Abstract
Laser tweezers or optical traps are established laser tools for optical noncontact manipulation of micron/submicron sized objects in liquids such as nonadherent biological cells in medium. Typical laser traps are based on optical gradient forces generated with high numerical aperture near-infrared (NIR) continuous wave (cw) lasermicroscopes. The laser-cell interaction is determined by a change of themomentum due to the beam direction being altered by refraction. In order to avoid laser absorption, NIR cw lasers such as the Nd:YAG laser at 1064 nm, the frequency doubled erbium:YAG fiber laser at 760 nm, the tunable cw Ti:sapphire ring laser, and laser diodes at wavelengths < 800 nmare employed. They are considered to be safe tweezer sources. However, two-photon absorption effects may occur due to the generation of high MW/cm2 laser intensities when using tightly focused cw laser beams at a power of 100mW or more. These nonlinear effects can be used for two-photon excited fluorescence spectroscopy of trapped objects. However, when using low-wavelength NIR (< 800 nm) traps, potential “UV-like” photodamaging effects have to be considered during cell manipulation. The use of the Nd:YAG laser at 1064nm is recommended when using laser tweezers for optical sperm transport for laser-assisted in vitro fertilization (IVF).