The interaction of light and matter lies at the heart of the principle of optoelectronic devices. By tuning this strength of the electric field component of the light wave, one can gain control over this interaction. A simple way of achieving this is by employing microcavities, which are one-dimensional photonic structures. These give rise to an effective quanitzation of the light field in one direction. The largest enhancements in the strength of light-matter coupling are achieved for cavities with dimensions on the order of the effective wavelength of light. As organic materials have the very large oscillator strengths required for light-matter coupling, as well as excellent thin film forming properties, they are ideal materials with which to exploit tunable electron-photon coupling. We demonstrate the influence of the optical field strength in organic microcavity photodiodes. Besides allowing tunability of the response spectrum by varying the effective resonator thickness, a large increase in the photocurrent sensitivity is observed below the absorption threshold of the optically active material. The microcavity induced field enhancement plays a particularly important role under two-photon excitation. In this case we observe a 500-fold increase in the photocurrent response with respect to a non-cavity device. This opens a range of applications for organic microcavity photodiodes as nonlinear detector elements.