Optical transfection is a biomedical technique that entails introducing nucleic acids (i.e. genetic material such as DNA) into cells using light. All cells are surrounded by a plasma membrane, which prevents many substances from entering or exiting the cell. Lasers can be used to burn a tiny hole in this membrane, allowing substances to enter. This is tremendously useful to biologists who are studying disease, as a common experimental requirement is to put things (such as DNA) into cells.
Typically, a laser is focussed to a diffraction limited spot (~1 µm diameter) using a high numerical aperture microscope objective.  The plasma membrane of a cell is then exposed to this highly focussed light for a small amount of time (typically tens of milliseconds to seconds), generating a transient pore on the membrane.  The generation of a photopore allows exogenous plasmid DNA, RNA, organic fluorophores, or larger objects such as semiconductor quantum nanodots to enter the cell.  In this technique, one cell at a time is treated, making it particularly useful for single cell analysis.
This technique was first demonstrated in 1984 by Tsukakoshi et al., who used a frequency tripled Nd:YAG to generate stable and transient transfection of normal rat kidney cells.  Since this time, the optical transfection of a host of mammalian cell types has been demonstrated using a variety of laser sources, including the 405 nm continuous wave (cw), 488 nm cw, or pulsed sources such as the 800 nm femtosecond pulsed Ti:Sapphire or 1064 nm nanosecond pulsed Nd:YAG.