PHOTOTHERMAL DISPLACEMENT SPECTROSCOPY OF SURFACES AND THIN FILMS

Abstract

We have developed a new technique, photothermal displacement spectroscopy, for studying the optical and thermal properties of surfaces. Photothermal displacement spectroscopy is based on optical detection of the thermal expansion of a sample as it is heated by absorption of electromagnetic radiation. Details of the experimental configuration are shown. An optical absorption spectrum is generated as an intensity modulated, tunable light beam (pump beam) is focussed onto the sample, and, depending on the optical cross section, some fraction of the radiation is absorbed. As the excited electrons decay nonradiatively, the sample is heated. The illuminated surface is then displaced as the sample expands. A probe beam reflected from the surface is deflected by the slope of the surface displacement. This deflection is measured by a position sensitive photodiode whose output is then amplified by a phase sensitive lock-in amplifier referenced to the pump beam modulation. Thermal information can be obtained by measuring the shape and phase of the photothermal displacement relative to the illumination as a function of the modulation frequency.