Acceptance Angle Influence on the Optimum Incident Spot Size for High-Finesse Microcavity Two-Photon Absorption Photodetectors

The influence of the limited acceptance angle of a high-finesse microcavity two-photon absorption photodetector on its response has been investigated. It is shown that the limited acceptance angle of the microcavity explains the observed asymmetry seen in the spectral dependence of the microcavity. The theory describing the influence of the acceptance angle allows for an optimum incident beam waist for any cavity structure to be calculated, with an optimum spot diameter of 7 ¿m having been calculated for the microcavity under test. The theory also enables the calculation of an optimum incident spot size for any microcavity. It is shown that a cavity with higher overall reflectivity require larger input spot sizes in order to optimize the level of generated two-photon absorption.