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 approximately 5 microm 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 approximately 10 K using an ultrafast optical pump pulse; the subsequent changes of the phase difference deltaDelta between p and s polarized reflectivity are tracked using off-null ellipsometry using a time-delayed probe pulse. We demonstrate a sensitivity of deltaDelta approximately = 3x10(-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 approximately 200 MHz frequency acoustic waves in O(2), N(2), and Ar at atmospheric pressure agree well with prior results obtained at much lower pressures and frequencies.