Optical analogue of 2D heteronuclear double-quantum NMR

Abstract

Heteronuclear multi-quantum spectroscopy is a powerful part of the NMR toolbox, commonly used to identify specific sequences of atoms in complex pulse sequences designed to determine the structure of complex molecules, including proteins. Optical coherent multidimensional spectroscopy (CMDS) is analogous to multidimensional NMR and many of the techniques of NMR have been adapted for application in the optical regime. This has been highly successful, with CMDS being used to understand energy transfer in photosynthesis and many body effects in semiconductor nanostructures amongst many other scientific breakthroughs. Experimental challenges have, however, prevented the translation of heteronuclear multi-quantum NMR to the optical regime, where capabilities to isolate signals in otherwise congested spectra, reduce acquisition times and enable more incisive probes of multi-particle correlations and complex electronic systems would have great benefit. Here we utilise a diffraction based pulseshaper to impose the two key avenues of selectivity required for the analogue of heteronuclear multi-quantum spectrocopy, - control of the individual pulse spectra, and control of the order and timing of interactions - while maintaining phase-locked excitation pulses. We utilise this approach to isolate the 2-quantum signal from a mixed two exciton state in a semiconductor coupled quantum well. The removal of other overlapping peaks allows analysis of the relevant peak shape and thus details of interactions with the environment and other carriers. An alternative pulse ordering also identifies a 2-quantum state associated only with a GaAs defect, the signature of which would otherwise be confused with other interaction induced effects. The experimental approach described here provides access to crucial details of excitonic interactions, which are important in a range of material systems