The single-particle approximation is ubiquitous: The total wave function is approximated as a product, leading to a separation of variables. Despite immense success, there are important phenomena in which multi-particle correlations are relevant. We develop techniques that can resolve such interactions. Combining coherent two-dimensional electronic spectroscopy (2DES) and fluorescence microscopy, we determine the exciton–phonon coupling strength of a 2D dichalcogenide at room temperature [1]. We also observe a new type of quasiparticle arising from the strong trifold coupling of excitons, phonons, and photons in a microcavity [2]. Taking fluorescence detection to the limit, we implement ultrafast single-molecule spectroscopy including spectral resolution [3]. For a spatial resolution of down to 3 nm, we combine 2DES with photoemission electron microscopy (PEEM). This allows us to determine the coupling between plasmons and electrons in a metallic nanoslit resonator by observing a plasmon–polariton quantum wave packet [4]. Lastly, we develop a general method to separate experimentally the nonlinear response terms of perturbation theory [5]. This solves the decades-old “annihilation problem” of femtosecond spectroscopy and provides access to multi-particle interactions in a systematic manner with applications for molecules, supramolecular aggregates, polymers, interfaces, plasmonic and hybrid nanosystems.
[1] D. Li et al., Nature Commun. 12, 954 (2021).
[2] D. Li et al., Phys. Rev. Lett. 128, 087401 (2022).
[3] D. Fersch et al., J. Phys. Chem. Lett. 14, 4923 (2023).
[4] S. Pres et al., Nature Phys. 19, 656 (2023).
[5] P. Malý et al., Nature 616, 280 (2023).