1. A taggant comprising: an inorganic crystal lattice which comprises at least a first rare earth active ion and a second rare earth active ion; wherein the first rare earth active ion absorbs exciting incident infrared radiation having a first wavelength of about 1300 nm to about 2200 nm, and transfers the absorbed infrared radiation to the second rare earth active ion, and the second rare earth active ion emits infrared radiation at a second wavelength of about 1400 nm to about 2200 nm that is greater than the first wavelength of the exciting incident infrared radiation.