Photonic devices with dynamic and continuously programmable states are important for optical computing, beam steering, and displays. A promising material platform is phase-change materials (PCMs), as these materials can be dynamically tuned to multiple optical levels by controlling the transition between their amorphous and crystalline phases. To design PCM-based optical switches with efficient and accurate multi-level control, the amorphous state dependent phase-transition kinetics must be well understood.

In this work, we demonstrate that the degree of structural disorder strongly effects the recrystallisation kinetics using modelling and experiments. Firstly, we show using a thermo-optical phase-change model that partially disordered states can be accessed in Ge₂Sb₂Te₅ by partial melting the nanocrystalline microstructure. Subsequent rapid quenching freezes-in the partially molten phase, with optical properties that depend on the energy applied during melting. The theory is supported by nanosecond laser switching where 16 different partially amorphized states are written into Ge₂Sb₂Te₅. Secondly, we study how the phase transitions in Sb₂Te₃ depend on crystallographic orientation and its degree of disorder. We find that the level of disorder in Sb₂Te₃ strongly depends on the laser pulse energy used to amorphize the crystal. Interestingly, the recrystallization rate tends to be faster when the initial state is more disordered, indicating that additional crystallization pathways may be created by the higher energy pulses.

Our results highlight the complex crystallization behaviour of phase-change materials and that crystallisation strongly depends on the way the amorphous state is prepared. These findings could potentially be utilized to design efficient PCM devices with fine control of the optical properties and that can switch at higher rates.