Nonlinear photoresponse of NaYF4:Yb,Er@NaYF4 nanocrystals under green CW excitation: a comprehensive study

One of the efficient and well-known upconverting nanomaterials is NaYF₄:Yb,Er@NaYF₄, which emits photoluminescence at 545 nm and 660 nm under an excitation of 980 nm. Here, the nonlinearity of β-NaYF₄:Yb,Er@NaYF₄ at 532 nm is investigated using three nonlinear approaches. For the first time, the nonlinear optical conjugation of NaYF₄:Yb,Er@NaYF₄ nanocrystals is observed using the degenerate four-wave mixing method. In the optical bistability study, the optical hysteresis of NaYF₄:Yb,Er@NaYF₄ is measured using the Mach–Zehnder interferometer nonlinear ring cavity, and the results of bistability loops show different behaviors at different power regimes. Finally, the Z-scan technique is used for determining the nonlinear absorption and refraction coefficients, which are calculated in the order of 10⁻⁴ (cm W⁻¹) and 10⁻⁸ (cm² W⁻¹), respectively. The results indicate that by increasing incident powers, optical behaviour changes in both optical bistability and Z-scan. Therefore, the results exhibit that the β-NaYF₄:Yb,Er@NaYF₄ nanocrystals have nonlinear photoresponses at both 980 and 532 nm, which could be promising for photonic devices based on NIR light and visible light.

NaYF₄:Yb,Er@NaYF₄ is an efficient and well-known upconverting nanomaterials at 980 nm, also it has strong optical nonlinearity at 532 nm related to energy states of the Yb/Er system which is determined by a unique approach.