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 nonvolatile 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 nonvolatile PCM-clad silicon photonic 1 × 2 and 2 × 2 switches with a low insertion loss of ∼1 dB and a compact coupling length of ∼30 μm 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.
Design of the 2 × 2 DC switch. (a) Schematic of the switch. (b, c) Normalized optical field intensity distribution in the device for (b) aGST and (c) cGST simulated by the 3D eigenmode expansion method (Lumerical) at 1550 nm.
Experimental results of the 2 × 2 DC switch. (a) Optical microscope image of the fabricated switch. (b) SEM image of the switch. (c) An enlarged view of the coupling region highlighted by the orange rectangle in (b) with the GST false-colored. (d, e) Measured transmission at the cross and bar ports with the GST in the (d) amorphous and (e) crystalline states.