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Fabry-PeÂ¿rot and critical angle geometries are examined experimentally in order to achieve sensitive optical detection of photogenerated carriers in silicon. Whereas modulation of an optical probe beam in the Fabry-PeÂ¿rot geometry is found to be dominated by changes in refractive index, the critical angle geometry is determined to be dominated by absorption. In the Fabry-PeÂ¿rot geometry, the maximum normalized transmission change as a function of absorbed pump energy is 0.58 per Â¿J . The response is measured for a 3-mm-thick silicon cavity with a finesse of 2.2, where parallel silicon-air interfaces are the reflective surfaces. In the critical angle configuration, a maximum normalized transmission change of 0.067 per Â¿J is measured. The response is measured for 13 probe beam internal reflections near the critical angle and a beam waist radius of 0.17 mm. While the high driver for sensitivity in the Fabry-PeÂ¿rot geometry is the cavity finesse, it is the beam waist radius that is important in the critical angle geometry.