Fig. 2: ML performance of MgO/MgF₂:Cr³⁺ heterostructures.

a Schematic illustration of the flexible composite ML film consisting of PET/LDPE layers embedded with MgO/MgF₂:Cr³⁺ phosphor. b Diagram of the custom-built ML measurement setup, comprising a mechanical rotation platform, pressure sensor, and NIR fiber optic spectrometer. c ML emission spectra of MgO/MgF₂:Cr³⁺ samples with varying MgO: MgF₂ molar ratios under 10 N load and 2000 rpm rotation speed. d Corresponding integrated ML intensities showing a maximum at a MgO: MgF₂ ratio of 0.4:0.6. Error bars represent standard deviations from ten repeated measurements. e ML intensity response under variable loads (5–15 N), showing load-dependent emission enhancement. f PSL intensity variation under different 980 nm laser powers (0.5–3.5 W), with insets showing corresponding emission spot images. g ML intensity profile under cyclic loading, demonstrating ML signal reproducibility and mechanical stability. h Repeated PSL response cycles under 1.0 W 980 nm laser excitation, demonstrating stable PSL behavior. Source data are provided as a Source Data file.