With optical tweezers, the potential photodamage to cells or other biological tissues makes it important to know local laser power. Because the trapping force is proportional to the laser power at the objective focus, power measurement must be performed in situ. In the Sept. 2 issue of Applied Physics Letters, a group of scientists at the Universidade Federal de Minas Gerais in Belo Horizonte and the Universidade Federal do Rio de Janeiro, both in Brazil, detailed a simple mercury-based microbolometer that can accomplish this task.

Using a microscope equipped with a high-numerical aperature objective, the researchers visualized a mercury droplet immersed in water that was heated by an IR laser. They reported that, for a given laser power, the droplet reached the boiling temperature of water and jumped at an easily determined time. Using known material parameters and droplet size, they were able to solve the heat equation for the system and subsequently determine the local power of the laser.