Fig. 1

Experiment. a Schematic of the experimental setup: The three laser beams are provided by the multidimensional optical nonlinear spectrometer (MONSTR)⁵⁷. Three beams labeled as A^*, B, and C are used to generate the FWM signal, where A^* corresponds to the phase conjugate beam. The beams are aligned in the three corners of a square. The FWM signal generated at the sample propagates along the missing corner (direction −\({\mathbf{k}} _{\mathrm{a}} + {\mathbf{k}} _{\mathrm{b}} + {\mathbf{k}} _{\mathrm{c}}\)) and is collected by the detector. The samples are kept at 10 K inside the magneto-optical cryostat. b The magnetic fields up to 25 T are applied perpendicular to the sample surface in Faraday geometry. c Pulse sequence leading to positive delay FWM signal. d When the phase conjugate pulse A^* arrives last, the negative delay signal is generated due to multiple exciton correlations