A study of waveguide modulation by excitonic screening

Reports a GaAs-AlGaAs single quantum well waveguide device which uses quenching of excitonic absorption and can provide the high contrast absorption modulation, low insertion loss and low voltage operation desirable for systems applications. The device operates in depletion mode which favours high speed modulation. Excitonic absorption, normally observed at the quantum well band edge, can be quenched by the presence of free carriers in the quantum well sub-band. The presence of electrons in the well inhibits excitonic absorption by band filling effects. In the device described electrons are provided by an n+ doped region adjacent to the quantum well. This fills the sub-band with carriers, ensuring that the modulator is initially in a low absorption state. The presence of a junction in the structure then allows a small reverse bias to influence the carrier density in the quantum well, hence enabling a modulation of the excitonic absorption