Non-intrusive temperature measurement of NSOM probes with thermoreflectance imaging

A non-intrusive optical measurement technique (i.e. the thermoreflectance method) was adopted to study the light-induced heating of near-field scanning optical microscope (NSOM) probes for regions ≳5 µm from the tip of the probes. The thermoreflectance measurements indicate that there is a hot spot at 20-30 µm away from the tip of the probe. The location of the hot spot is not a strong function of laser wavelength for a fixed type of NSOM probe. The temperature of the NSOM probes increases almost linearly with respect to the input laser power. The shorter wavelength light (405 nm) provides stronger heating of the gold-coated NSOM probes compared with the longer wavelength light (650 nm). The modulation rate of the input laser also contributes to the heating of the NSOM probes: the temperature variation of the NSOM probes decreases with increase in the modulation frequency of the input light. In addition to laser conditions, the NSOM probe with a smaller aperture suffers stronger heating compared with that with a larger aperture. (paper)