Fig. 1. 3D integration enables ultralow-noise isolator-free lasers in silicon photonics^[22]

Fig. 2. Progress of silicon nitride based photonic integration, with emphasis on demonstration marked by release year^[44]

Fig. 3. Schematic of the smart-cut process for LNOI wafer^[45]

Fig. 4. Schematic diagrams of crystal structure of typical 2D materials^[55]. (a) Graphene; (b) transition metal dichalcogenides; (c) black phosphorus; (d) hexagonal boron nitride

Fig. 5. Process flow classification of heterogeneous integration technology

Fig. 6. Wafer bonding^[21]. (a) Schematic diagram of the key components of a wafer bonding machine; (b) types of wafer bonding processes currently used in semiconductor technology

Fig. 7. Flow chart of wafer bonding process^[21]. (a) Direct (O₂ plasma-assisted/SiO₂ covalent) wafer bonding; (b) intermediate layer wafer bonding (DVS-BCB)

Fig. 8. Schematic diagram of micro-transfer technology^[85]

Fig. 9. On-chip heterogeneous integrated optical waveguides. (a) Lithium niobate-amorphous silicon optical waveguide based on monolithic integration^[90]; (b) silicon-lithium niobate optical waveguide based on wafer bonding^[91]; (c) lithium niobate-silicon nitride optical waveguide based on monolithic integration^[92]; (d) silicon nitride-lithium niobate optical waveguide based on wafer bonding^[93]; (e) lithium niobate-polymer optical waveguide based on monolithic integration^[95]

Fig. 10. On-chip heterogeneous integrated passive devices. (a) Silicon nitride-αSi grating coupler^[98]; (b) silicon nitride-assisted edge coupler^[99]; (c) arrayed waveguide grating based on polymer loaded lithium niobate^[100]; (d) silicon nitride-loaded lithium niobate-based power beam splitter^[101]; (e) wavelength-division multiplexer (WDM) based on subwavelength grating^[102]; (f) mode multiplexer^[103]; (g) polarization beam splitter rotator^[103]; (h) polarizer based on subwavelength grating^[104]; (i) polarization rotator^[105]

Fig. 11. On-chip heterogeneous integrated lasers. (a) Pokels laser fabricated by horizontal aligning technique^[107]; (b) on-chip laser fabricated by horizontal aligning technique^[109]; (c) LN-Ⅲ-Ⅴ heterogeneous integrated laser fabricated by photonic wire bonding^[110]; (d) narrow linewidth laser through Ⅲ-Ⅴ/Si/Si₃N₄ multi-layer bonding^[111]; (e) high speed evanescent quantum-dot lasers on silicon based on wafer bonding^[113]; (f) silicon Ⅲ-Ⅴ heterogeneous integrated lasers fabricated using wafer bonding^[114]; (g) DFB laser implemented on silicon substrate using micro-transfer technology^[116]; (h) Ⅲ-Ⅴ DFB laser implemented on a silicon substrate by using micro-transfer technology^[117]; (i) Ⅲ-Ⅴ on lithium niobate substrates using micro-transfer technology lasers^[118]; (j) 1.3 μm quantum dot DFB lasers directly grown on silicon substrates^[120]; (k) InP sub-micron line and large size InP film lasers selectively grown on SOI wafers^[121]; (l) direct growth of group Ⅲ-Ⅴ quantum dot lasers on SOI trenches^[122]

Fig. 12. On-chip heterogeneous integrated EO modulators. (a) Germanium-silicon electro-absorption modulator^[123]; (b) silicon-based graphene electro-absorption modulator^[124]; (c) silicon-graphene-hBN electro-absorption modulator^[125]; (d) silicon-lithium niobate heterogeneously integrated EO modulator^[96]; (e) silicon-lithium niobate heterogeneous EO modulator^[127]; (f) silicon nitride-loaded lithium niobate heterogeneously integrated EO modulator^[92]; (g) silicon nitride-lithium niobate heterogeneous integrated EO modulator based on micro-transfer technology^[129]; (h) lithium tantalate-amorphous silicon topological photonic heterogeneous integrated EO modulator^[130]; (i) silicon nitride-Ferroelectric PZT thin film heterogeneous integrated EO modulator^[131]; (j) silicon-BTO heterogeneous integrated EO modulator^[133]; (k) BTO-silicon nitride heterogeneously integrated micro-ring modulator^[135]; (l) silicon-polymer heterogeneously integrated racetrack micro-ring modulator^[136]

Fig. 13. On-chip heterogeneously integrated detectors. (a) First high-speed waveguide-coupled photodetector based on germanium-silicon heterogeneous integration^[139]; (b) silicon germanium photodetector with bandwidth up to 265 GHz^[140]; (c) high-speed waveguide-coupled Ge/Si impedance resonance APD^[141]; (d) Ⅲ-Ⅴ type photodetectors grown directly on InP-SOI platforms parallel to the silicon device layer^[143]; (e) InGaAs bonded to thin-film lithium niobate single line-wave photodiode^[145]; (f) plasma-enhanced waveguide integrated graphene photodetector^[147]; (g) hybrid silicon-BP waveguide photodetectors^[148]; (h) Van der Waals heterostructured photodiodes consisting of BP and MoS₂ layers^[149]

Fig. 14. On-chip heterogeneously integrated photonic systems. (a) Silicon-silicon nitride heterogeneous integrated OPA based on PN junction phase shifter^[151]; (b) silicon-silicon nitride heterogeneous integrated OPA based on TO phase shifter^[152]; (c) lithium niobate-silicon nitride hybrid-integrated EO OPA^[154]; (d) fully integrated hybrid microwave photonics receiver^[156]; (e) fully integrated microwave photonics filter^[157]; (f) lithium niobate-silicon nitride heterogeneous integrated microwave photonic multiparameter measurement system; (g) mode-locked laser with wafer bonding integration of silicon nitride and Ⅲ-Ⅴ semiconductor optical amplifier wafers^[161]; (h) Ⅲ-Ⅴ-LN heterogeneous integrated microcomb lasers^[163]; (i) platform-independent lasers utilizing micro-transfer technology^[164]; (j) optical neural network based on phase change materials^[167]; (k) germanium-based silicon photodiode-based nonlinear activator^[169]; (l) MoTe₂-based all-optical nonlinear activator^[171]