Resonance of Electronic States and Optical Non-linearity in Coupled Quantum Well Structures

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The purpose of the research is to clarify the optical nonlinearity in a strongly coupled triple quantum well structure, where the application of an external field changes the situation of resonance of the electronic states. Thus the exciton oscillator strength should be a strong function of the external field. Since the electrons and holes have different tunneling escape times, after the excitation of carriers in a well by a femtosecond laser pulse, electrons will escape to the other well faster than holes, resulting the build up of internal field across the wells. This induced field could affect the change of the oscillator strength of excitons. Since the induced field will increase by increasing the input optical power, we can expect strong optical nonlinearity which will be responsible in the femtosecond region.A GaAs/AlGaAs triple quantum well structure was made by MBE on a semiinsulating GaAs. The optical spectra was investigated using femtosecond laser pulse. The transmission of the laser light through the sample was investigated at various wave lengths. The transmittance was found to be a strong function of the input intensity. Above a critical power, the transmittance is decreased as the increase of the input power. The nonlinear behavior is enhanced near the anti-crossing energy associated with the triple resonance, which is in good agreement with the theoretical prediction.