Benchmarking machine-learning surrogates for rapid prediction of the reflectance-defined stopband width of finite dielectric Bragg reflectors

Predicting the stopband width of finite one-dimensional dielectric Bragg reflectors requires a transfer-matrix calculation followed by spectrum-level edge extraction for each candidate stack. Although an individual wavelength-vectorized TMM evaluation is inexpensive, this calculation is repeated throughout large screening campaigns. Five supervised regressors are benchmarked as surrogates for the reflectance-defined stopband width Δλ Stop of finite multilayers at normal incidence, in the lossless, non-dispersive limit. TMM spectra were generated for stacks parameterized by the refractive indices n L and n H , the layer thicknesses d L and d H , and the number of bilayers N. For each spectrum, Δλ Stop was taken as the widest continuous interval with R(λ) ≥ 0.95. Designs with no such interval, or with a widest interval below 5 nm, were excluded, retaining 1660 of 2000 sampled designs. Hyperparameters of RF and SVR were selected by 10-fold cross-validation on the 1411-structure development set, while MLP depth was examined from one to ten hidden layers under the same test-isolated protocol. The selected six-hidden-layer MLP (128,64,32,16,16,16) achieved R 2 = 0.9862, RMSE = 10.47 nm, and MAE = 5.92 nm on the independent 249-structure test set. On a structured external set of 120 quarter-wave designs from four dielectric pairs, the final MLP achieved R 2 = 0.9688, RMSE = 5.67 nm, and MAE = 4.44 nm against the TMM reference; 94.17% of the cases had absolute errors below 10 nm and all 120 cases remained below 20 nm. Under an accuracy-controlled benchmark, the wavelength-vectorized TMM required 0.856 ms per structure at the selected 201-point spectral grid, whereas end-to-end single-sample inference with the final MLP, including input standardization, required 0.328 ms, corresponding to a 2.60× speedup. The surrogate is intended as a bounded-domain screening tool complementing, not replacing, full TMM analysis.