A spectroscopic ellipsometer SE 800 (SENTECH instrument GmbH, Germany) with Spec-traRay/4 software was employed for the measurement and data analysis. Spectra of the ellipsometric ∆ angle were recorded at an incident angle of 80 • for Au(111) and after each modification step and analysed in the range where the SAM was found to be transparent: 600 nm to 825 nm. Assuming an isotropic and homogeneous layers, an optical model of depending on the sample sensitivity and measurement conditions. For measuring protein single molecules an off-resonance tuning scheme was employed. The laser power was optimized for obtaining a good signal but sometimes had to be lowered in cases of very high IR amplitudes due to a loss of linearity of the photothermal signal as reported earlier. The laser power were between 3.8 % to 64 % of the maximum peak power for the four modules of QCL, i.e. 0.2 mW to 9 mW and for the OPO laser from 0.1 mW to 1.5 mW (2 % to 6 %).