We describe a sensitive method for measuring time-dependent changes in refractive index within ∼5 μm of an interface using off-null time-resolved ellipsometry and a dual-cavity femtosecond laser. The sensitivity to changes in refractive index is two orders of magnitude higher than conventional picosecond interferometry. A thin metal film on a sapphire substrate is heated by ∼10 K using an ultrafast optical pump pulse; the subsequent changes of the phase difference δ Δ between p and s polarized reflectivity are tracked using off-null ellipsometry using a time-delayed probe pulse. We demonstrate a sensitivity of δ Δ≈3× 10-7 deg/Hz using interfaces between Au and water, and Au and various gases including R134a, a common refrigerant. Our data for the damping rate of ≈200 MHz frequency acoustic waves in O2, N 2, and Ar at atmospheric pressure agree well with prior results obtained at much lower pressures and frequencies.
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