Background-free measurement of exciton-exciton annihilation by two-quantum fluorescence-detected pump-probe spectroscopy

TL;DR

This paper presents a novel two-quantum fluorescence-detected pump-probe spectroscopy technique that isolates background-free signals of exciton interactions, enabling clearer analysis of ultrafast multiparticle dynamics.

Contribution

The authors develop a pulse-shaper-based collinear setup with phase cycling and post-processing to remove incoherent mixing in 2Q F-PP spectroscopy, improving signal clarity.

Findings

Successfully applied to squaraine systems to isolate doubly excited states.

Eliminates incoherent mixing and parasitic signals in 2Q F-PP measurements.

Provides background-free spectral and dynamical information of excitonic states.

Abstract

We introduce two-quantum (2Q) fluorescence-detected pump-probe (F-PP) spectroscopy as a tool to probe ultrafast multiparticle interactions in many-body systems. We describe a pulse-shaper-based fully collinear setup utilizing phase cycling to capture the 2Q F-PP signal simultaneously with the one-quantum (1Q) F-PP signal. Thus, we investigate the dynamics of energy transfer and diffusion-limited annihilation. We apply a data post-processing strategy to isolate excited-state dynamics from spurious background. The technique is applied to a squaraine heterodimer and a squaraine copolymer to demonstrate the removal of so-called incoherent mixing that generally plagues action-detected nonlinear spectroscopy on multichromophoric systems. Specifically, we show that this approach is not only applicable to 1Q but also to 2Q F-PP signals, eliminating incoherent mixing contributions as well as other "parasitic" signals that result from pulse-overlap ambiguities. As a result, we retrieve background-free spectral and dynamical information of doubly excited electronic states.