Fig. 1. Mid-infrared group-IV photonics and its applications

Fig. 2. Structural diagrams of germanium wafers and scanning electron microscope (SEM) images of germanium waveguides. (a) Structural diagrams of germanium wafers^[51-54]; (b) SEM image of GOS waveguide^[51]; (c) SEM image of GOSOI waveguide^[52]; (d) SEM image of GOSiN waveguide^[53]; (e) SEM image of GOI waveguide^[56]

Fig. 3. Germanium grating couplers. (a) SEM image of GOS grating coupler^[57]; (b) SEM image of GOS grating coupler with low-reflectivity^[65]; (c) SEM image of GOSOI grating coupler^[66]; (d) SEM image of GOI focusing subwavelength grating (SWG) coupler^[56]; (e) simulated and experimental results of GOI focusing SWG^[56]

Fig. 4. Germanium microring resonators. (a) SEM image of GOI vernier-effect microring resonator^[73]; (b) SEM image of GOI microring resonator^[67]; (c) SEM image of GOI high-Q factor microring resonator^[75]; (d) free spectral range measurement of GOI high-Q factor microring resonator^[75]; (e) Q factor measurement of GOI high-Q microring resonator^[75]

Fig. 5. Germanium photonic crystal cavities. (a) SEM image of germanium photonic crystal cavity^[81]; (b) simulated results of germanium photonic crystal cavity^[81]; (c) measurement results of germanium photonic crystal cavity^[81]; (d) SEM image of germanium photonic crystal nanobeam cavity^[82]; (e) measurement results of germanium photonic crystal nanobeam cavity^[82]

Fig. 6. Germanium waveguide active devices. (a) Structural diagram and SEM image of low-threshold germanium nanowire laser^[92]; (b) SEM image of germanium thermal phase shifter^[95]; (c) SEM image of germanium modulator with PIN diode^[98]; (d) SEM image of germanium all-optical modulator^[99]

Fig. 7. Germanium waveguide devices used for biochemical sensors. (a) Experimental principle of infrared absorption spectroscopic measurement of bovine serum protein (BSA) using germanium waveguide ^[104]; (b) system image for infrared absorption spectrum measurement of BSA and aggregates using germanium waveguide ^[105]; (c) schematic of infrared absorption spectrum measurement of BSA and aggregates using germanium waveguide ^[105]

Fig. 8. Nonlinear optical study of germanium waveguide devices. (a) Measurement results of two-photon absorption coefficient of GOS waveguide^[100]; (b) schematic of stimulated Brillouin scattering^[111]; (c) structural diagram of Raman laser^[113]; (d) experimental results of supercontinuum generation^[118]

Fig. 9. Simulated results of mid-infrared germanium Kerr frequency comb. (a) Structural diagram of germanium Kerr frequency comb model; (b) temporal and spectral curves of intra-cavity field with pump wavelength of 2.8 μm; (c) temporal and spectral curves of intra-cavity field with pump wavelength of 3.5 μm