Coherent measurements of high-order electronic correlations in quantum wells

Three's the limit for multiple excitons Long-range correlations between charge particles (electrons and 'holes') in semiconductors lead to many-body effects, which are of fundamental interest and also of importance in optoelectronic applications. The exciton state, in which an electron and hole are paired, has been extensively studied, but the properties of multiple exciton states involving three or more charge particles are largely unknown as they are challenging to observe experimentally. Daniel Turner and Keith Nelson have extended a spectroscopy technique called multidimensional Fourier transform optical spectroscopy, and demonstrate its ability to generate and characterize bi-excitons, tri-excitons and other unbound correlations in a gallium arsenide nanostructure. This experiment involves controlling the geometry, temporal delays and optical phases of up to seven light fields simultaneously. The findings are of particular interest as it was previously not known whether tri-excitons — involving correlations between six particles — could exist at all. The authors also present clear evidence that four-exciton states do not exist, indicating an upper limit for many-body correlations in this type of semiconductor system.