Up‐Conversion from Er 3+ –Er 3+ Pairs in CdF 2 Crystals

The effect of preferential clustering on the properties of infrared to visible conversion of Er 3+ (between 1 to 4 mol%) is investigated in CdF 2 at 77 K. Without preferential clustering, the green emission is predominant. If there is preferential clustering, the red emission is predominant. This emission mainly results from the non‐resonant cross relaxation process in an Er 3+ –Er 3+ pair doubly excited in the state ( 4 I 11/2 , 4 I 13/2 ). The efficiency of this up‐conversion process is two orders of magnitude higher than that of the single Er 3+ sites. It is suggested to use the up‐conversion as a technique to inquire about the degree of association of the Er 3+ ions in the host lattices.