  An optical equivalent of the field-programmable gate array (FPGA) is of great
interest to large-scale photonic integrated circuits. Previous programmable
photonic devices relying on the weak, volatile thermo-optic or electro-optic
effect usually suffer from a large footprint and high energy consumption. Phase
change materials (PCMs) offer a promising solution due to the large
non-volatile change in the refractive index upon phase transition. However, the
large optical loss in PCMs poses a serious problem. Here, by exploiting an
asymmetric directional coupler design, we demonstrate PCM-clad silicon photonic
1 \times 2 and 2 \times 2 switches with a low insertion loss of ~1 dB and a
compact coupling length of ~30 {\mu}m while maintaining a small crosstalk less
than ~10 dB over a bandwidth of 30 nm. The reported optical switches will
function as the building blocks of the meshes in the optical FPGAs for
applications such as optical interconnects, neuromorphic computing, quantum
computing, and microwave photonics.
