Sample heating in near-field scanning optical microscopy

Heating near the aperture of aluminum coated, fiber optic near-field scanning optical microscopy probes was studied as a function of input and output powers. Using the shear-force feedback method, near-field probes were positioned nanometers above a thermochromic polymer and spectra were recorded as the input power was varied. Excitation at 405 nm of a thin polymer film incorporating perylene and N-allyl-N-methylaniline leads to dual emission peaks in the spectra. The relative peak intensity is temperature sensitive leading to a ratiometric measurement, which avoids complications based solely on intensity. Using this method, we find that the proximal end of typical near-field probes modestly increase in temperature to 40-45°C at output powers of a few nanowatts (input power of ∼ 0.15 mW ). This increases to 55-65°C at higher output powers of 50 nW or greater (input power of ∼ 2 - 4 mW ). Thermal heating of the probe at higher powers leads to probe elongation, which limits the heating experienced by the sample.