We report on the enhanced quantum-confined Stark effects (QCSE) in ZnSe/ZnMgSSe asymmetric coupled quantum wells (ACQWs), and its application for blue-ultraviolet optical modulator. A new electronresonant-type of ACQW structure: [ZnSe(6ML)/ZnMgSSe(6ML)/ZnSe(12ML)] is designed and fabricated by molecular beam epitaxy (MBE). This new structure device exhibits a strong coupling of the electron envelope wavefunctions and a large Stark shift of ∆E ∼ 28 meV at room temperature. This field enhanced Stark shift is found to be superior to the usual heavy-hole-resonant type ACQW (∆E ∼ 21 meV), which is consistent with the theoretical simulation based on the effective mass approximation.1 Introduction A quantum confined Stark effect (QCSE) of excitonic transition is attracting for optical functional devices such as high speed light modulators, switches or self electro-optic effect devices (SEEDs) [1,2]. The exciton binding energy in theII-VI widegap semiconductors is much larger than that of III-V compounds, and therefore, these widegap compounds have a large potential for QCSE devices in new short wavelength region. Furthermore, asymmetric-coupled quantum wells (ACQWs) of II-VI widegap compounds have a big advantage on the large Stark shift under low electric field [3].In this paper, enhanced Stark shifts in ZnSe/ZnMgSSe ACQWs are proved experimentally, and the physical mechanism is examined by theoretical analysis. The MQW optical modulators used in this study are p-i-n structure diodes including 50 periods ACQWs for the active layers, grown by molecular beam epitaxy (MBE) on n + -GaAs(100) substrates. Excitonic transition energies and Stark shifts were determined by electroreflectance (ER) and electroabsorption (EA) experiments. The theoretical analysis of ACQWs under high electric fields are based on the finite element method in the framework of the effective mass approximation [4,5].