Fig. 1. Photoinduced nonreciprocity in a TMD monolayer.

a, b Schematic illustration of nonreciprocal reflection due to valley-selective response induced by a CP pump. A clockwise (CW) pump is shown as an example. c, d Increased exciton population density at K′ valley due to σ⁻ pump is probed by beams of different helicities incident from the opposite directions. e Schematic of the experimental set up to probe reflection of a TMD monolayer under CP pump (as in a and b) in wavevector inversion geometry. Here, M mirror, FM flip mirror, L Lens, TL tube lens, BS beam splitter, ND variable neutral density filter. f Nonreciprocal reflectance response of WS₂ monolayer for σ⁻ probe beams propagating in +k_z (red curve) and −k_z (blue curve) directions for intense σ⁻ pump incident from one side (+k_z direction).