Laser tweezers and optical microsurgery in cellular and molecular biology. Working principles and selected applications.

After focusing in a microscope, light can be used for micromanipulation of (sub-)micrometer sized objects. Focused beams of classical light ablate elements of the cell division machinery and switch the beating of hearts on a cellular basis. Focused lasers (laser microbeams or optical scissors) allow in addition very precise nanomachining in a wide field of applications, from developmental biology to plant biotechnology. While in microbeam work high power densities and efficient light-tissue interactions are required, optical tweezers work in a complementary way: Moderate power densities and small absorption of the laser by the biological material is needed. With light pressure and optical gradient forces optical tweezers can be used to move microscopic objects, even in the interior of closed cells. In total, most mechanical micromanipulation techniques known from cellular and molecular biology can be replaced by their optical correlate and some applications are possible which cannot be performed micromechanically. When these optical microtools are operated at their maximum performance, the physical effects are as interesting as their biological applications: The laser microbeam can generate extreme local temperatures, which however are dissipated within a few tens of nanoseconds and therefore cause damage only very locally. The optical tweezers with a working wavelength in the optical window of biological material (600-1100 nm) exert piconewton forces without any mechanical contact. The present article discusses some quantitative physical aspects of microbeams and optical tweezers and gives a few selected examples of applications.