Hierarchical Structural-Coherent Inverse Design of a Guided MIM Dual-Window Spectral Power Processor

Abstract A guided Ag–TiO₂–Ag metal–insulator–metal (MIM) structure is investigated as a dual-window spectral power processor in which spectral placement and power allocation are treated as separate control tasks. The resonator geometry is first selected to establish the required short- and long-wavelength transmission windows, after which port activation and relative input phase are used to redistribute power without modifying the structure. A finite-element database of 27 geometries was used to construct a compact Gaussian-process surrogate for the structural inverse-design step, with the final candidate verified independently by full-wave FEM. For the representative 850/1310-nm case, the resulting geometry produced sampled transmission maxima at 850 and 1310 nm, while local interpolation located the resonances at approximately 850.44 and 1309.96 nm. Once the geometry was fixed, the port states provided distinct coarse weighting of the two windows. Continuous variation of the middle-port phase then shifted the operating condition from 1310-nm priority to near-exact equalization and finally to 850-nm priority. At ϕ=2.105 rad, FEM verification gave T850=0.74944 and T1310=0.74950, corresponding to a residual imbalance of about 0.00035 dB. Mesh convergence, field distributions, and ±2-nm dimensional perturbations were also examined. Unlike conventional geometry-fixed or phase-switched MIM devices, the proposed framework assigns spectral positioning and post-design power allocation to separate structural and coherent control stages on the same nanophotonic platform.