(color online) (a) The schematic diagram of the CDQW MZ modulator. Two red regions denote two arms (arm I and arm II) constructed by the CDQW waveguides. Two grey strips denote electrodes, located at the top and bottom of the arm II with an applied bias voltage V, respectively. The blue arrow denotes incident field with electric-field polarization along the z direction. (b) The AlGaAs/GaAs CDQW configuration without (arm I) and with (arm II) an applied electric field, and confinement direction, where W_L, W_R, and W_B denote the left-well, right-well, and mid-barrier width, respectively.