Robust design of a single-arm Sb2Se3 directional coupler on thin-film lithium niobate by Bayesian optimisation, with an integrated transparent microheater

A non-volatile directional coupler switch on thin-film lithium niobate is presented in which a single arm carries an Sb2Se3 film driven between its amorphous and crystalline states by an indium tin oxide (ITO) microheater 50 nm above the film. The coupler geometry was found by Bayesian optimisation scored on the worst corner of a fabrication-tolerance box rather than on nominal performance. The optimiser selects a film 33 nm thick rather than the 20 nm of a manually designed device, because the thicker film holds its extinction through a 2 nm thickness error where the manual design falls from 22 to 6 dB. The heater is then designed into the optical cross-section: it moves the phase-matched arm width by about 20 nm, and the re-trimmed coupler is verified in three-dimensional FDTD. The final device is 12.5 µm long and gives 26.2 dB extinction in its weaker state at 1550 nm, at least 13.1 dB over the whole tolerance box, and more than 15 dB over an 85 nm band, with excess insertion loss of 0.55 dB (cross) and 0.06 dB (bar). A closed flux balance shows where the cross-state loss goes: 3.8% is absorbed in the heater and about 8% is scattered at the film ends. Transient heat simulation gives crystallisation with a 10.7 mW, 2 µs pulse (21.5 nJ) and re-amorphisation with a 62.5 mW, 400 ns pulse (25.0 nJ) quenched at 0.93 K/ns.