In situ measurement of laser power at the focus of a high numerical aperture objective using a microbolometer

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Abstract
We built a simple mercury-based microbolometer to measure “in situ” laser power at the focus of high numerical aperture objectives used in optical microscopy and optical tweezers. With an optical microscope equipped with a high numerical aperture objective, we visualize a mercury droplet immersed in water that is heated by an infrared laser, whose power we want to determine. For a given laser power the mercury droplet reaches the boiling temperature of the water and then jumps, making a precise determination of this point very easy. We solve the heat equation for this system and, using known material parameters and the size of the mercury droplet, we determine the local power of the infrared laser.