such as pump-probe spectroscopy provide access to the nonlinear optical response of these systems. In the perturbative regime, the dominant contribution is the third-order response, which encodes key information on single-exciton dynamics and relaxation pathways. However, in extended systems or at pump intensities exceeding the perturbative limit, many-body effects arising from exciton–exciton interactions (such as exciton–exciton annihilation) can emerge, contaminating the effective third-order signal and adding nuances to the optical properties of the material in operando conditions.
In this work, we present a recently introduced approach to disentangle these higher-order contributions and isolate specific nonlinear orders by analyzing signals acquired at different pump intensities [1,2]. A central step of this methodology is a quantitative understanding of the commonly observed experimental phenomenon of signal saturation with pump intensity, which is crucial for determining which pump intensities enable reliable order separation. Our analysis is based on an open-quantum-system modeling of exciton systems, combining coherent light–matter coupling with dissipative processes, and numerical simulations of pump–probe signals for prototypical molecular aggregates. We identify three distinct saturation regimes: (i) a coherent regime dominated by Rabi oscillations, (ii) an incoherent short-pulse regime in which signal saturation follows an exponential law, and (iii) an incoherent long-pulse regime characterized by a saturable absorption law.
[1] P. Malý, J. Lüttig, P. A. Rose, A. Turkin, C. Lambert, J. J. Krich, and T. Brixner, Separating Single- from Multi-Particle Dynamics in Nonlinear Spectroscopy, Nature 616, 280 (2023).
[2] J. J. Krich, L. Brenneis, P. A. Rose, K. Mayershofer, S. Büttner, J. Lüttig, P. Malý, and T. Brixner, Separating Orders of Response in Transient Absorption and Coherent Multidimensional Spectroscopy by Intensity Variation, J. Phys. Chem. Lett. 5897 (2025).
| Keyword-1 | Pump-probe spectroscopy |
|---|---|
| Keyword-2 | Intensity cycling |
| Keyword-3 | Molecular exciton |