Fig. 1. Band structures of different materials. (a) Si; (b) Ge

Fig. 2. QCSE in Ge/SiGe quantum well. (a) Without electric field; (b) with electric field applied

Fig. 3. Epitaxial structure and test results of Ge/SiGe multiple quantum wells. (a) Epitaxial structure; (b) absorption spectra of Ge/SiGe quantum wells^[45]

Fig. 4. Schematic diagram of oblique incidence photoelectric modulator^[49]

Fig. 5. Side oblique incidence Ge/SiGe multiple quantum well modulator. (a) Absorption spectrum at 100 ℃; (b) schematic diagram of the structure^[50]

Fig. 6. Waveguide type Ge/SiGe multiple quantum well electro-absorption modulator. (a) Epitaxial structure; (b) SEM image; (c) absorption spectra at different voltages; (d) extinction ratio at different operating voltages^[28]

Fig. 7. Test results of Ge/SiGe multiple quantum wells. (a) Absorption spectrum; (b) extinction ratio at different working voltages^[31]

Fig. 8. Absorption spectra of Ge/SiGe multiple quantum wells. (a) Absorption spectra of Ge/SiGe multiple quantum wells with different well widths; (b) absorption spectra of Ge/SiGe multiple quantum wells at different voltages^[29]

Fig. 9. Structure and simulation results of low bias voltage Ge/SiGe multiple quantum well electroabsorption modulator. (a) Structural design; (b) TM polarization absorption coefficient; (c) TE polarization absorption contrast at different voltages^[51]

Fig. 10. Simulation results of Ge/SiGe multiple quantum well electro-absorption optical modulator. (a) Waveguide structure; (b) overall structure; (c) photocurrent response at different voltages when uniaxial tensile strain is introduced; (d) photocurrent contrast at different voltages^[52]

Fig. 11. Structure of the modulator. (a) Epitaxial design of multiple quantum wells; (b) overall structure of the device; (c) distribution of biaxial tensile strain^[53]

Fig. 12. Electroabsorption light modulation extinction ratio of the device under different reverse voltages. (a) 0 V/2 V; (b) 0 V/4 V; (c) high frequency response characteristic curve of the device under TE polarization; (d) high frequency response characteristic curve of the device under TM polarization^[53]

Fig. 13. Test results of Ge/SiGe multiple quantum wells under different bias voltages. (a) Absorption spectrum; (b) secondary electro-optic coefficient^[54]

Fig. 14. Test results of Ge/SiGe asymmetrically coupled multiple quantum wells. (a) FP interference comb spectra at different bias voltages; (b) optical power spectra at different bias voltages; (c) modulation extinction ratios at different operating voltage swings; (d) normalized high-frequency response curve^[57]

Table 1. Performance of Ge/SiGe multiple quantum well modulators based on electroabsorption effect

Table 2. Performance comparison of Ge/SiGe multiple quantum well modulators based on electrorefractive index effect