Fig. 3: Multimodal ML performance and mechanism of MgO/MgF₂:Cr³⁺ heterostructures.

a ML emission spectra and corresponding integrated intensities of MgO/MgF₂:Cr³⁺ (MgO: MgF₂ = 0.4:0.6) under different rotation speeds (2000–6000 rpm). Error bars represent standard deviations from ten repeated measurements. b Photographs of ML emissions at varying rotation speeds and applied loads, demonstrating bright NIR luminescence under increasing mechanical stimuli. c The time-load triangle wave and their corresponding ML intensities under different load. Histogram of ML intensity as a function of load, showing a positive correlation. d Time-lapse images of ML response under 10 N load, captured from 0.5 to 2.5 s, showing temporal evolution of ML intensity. e ML intensity stability during 10 consecutive cycles under repeated triangular load input, confirming excellent mechanical durability and repeatability. f Piezoelectric amplitude–bias curves of MgF₂:Cr³⁺ and MgO/MgF₂:Cr³⁺, both displaying butterfly-shaped hysteresis loops, indicative of intrinsic piezoelectric behavior. g Real-time output current and voltage signals of MgO/MgF₂:Cr³⁺ under 5 N cyclic loading, supporting its self-powered sensing capability. h Long-term cyclic ML performance at 700 rpm roation speed and 7 N load, showing consistent emission over >1000 cycles. Inset: Image of the mechanical measurement setup. i Comparison of ML generation rates among various NIR ML materials. MgO/MgF₂:Cr³⁺ heterostructure exhibits the quickest ML generation rate, indicating its capability for real-time dynamic sensing and fast-response applications. Source data are provided as a Source Data file.