Looking back at Eq. (1), the upconversion emission rate is directly proportional to the excitation rate. The excitation rate (per unit volume per second) in a linear photonic material system at a given point is equal to the product of absorption coefficient (per unit length, normally denoted by ) times photon flux (per unit area per second) at that point. In the case of two-photon upconversion, a second-order nonlinear medium, the absorption coefficient itself, is function of photon flux. In order to decouple the complexity of 3D-FDTD analysis from the complexity of a nonlinear and nonhomogenous medium, we use a narrowband effective medium approximation, using a normalized Lorentz-Lorenz formulation, described in 38, with an effective real refractive index of and an effective imaginary refractive index of , which relates to the absorption coefficient () by . For the PMMA medium dispersed with -doped nanoparticles, we use an value of 1.6.39 The value, however, is a function of the intensity of the electromagnetic field. As a result, we adopt a reference imaginary refractive index value of , which is reported for the low-power regime in 39, for the : 3% Er, 17% Yb material. For a nanoparticle/PMMA composite with a nanoparticle volumetric ratio of 20%, the effective medium approximation leads to effective and values of 1.501 and , respectively. We use as the base imaginary refractive index and use a normalization coefficient of such that . In this study, we sweep the values over a large window to assess the interaction of light–plasmon–nanoparticle as well as the light absorption by nanoparticles and plasmonic resonance. The reference value leads to an absorption coefficient of at 980 nm and penetration depth of 1.67 mm. Such small and values require a very thick (millimeter range) layer of upconversion material. This may not be a cost-efficient method for implementing an upconversion layer, as this layer uses a number of rare-earth elements in high concentration. As demonstrated in Sec. 4, field enhancement by plasmonic resonance can enhance upconversion efficiency; however, plasmonic field enhancement is confined to close proximity to the metal surface. We use a plasmonically enhanced upconversion process through nanostructured metal surfaces to assess whether efficient upconversion can be generated from upconversion layers with a thickness of the order of 1 μm. Figure 9 shows the structure of the volume simulated for absorption and reflection analysis, in which a 1-μm-thick homogenous upconversion layer covers the surface of the gold photonic crystal.