Asymmetric line shape of near-field luminescence spectrum induced by stress in Al₂O₃

The luminescence spectroscopy of Al₂O₃ is widely used for sensing the stress in Al₂O₃based materials. One promising approach that improves the spatial resolution of luminescence spectroscopy is the use of a near-field optical system, which is effective for assessing the localized stress in nano/microsize structural components. Here, we perform the spectral analysis of near-field excited luminescence for stressed Al₂O₃, which shows an abnormal asymmetry in line shape, which is not observed in a conventional far-field measurement. For analysis, we employ two models that include different effects. The model of stress-gradient-based spectrum broadening fails to account for the asymmetry of the luminescence spectrum. In contrast, the model including vibronic state transition yields good matching of the measured spectrum line shape, indicating that the electron-phonon coupling in Al₂O₃ affects the appearance of the asymmetric line shape. The magnitude of electron-phonon interaction as well as the effect of the excitation method on the line shape are discussed.