“…The behavior that all slopes are close to 2.5 is attributed to the effect of self-focusing instead of cross relaxation process. 25 The studies on the optical characteristics of Er:LiNbO 3 crystals codoped without and with In 3+ ions indicate that the n values of 2.2 and 2.3 are considered as the two-photon process involved in the UC mechanism responsible for populating the green emission in Er:LiNbO 3 crystals codoped with and without the In 3+ ions. 26 Consequently, the two-photon process is only considered for populating the green and red UC emissions in the Er:CLN crystal here.…”