Time-resolved ellipsometry for studies of heat transfer at liquid/solid and gas/solid interfaces.

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 δΔ between p⁁ and s⁁ polarized reflectivity are tracked using off-null ellipsometry using a time-delayed probe pulse. We demonstrate a sensitivity of δΔ≈3×10^-7 deg/[Square_Root]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, N₂, and Ar at atmospheric pressure agree well with prior results obtained at much lower pressures and frequencies. [ABSTRACT FROM AUTHOR]