“…Amongst the RE 3+ ions, Er 3+ , Ho 3+ , and Tm 3+ are used in most of the studies, as they are known to provide good emission intensity in the visible region of the electromagnetic spectrum. A list of RE ions in various host material [ 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 ] with their different UC emission bands upon 980 nm excitation is shown in Table 1 . The energy levels of Er 3+ — 4 I 15/2 , 4 I 13/2 , 4 I 11/2 , 4 F 9/2 and 2 H 11/2 / 4 S 3/2 are suitable for 800 nm or 980 nm laser light excitation.…”