Field-induced optical effect in a five-step asymmetric coupled quantum well with modified potential

A five-step asymmetric coupled quantum well (FACQW) is demonstrated and analyzed for its field-induced optical effect. A strong combined exciton absorption peak is caused by e1hh2 and e2hh1 transitions for the TE mode and e1lh2 and e2lh1 for the TM mode at a small negative applied electric field, while two separated exciton absorption peaks appear at positive applied electric field. The large change of the exciton absorption strength produces large positive refractive index change whose value is larger by over one order of magnitude compared to that in a rectangular quantum well (RQW) when the operation wavelength is away from the absorption edge. A tensile-strained FACQW is employed to improve the polarization dependence. The difference of the refractive index change for TE and TM modes is under 2% while that in the unstrained FACQW is larger than 50%. A modified FACQW structure is also studied for a negative refractive index change. The strong combined exciton absorption peak without a red shift is observed in the measurement of photocurrent spectra of the FACQW at room temperature. The absorption properties of the FACQW under high applied electric field are also observed and analyzed. The measured results are in good agreement with bur numerical analysis. Finally, ridge-waveguide Mach-Zehnder modulators have been fabricated by using the FACQW and RQW structures. The half-wave voltage of the FACQW modulator is as low as 3 V while that of the RQW modulator is 8 V. The result proves that the FACQW structure indeed has larger refractive index change than that of the RQW structure. These properties obtained with the FACQW family have a great potential for application to ultrafast and low-voltage optical modulators and switches.