In situ laser power measurement at the focus of microscope objectives used in optical tweezers.

We discuss measurements of the laser power at the focus of high numerical aperture objectives used in optical microscopy and optical tweezers. For a given power, the focused incident laser beam heats a small mercury bead that jumps when it reaches the boiling temperature of water, the medium used in the experiments. From the size of the mercury beads, the heat conductivities of water and glass-slide, and the absorption coefficient of mercury for the laser wavelength used, the incident power can be obtained using the solution of the associated heat problem. This problem is mapped onto the problem of a charged conducting sphere embedded in a semi-infinite medium of dielectric constant ϵ1, separated from another semi-infinite medium of dielectric constant ϵ2. [ABSTRACT FROM AUTHOR]