shows that the core-shell monolith induces a spatially heterogeneous near-field absorption profile. The absorption profile is normalized to the average value of the average planar absorption of a null structure, or an SiO2 monolith without any nanosphere or TiO2 layers included. Figure compares the FDTD-simulated, total integrated absorption in the GaAs layer for a 350 nm core-shell monolith for three different base layer/shell layer thicknesses: 0 nm/0 nm, 200 nm/200 nm, 400 nm/400nm. The absorption spectra for each core-shell monolith were compared to a planar monolith that includes the equivalent thickness of TiO2, though without the nanosphere structuring. Even though the PS nanosphere diameter is fixed in these simulations, the resonant wavelengths and corresponding absorption enhancements were dependent on the base layer and shell layer thicknesses. For example, the 200 nm/200 nm core-shell monolith (Figure ) exhibited a sharp resonance at λ = 550nm that generated roughly 30 times more absorption than the planar monolith. The absorption at λ = 550nm in the 400 nm/400 nm core-shell monolith (Figure ) was roughly half that observed for the planar monolith with the same total TiO2 thickness. Given the observed relationship between the monolith structure and the resonances, we studied the converse question: What is the optimal core-shell monolith structure to 0 nm base layer 0 nm shell layer 200 nm base layer 200 nm shell layer 400 nm base layer 400 nm shell layer maximize absorption in a thin film absorber for a targeted wavelength? Specifically, we used FDTD simulations to identify the optimal theoretical core-shell monolith structure for maximizing absorption in the 40 nm GaAs-on-Si (Figure ) absorber at λ = 450 nm.