Control of the cool/warm white light generation from lanthanide ions in monoclinic double tungstate crystals

We studied the physical effects arising from the combination of several concentrations of luminescent ions, Ho, Tm and Yb in a biaxial crystal to achieve the optimum multicolour emission for cool or warm white light generation. White light is generated by the simultaneous emission and combination of three photons at wavelengths, 475 (blue), 542 (green) and 651 nm (red) with associated energies, 21,053 cm⁻¹, 18,450 cm⁻¹ and 15,361 cm⁻¹, respectively. The physical mechanism that generated the white light involved the absorption of infrared photons and after partial energy transfer to neighbouring ions, excited state absorption and upconversion phenomena took place to combine the emerging photons at the above wavelengths. White light is generated with rather high efficiency and very low excitation threshold. The control of simultaneous light generation is accomplished with the amount of dopant ions in the crystal.