Upconversion luminescence of an insulator involving a band to band multiphoton excitation process.

A multiphoton process to the conduction band of the insulator Er2O3 is reported, which occurs in vacuum under near infrared excitation. The enormous upconversion intensity is two orders of magnitude greater than that in air, and also the intensity ratio for green and red emission bands is inverted. The mechanism is probed by experiments of laser power dependence, cathodoluminescence, photoconductivity dependence and chemical dilution, and it involves cross-relaxation energy transfer upconversion of erbium ions to reach the conduction band. The upconversion color is tunable by changes in pressure, laser diode intensity and wavelength, or dopant ion concentration.