New technique to trace fast surface phenomena
Reflection of light provides a powerful probe for studying surface properties. Due to its simplicity, ellipsometry has been extensively used for investigating surface dielectric properties in practical applications ranging from microelectronics to biology. Two industrial partners from France and the United Kingdom and a research centre from Greece joined forces to further improve mostly the time response of this technique. With EU funding of the project 'SOPRA-FORTH partnership for the development of a microsecond ellipsometer' (SOFORT), project partners produced a small portable pre-commercial ellipsometer prototype to demonstrate its potential for studying ultra-fast surface processes. Combining the established technique of ellipsometry with the emerging one of cavity ring-down, the prototype contained a laser of high pulse repetition rate, a cavity and a detector. Improvement in the sensitivity and spectral and time resolutions was the main pillar of project efforts. A prism was inserted into the cavity and the laser light underwent internal reflections. Tailored dielectric coatings at the surface prism enhanced the sample sensitivity to any variation of refractive index by a factor of five. The ellipsometer set-up allowed performing spectroscopic measurements over a large wavelength range. It makes use of a broadband white-light laser source and broadband mirrors. A grating is used to separate the various optical frequencies in the cavity output, while use of a photomultiplier array serves to detect 32 channels simultaneously. Project partners developed a fast data acquisition system relying on a field-programmable gate array that allowed exploiting the laser repetition rate (100 kilohertz to 1 megahertz (MHz)). Significantly improving the rate at which ellipsometric measurements can be obtained (up to 1 MHz), the system enabled real-time analysis and recording of the measurements. Project partners used this system to monitor phenomena on the microsecond timescale, demonstrating for example how dye molecules vary the surface refractive index at different temperatures. The demonstrated technique substantially enhanced the time resolution of the classical ellipsometer by orders of magnitude, while the set-up remained technically simple. Project findings were reported in publications.
Keywords
Fast surface phenomena, ellipsometer, cavity ring-down, microsecond resolution, surface properties
