Detection of ultrafast phenomena by use of a modified Sagnac interferometer

We describe a time-division interferometer based on the Sagnac geometry for monitoring ultrafast changes in the real and the imaginary components of t he refractive index as well as phase changes that are due to surface displa cement. Particular advantages of this interferometer are its simple common- path design and operation at normal incidence with a microscope objective f or both pumping and probing. Operation is demonstrated by detection of temp erature changes and coherent phonon generation in a gold film. (C) 1999 Opt ical Society of America. OCIS codes: 320.7130, 120.3180, 320.7120.