Fig. 1. (a) Thermo-optical (TO) modulator based on Ge-on-SOI waveguide platform ^[6]; (b) Si-on-sapphire TO modulator with PhC (photonic crystal) waveguide^[7]; (c) TO modulator based on SOI with spiral waveguides arms. Inset shows the heater ^[8]; (d) 2×2 MZI TO switch^[10]and; (e) Dual mode TO switch based on SOI MZI structure at 2 μm band^[11]

Fig. 2. Thermo-optic modulator with doped-silicon heater^[12]. (a) Microscope image, (b) the static response and (c) the dynamic response of the MZI modulator; (d) Microscope image, (e) the static response and (f) the dynamic response of the MRR-based modulator

Fig. 3. (a) Diagrams of silicon photonic circuits integrated with LiNbO₃ thin plate, wafer, single die and basic circuit and the cross-section illustration of bonding^[16]; (b) Integrated Ta₂O₅ -LiNbO₃ rib waveguide^[17]; (c) Mid-infrared electro-optic modulator based on Si-on-LiNbO₃ waveguide^[18]

Fig. 4. Changes of (a) refractive index and (b) absorption coefficient of silicon and germanium in the Mid--IR caused by free carrier; (c) Wavelength scaling of −Δn/Δk^[23], the carrier concentration is fixed at 5×10¹⁷ cm⁻³

Fig. 5. (a), (b) carrier-injection modulators^[24,26] and (c) carrier-depletion modulators based on SOI ^[27]; (d) Electro-optic modulator and electro-absorption modulator on Ge-on-Si ^[28]

Fig. 6. Graphene-chalcogenide modulator^[4]. (a) The structure and working principle of the graphene Mid-IR waveguide modulator; (b) Measured and (c) simulated modulation depth of the device versus wavelength and bias （Unit: dB/mm）

Fig. 7. Performance of Mid-IR graphene-chalcogenide modulators^[38]. (a) The Fermi-level-related optical absorption of a graphene layer across the Mid-IR band evaluated by the surface conductive model; (b) Simulation result of modulation depth for graphene mid-IR electro-absorption modulators （The white dashed line is zero modulation depth）; (c) FOM (modulation depth/insertion loss) of the modulator （The black dashed line represents unity FOM）

Fig. 8. Mid-IR extrinsic silicon photodetectors. (a) B doped SOI detector; (b) SEM of the same device (Inset: Schematic diagram of a silicon waveguide); (c) Relationship between the reverse bias voltage and the photocurrent/response of the detectorat two micron wavelength range; (d) Eye diagram of the detector with voltage of 27 V^{[56, 69]}

Fig. 9. Mid-infrared Black Phosphorus(BP) detector. (a)-(b) Structure of the silicon-based BP slow-light integrated detector; (c) Relationship between the power and the responsivity of the BP detector at three different wavelengths; (d) Current noise power density of the photonic crystal waveguide and the subwavelength grating waveguide BP photodetectors, respectively^[52]

Table 1. Hybrid integrated photodetectors with different active materials on silicon