Low-loss multilevel operation using lossy phase-change material-integrated silicon photonics

. Chalcogenide phase-change materials (PCMs) offer paradigms for programmable photonic integrated circuits thanks to their zero static energy and significant refractive index contrast. However, prototypical PCMs, such as Ge 2 Sb 2 Te 5 (GST), are lossy in their crystalline phase, albeit transparent in the amorphous state. Moreover, electrically switching PCMs to intermediate states is a stochastic process, limiting programming accuracy. As a result, achieving both low-loss and deterministic multilevel operations with GST remains challenging. Although low-loss PCMs, such as Sb 2 S 3 and Sb 2 Se 3 , have been discovered in recent years, they are much less technologically mature. We propose a design with multiple GST segments to overcome the challenge of deterministic multilevel operation. GST segments are individually controlled by interleaved silicon p++-doped-intrinsic-n+ +-doped diode heaters in a binary but reliable fashion, and multiple levels are encoded in their phase sequence. A 1 × 1 programmable unit with two unequal GST segments is experimentally demonstrated, showcasing four distinct operation levels and negligible thermal crosstalk with only one pair of metal contacts. We then extend the design to 1 × 2 and 2 × 2 programmable units. For the 2 × 2 programmable unit design, we propose a phase-detuned three-waveguide directional coupler structure to mitigate the absorption and radiation loss, showing < 1 . 2 dB loss and three splitting ratios. We provide a new path toward low-loss and multi-level optical switches using lossy PCMs.