Fig. 1. Phase change mechanism, bonding mechanism, and optical properties of PCM. (a) Phase change mechanism; (b) bonding mechanism^[36]; (c) real and imaginary parts of the dielectric constants of PCM^[38]

Fig. 2. Photonic waveguide design and optical switching^[21]. (a) GST waveguide device; (b) SEM image; (c) write-erase operation; (d) multilevel storage

Fig. 3. Neural synaptic devices and arrays. (a) Neural and synaptic structures^[22]; (b) GST-based integrated photonic neural synaptic device^[22]; (c) all-optical neural network architecture design^[27]; (d) optical microscopic image of neural network with optical waveguides^[27]

Fig. 4. All-optical analog-to-digital conversion system based on GST-decorated silicon photonic device^[48]. (a) Photonic analog-to-digital converter chip; (b) cross-sectional view of phase-change waveguide device; (c) 65 distinct levels

Fig. 5. Electrically controlled photonic computing architecture^[29]. (a) Schematic of electrically controlled photonic waveguide memory cell; (b) multilevel switching; (c) parallel computing using GST cell; (d) image processing results

Fig. 6. Ultrafast photonic waveguide memory based on SST. (a) Switching speeds of phase-change random-access memory devices based on SST and GST^[50]; (b) crystalline state of SST at 0 ps^[50]; (c) crystalline state of SST at 600 ps^[50]; (d) schematic diagram of SST-based photonic waveguide memory^[57]; (e) multilevel storage^[57]; (f) cycling test^[57]

Fig. 7. Biphasic neural synaptic device based on AIST. (a) Crystallization growth at the amorphous-crystalline interface of AIST^[62]; (b) schematic diagram of photonic synapse, which is composed of the AIST dual devices (AIST 1 and AIST 2) on the Si₃N₄ waveguide^[64]; (c) optical microscopic images of AIST devices (with scale bar of 2 μm)^[64]; (d) biphasic storage behavior of single-node dual AIST devices^[64]

Fig. 8. First-principle calculation and simulation of optical waveguide device based on Sb₂Te. (a) Atomic ordering process and crystal structure of Sb₂Te (yellow spheres represent Sb atoms and green spheres represent Te atoms)^[70]; (b) HAADF image of Sb₂Te and EDS map of Sb and Te atoms^[69]; (c) transmission spectra of waveguide device in amorphous phase (amor.), crystalline phase (cryst.), and ground state (A7)^[70]; (d) electric field profiles of waveguide^[70]

Fig. 9. Photonic waveguide devices based on ultra-thin Sb. (a) Schematic diagram of SOI waveguide integrated with 3 nm-thick Sb^[75]; (b) schematic diagram of microwave photonic filter based on Sb thin film^[76]; (c) reversible and repetitive switching of 10 µm-long Sb thin film between amorphous state and crystalline state^[76]; (d) multilevel storage^[76]

Fig. 10. Ultra-low loss photonic waveguide devices based on novel PCM. (a) Refractive indices of amorphous GSST^[77]; (b) refractive indices of crystalline GSST^[77]; (c) cumulative crystallization of electrically controlled GSST device^[31]; (d) refractive indices of amorphous (dashed line) and crystalline (solid line) Sb₂Se₃^[84]; (e) electrically controlled phase shifter based on Sb₂Se₃^[85]; (f) transmission spectra of MZI based on Sb₂Se₃^[85]