Fig. 2. Tuning of an MZI by switching of an ultralow-loss PCM for selective optical phase control.

(A and B) Spectral response of an MZI during phase tuning using laser annealing of a cladding PCM from the initial state (blue) through intermediate states (gray and black) to the final spectrum (red), vertically offset (top), and normalized to the initial state (bottom). A reversible 2π phase shift (A) and 10π tuning (B) with no change to modulation depth or insertion loss. (C) Transmission of an MZI for a fixed wavelength of 1553 nm, as a 50-μm patch is amorphized (blue) and recrystallized over 20 μm (red), followed by a second amorphization (blue), step size 500 nm per data point. Inset: Optical microscope image of the Sb₂Se₃ patch. Scale bar, 20 μm. (D) Transmission through an MZI as a single spot is amorphized and recrystallized 600 times (top). The same data with the drift normalized using a 50-point moving average (bottom). Photo credit: Ioannis Zeimpekis, University of Southampton.