Because laser light is nearly perfectly collimated, coherent and monochromatic it is possible to focus a laser beam down to diffraction limited spot sizes thereby concentrating large amounts of energy on a small area.
The high energy density in the focus permits the breaking of molecular bonds and the vaporisation of those materials absorbing the respective laser wavelength. The controlled removal of material by vaporisation or molecular fragmentation is called ablation. Thus lasers can be used as extremely sharp and most precise knifes. If used in combination with high quality focussing optics it is possible to precisely cut out and isolate cells or even organelles (microdissection) as well as to destroy or only bleach defined areas of a specimen (see the FluoView SIM Scanner concept).