Compositional variations in optical characteristics of Mn doped spinel crystals

Mn doped spinel (MgAl 2 O 4 ) crystals have been grown by floating zone (FZ) techniques with various conditions of O 2 concentrations in the growth atmosphere from 0 to 75 vol%, with a starting composition of molar ratio x= MgO/(MgO+Al 2 O 3 ) from 0.3 to 1.0 and/or Mn concentrations from 1.0 to 6.0 at%. Optical absorption spectra and photoluminescence spectra were evaluated using crystals grown under various growth conditions. The color of the crystals is found to vary with their composition and the O 2 concentration in the atmosphere. Crystals grown under a 100 vol% Ar atmosphere, were a pale green color and emitted a strong green luminescence at λ =520 nm. Colors of stoichiometric crystals, x =1.0, and Mg-poor crystals grown under an oxidizing atmosphere are yellow and red, respectively. Both optical absorption spectra and photoluminescence spectra of Mg-poor, x 2 –Ar) atmosphere are found to vary greatly from those of the stoichiometric crystals.