NONLINEARITY IN PHOTOTHERMAL RADIOMETRIC IMAGING

Non-linear effects occurring in photothermal microscopes based on prin ciple of photothermal radiometry are studied as a function of the lase r power and of the modulation frequency. For stainless steel, a second harmonic component of up to 11% of the fundamental amplitude is found . Phase shifts of up to 20 degrees are observed as a function of the m odulation frequency. The nonlinearity is mainly due to Stefan-Boltzman n's law. The experimental results are in good agreement with a theoret ical model based on the static and oscillating temperature distributio n.