Fig. 2: Photon echo from trions at zero magnetic field.

a Energy level diagram and optical transitions for the trion X⁻. H and V indicate the polarization of optical transitions. b Polar plots of two-pulse photon echo (PE) amplitude in HRH (solid line) and HRV (dashed line) polarization configurations at t = 2τ₁₂ = 132 ps as function of polarization angle φ₂ of the second pulse. H and V correspond to linear polarizations along x and y axes, respectively. Linear polarization R is defined by the angle φ₂ with respect to the x-axis. c Decay of the two-pulse (2PE) and three-pulse (3PE) photon echoes as function of 2τ₁₂ and τ₂₃. In the three-pulse photon echo the delay time τ₁₂ = 33.3 ps. The two-pulse photon echo decays exponentially with T₂ = 0.45 ns (blue circles). The three-pulse PE shows exponential decay with  a short time constant of T₁ = 0.26 ns superimposed on the long-lived offset (green triangles). Dashed red curves show the corresponding exponential fits.