Fig. 1: NIR pump–MIR probe spectroscopy of twisted WSe₂ bilayers.

a Optical microscope image of a representative twisted homobilayer (BL) on a diamond substrate. The blue frame indicates the lower WSe₂ monolayer, which is covered by a second WSe₂ monolayer (orange frame) forming a twisted BL in the overlap region (black frame). Scale bar: 50 µm. Inset: component of the second harmonic I_2ω,|| polarized parallel to the pump laser light. The characteristic patterns of sixfold symmetry indicate the armchair directions of the bottom (blue dots) and top (orange dots) monolayers, enclosing a twist angle, θ = 53°, close to the natural 2H configuration. b Normalized photoluminescence spectra for samples with different twist angles θ, recorded at a temperature of 5 K (vertically offset for clarity). The black dotted line serves as a guide to the eye highlighting the evolution of the K-Λ transition. c Sketch of the experiment, where a near-infrared (NIR) pump pulse (orange intensity envelope) optically injects 1s A excitons in the WSe₂ BLs. The excited sample is probed by a mid-infrared waveform (MIR, red curve). The exciton dispersion for a given orbital quantum number is depicted as a function of the center-of-mass momentum q (gray paraboloids). Red arrows indicate the 1s–2p transitions of the exciton, interrogated by the MIR photons at an energy ħω_MIR.