Nonlinear optical (NLO)Optical nonlinear characterization techniques have emerged as essential tools to investigate the optical behavior of advanced materials, especially those doped with REIs. These materials exhibit a wide range of photonic applications, including laser systems, optical amplifiers, up-conversionUp-conversion devices, and optical limiters. Although linear optical measurements provide valuable information under low-excitation conditions—such as absorption spectra, emission profiles, and refractive indexRefractive index—nonlinear techniques enable deeper exploration of intensity-dependent optical phenomena. Among the various NLO methods, the Z-scanZ-scan technique stands out as a powerful and widely used tool to investigate third-order nonlinearities in solid-state materials, glasses, crystals, and nanostructures. Introduced by Sheik-Bahae and colleagues in 1990, the Z-scanZ-scan technique allows the simultaneous determination of the nonlinear refractive indexRefractive index ( \({n}_{2}\) ​) and the nonlinear absorption coefficient, using a single focused laser beam and a relatively simple experimental setup. In rare-earth-doped systems, the Z-scanZ-scan technique is particularly useful due to the distinctive electronic configuration of REI. Their partially filled 4f orbitals are shielded from the crystal field by outer 5 s and 5p electrons, leading to sharp, well-defined energy levels that enable a variety of optical transitions. Under high-intensity excitation, these transitions can give rise to nonlinear responses through mechanisms such as multiphoton absorption, excited-state absorption, energy transferEnergy transfer up-conversionUp-conversion, and changes in population distributions among energy levels. In this chapter, we focus specifically on the origin of the nonlinear refractive indexRefractive index associated with changes in the population of excited states induced by REI doping—a mechanism often referred to as the population lens effectLens effects. This phenomenon arises when intense laser irradiation modifies the distribution of ions between the ground and excited states, leading to a spatially varying refractive indexRefractive index. As a result, the material behaves like a self-induced lens, whose focusing or defocusing action depends on the population dynamics of the dopant ions. This chapter introduces the fundamental concepts of nonlinear optics relevant to rare-earth-doped materials and presents the theoretical and experimental foundations of the Z-scanZ-scan technique, with an emphasis on the population lens effectLens effects. We discuss both open-aperture (OA) and closed-aperture (CA) Z-scanZ-scan configurations and explore how this technique can be applied to extract third-order susceptibility parameters. Examples from recent studies will be presented to demonstrate the utility of the Z-scanZ-scan technique in understanding nonlinear refractive behavior driven by excited-state population effects, thereby supporting the design of next-generation photonic materials and devices.