Fig. 4: Lattice heating and thermal relaxation of the metastable (4 × 1) phase, driven by optical excess energy.

a, Integrated (8 × 2) diffraction spot intensity as function of pump–probe delay Δt at incident photon energies of 0.8 eV and 1.55 eV on a stepped wafer. The light polarization was chosen parallel and perpendicular to the nanowire orientation, respectively, corresponding to the maximum switching efficiency at fluences of F_0.8 = 1.95 mJ cm⁻² and F_1.55 = 3.12 mJ cm⁻². For identical intensity suppression, the relaxation into the (8 × 2) ground state accelerates with photon energy due to increasing thermal lattice fluctuations, following the phase transition (see the inset). b, Corresponding (4 × 1) diffraction spot intensity. Increased lattice heating at 1.55-eV excitation manifests in an intensity reduction due to the Debye–Waller effect (ΔI_DBW). c, Pump-induced dynamic disorder increases the diffuse background intensity, counteracted by a reduction in the static disorder from enhanced phase homogeneity. d, Left: real-space sketch of indium atomic wires in the (8 × 2) phase, showing characteristic phase defects (alongside phase boundaries⁴⁶), which cause diffuse background scattering. Right: the photoinduced (4 × 1) structure exhibits an increased phase homogeneity. Thermal lattice fluctuations cause an accelerated relaxation for increasing photon energy.