Heating by absorption in the focus of an objective lens

We derive an integral solution for the local heating of a linearly absorbing, uniform medium exposed to strongly focused light. Numerical results for local heating under typical multiphoton microscopy and optical trapping conditions are presented for various aperture angles. In contrast with common Gaussian beam approximations, our model employs the focal-intensity distribution as described by the point spread function of the lens. In this way, the model also accounts for axial heat transportation, which results in a lower prediction for the temperature increase. For an aperture of 1.2 (water immersion), irradiation with 100 mW of 850-nm light for 1 s increases the local temperature of water by 0.2 K. Heating of water by linear absorption can be ruled out as a limiting factor in standard multiphoton-excitation microscopy.