SOFORT · SOPRA-FORTH Partnership for the Development of a Microsecond Ellipsometer

SOPRA-FORTH Partnership for the Development of a Microsecond Ellipsometer

The main goal of the SOFORT project was the development of a microsecond-resolved ellipsometer, based on a high-repetition-rate pulsed-laser cavity ring-down instrument, for the study of fast surface processes. The combination of the expertise of the three partners was well-matched for this goal; one in academia: IESL-FORTH, Crete (pulsed-laser cavity-ringdown spectroscopies); and two in industry: SOPRA, Paris (conventional ellipsometry), and PHOTEK, UK (fast photodetectors and fast data acquisition). The cavity ring-down ellipsometer was developed and improved along three directions (a) sensitivity, (b) spectral resolution, and (c) time resolution. Specifically: (a) The ellipsometric sensitivity was improved by using tailored dielectric coatings at the prism surface, which enhanced the senstivity of the measured ellipsometric phase angle to the refractive index of the sample. Enhancements of a factor of 5 were demonstrated, and future work aims to improve this further. (b) A supercontinuum “white-light” laser source (500-2000 nm) was used, along with broad-band laser mirrors (500-1500 nm), a grating and a 32-channel photomultiplier, which allowed ellipsometric measurements to be performed over a large spectral range, simultaneously for each laser shot. (c) We used both monochromatic and “white-light” laser sources, which both operated between 100 kHz and 1 MHz repetition rates, allowing cavity ringdown measurements every 1-10 microseconds (whereas data points were acquired every 4 ns). FPGA (field-programmable gate array) based data acquisition was developed that allows the ellipsometric measurements to be analyzed and recorded in real time. We have demonstrated this new fast ellipsometric system by measuring temperature-induced refractive index changes at surfaces caused by the absorption of ns laser pulses by dye molecules. A small, portable, precommercial prototype of the ellipsometer was built (50x30x20 cm), which contains the cavity, laser, and detector. It will be used to demonstrate the capabilities of fast ellipsometry to potentially interested ellipsometry companies. The expertise that was developed through the SOFORT project, in evanescent-wave cavity ring-down polarimetry measurements, was recently extended to the measurement of chiral optical rotation in evanescent wave for the first time, as shown in the publication by Sofikitis et al., Nature 76, 514 (2014). More information about SOFORT can be found at the project website: http://sofortproject.webs.com/

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