Quantum Confined Stark Effect can be observed in Quantum Wells due to the presence of excitonic states in the bandgap of the semiconductor structure which enhance the absorption of photons with energy lower than that of bandgap. This effect is useful in devices that uses changes induced by electroptic effects to operate. devices such as absorption modulators and even those refractive modulators benefit from QCSE effect. excitons exist in bulk semiconductor devices at very low temperature, but if you increase confinement by introducing quantum well structure excitons can exist even above room temperature. by the way excitons are electron-hole bound state which tends to lower the band energy. In Mach-Zehnder modulators the passive waveguide can incorporate QWs to enhance their refractive index change due to electro-optic effects...using materials with larger conduction band offset and smaller valence band offset one can achieve higher modulation speed and bandwidth.
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