Field-induced modulation of absorption in waveguides with InGaN quantum well active region

Bias-dependent modal absorption and gain in InGaN/GaN multi-section Fabry–Pérot ridge laser diodes emitting between 415 and 470 nm are investigated. Optical gain spectra are extracted using the Hakki–Paoli method, allowing the modal absorption of the absorber section to be determined under forward and reverse bias conditions. The absorption decreases with increasing reverse bias and reaches a minimum near the flatband condition, where the applied field compensates the internal polarization field and reduces the quantum-confined Stark effect (QCSE). The extracted flatband voltage shifts toward larger reverse bias for longer emission wavelengths. In addition, the transparency-energy shift increases with emission wavelength, indicating stronger QCSE in higher-indium-content quantum wells. These results provide insight into field-induced modulation of optical transitions in InGaN quantum wells and differential absorption as a figure of merit for electro-absorption modulators.