We propose a novel design for a semiconductor microcavity nonlinear detector, based on two-photon absorption, for high-speed signal processing applications in future wavelength division multiplexing (WDM) systems. By inserting a phase section and a gain section in the resonant cavity, we could achieve wavelength and bandwidth tunability of the nonlinear detector. Theoretical investigation is carried out in the frame of an extended transfer matrix formalism, leading to the selfconsistent calculation of the device properties. We model the design and characteristics of a GaAs/AlGaAs microcavity suitable for selective processing of different WDM channels, operating at various data rates and modulation formats.