Discovery of Compositional Design Windows in Ge–Sb–Te–S–Se Phase-Change Materials Enabling Programmable Nonlinear Activation Functions in Integrated Photonics

Optical phase-change materials (OPCMs) are promising nonvolatile tuning media for photonic integrated circuits, yet at telecom wavelengths their practical use remains constrained by the trade-off between refractive-index modulation and optical absorption. Here, we establish a high-throughput combinatorial library across the Ge–Sb–Te–S–Se quinary compositional space and construct composition-optical constant maps that reveal device-relevant compositional windows for simultaneously targeting strong index modulation and low loss. At 1550 nm, the maps identify a high-contrast window (Δ n ≈ 1.5–1.7, k c ≈ 0.13–0.27) and a low-loss window ( k c ≈ 0–0.06 with Δ n ≈ 0.9–1.2), thereby enabling application-oriented materials selection. From the high-contrast window, we select Ge 14.6 Sb 29.4 Se 35.3 S 11.3 Te 9.4 (GSSST-1), which switches at 1550 nm from ( n, k ) = (3.099, ∼0) to (4.721, 0.227). When integrated into hybrid silicon microring resonators (MRR), GSSST-1 yields a crystalline propagation loss of 0.282 dB/μm, corresponding to an ∼26.9× reduction relative to a same-thickness GST225 top-cladded MRR, while supporting 22 distinguishable intermediate states. Moreover, under single-wavelength readout, multilevel state programming enables five nonlinear transfer functions from the same device (ReLU, Softplus, ELU, RBF, and Half-sigmoid), which improve FashionMNIST classification performance relative to a linear baseline. Overall, this work connects high-throughput materials discovery with resonator-level benchmarking and programmable nonlinear photonic functionality, providing a materials-device codesign basis for low-loss OPCMs in scalable integrated photonic computing and reconfigurable circuits.