  The switchable optical and electrical properties of phase change materials
(PCMs) are finding new applications beyond data storage in reconfigurable
photonic devices. However, high power heat pulses are needed to melt-quench the
material from crystalline to amorphous. This is especially true in silicon
photonics, where the high thermal conductivity of the waveguide material makes
heating the PCM energy inefficient. Here, we improve the energy efficiency of
the laser-induced phase transitions by inserting a layer of two-dimensional
(2D) material, either MoS2 or WS2, between the silica or silicon and the PCM.
The 2D material reduces the required laser power by at least 40% during the
amorphization (RESET) process, depending on the substrate. Thermal simulations
confirm that both MoS2 and WS2 2D layers act as a thermal barrier, which
efficiently confines energy within the PCM layer. Remarkably, the thermal
insulation effect of the 2D layer is equivalent to a ~100 nm layer of SiO2. The
high thermal boundary resistance induced by the van der Waals (vdW)-bonded
layers limits the thermal diffusion through the layer interfaces. Hence, 2D
materials with stable vdW interfaces can be used to improve the thermal
efficiency of PCM-tuned Si photonic devices. Furthermore, our waveguide
simulations show that the 2D layer does not affect the propagating mode in the
Si waveguide, thus this simple additional thin film produces a substantial
energy efficiency improvement without degrading the optical performance of the
waveguide. Our findings pave the way for energy-efficient laser-induced
structural phase transitions in PCM-based reconfigurable photonic devices.
