Since the 1970s, researchers have struggled with the same problem: Shining a laser on a sample can excite it not just once, but several times per experiment. As a result, measurement results of the single excitation and the multiple excitations overlap and often cannot be separated. This makes it difficult to draw conclusions about the material being studied. To remedy this, the laser power is often reduced to such an extent that multiple excitations are less likely than single excitations. However, using laser spectroscopy, multiple excitations cannot be completely avoided and therefore lead to erroneous interpretation of the data produced. Even when multiple excitations are themselves the subject of investigation, it is still difficult to distinguish between two, three, four, or more excitations. A team of physicists and physical chemists from the Julius Maximilians-Universität Würzburg (JMU) and the University of Ottawa has found a solution to this issue. In an experiment conducted by Würzburg professor Tobias Brixner’s group, the researchers used the nonlinear spectroscopy method of transient absorption to track very fast changes in various materials that occur in a millionth of a millionth of a second. In the newly developed method, laser pulses of different power (green) are combined in such a way that single excitation (blue), double excitation (red), and triple excitation (yellow) can be distinguished, for example, in biological light-harvesting complexes. Courtesy of Julian Lüttig/Universität Würzburg. While the standard method uses a single laser power, the researchers used several different powers and combined the data according to a newly derived formula. In this way, they were able to systematically separate the effects from single to sixfold excitation. “Not so long ago, I wouldn't have thought that such a distinction was even possible,” Brixner said, “especially with such a simple procedure that any spectroscopic research group can implement and use without much additional effort.” "The fact that this method works for virtually any sample you want to study really surprised all of us,” said theorist and collaborator professor Jacob Krich of the University of Ottawa. The researchers obtained clean single-particle dynamics even at high excitation intensities and could systematically increase the number of interacting particles, infer their interaction energies, and reconstruct their dynamics, which are not measurable via conventional means, the researchers said. In extracting single and multiple exciton dynamics in squaraine polymers, the researchers found that the excitons, on average, meet several times before annihilating, a finding valuable for efficient organic photovoltaics. The method has a wide range of potential applications. “Separation of signals from single and multiple excitations is particularly useful for large systems with densely packed light absorbers, such as natural photosynthetic complexes or organic materials,” said first author Pavel Malý, who served as a postdoctoral fellow with Brixner at the time of the study; he is now a researcher at Charles University in Prague. The team envisions future applications in the probing of interactions in plasmonics, Auger recombination, exciton correlations in quantum dots, exciton interactions in two-dimensional materials and in molecules, multiphonon scattering, and other areas. The research was published in Nature (www.doi.org/10.1038/s41586-023-05846-7).