Nonlinear cathodoluminescence from insulators

White emission of high luminance is observed at room temperature for powder layers of various insulators under intense electron-beam irradiation. This emission appears at a threshold current density, typically about 0.02–0.2 mA/cm2 at 25 kV accelerating voltage. Its intensity increases steeply with an increase in the beam current. Spectral distribution of the emission is very broad with a peak in the near-infrared region. However, the nonlinear current dependence of the luminescence intensity indicates that the emission is not thermal radiation. This emission was observed for some phosphors, e.g. SrHfO3:Tm, Y2O3:Eu and Y2O2S:Eu, and also for materials without activator ions, but not for materials with relatively high conductivity, such as ZnO or indium tin oxide. The emission appears strongly at a low threshold on edges or rims of grooves or holes made on a screen. These results indicate that high electric field induced by charges on insulator tips gives rise to the emission.