Separating pathways in double‐quantum optical spectroscopy reveals excitonic interactions

Techniques for coherent multidimensional optical spectroscopy have been developed and utilised to understand many different processes, including energy transfer in photosynthesis and many‐body effects in semiconductor nanostructures. Double‐quantum 2D spectroscopy is one variation that has been particularly useful for understanding many‐body effects. In condensed matter systems, however, there are often many competing signal pathways, which can make it difficult to isolate different contributions and retrieve quantitative information. Here, a means of separating overlapping pathways while maintaining the fidelity of the relevant peak/s is demonstrated. This selective approach is used to isolate the double‐quantum signal from a mixed two exciton state in a semiconductor quantum well. The removal of overlapping peaks allows analysis of the relevant peak‐shape and thus details of interactions with the environment and other carriers to be revealed. An alternative pulse ordering identifies a double‐quantum state associated only with GaAs defects, the signature of which has previously been confused with other interaction induced effects. The experimental approach described here provides access to otherwise hidden details of excitonic interactions and demonstrates that the manner in which the double‐quantum coherence is generated can be important and provide an additional control to help understand the many‐body physics in complex systems. Interactions fundamentally define the macroscopic properties and functionality of solid‐state materials. Double‐quantum 2D‐spectroscopy is particularly sensitive to exciton interactions, however, overlapping peaks can confound the interpretation. In this work, specific signal pathways are selected, and overlapping peaks removed, by controlling both the spectrum and timing of the excitation pulses. Detailed analysis of the isolated peak then provides understanding of excitonic interactions and details of the individual states involved.