THE LIGHT-MICROSCOPE ON ITS WAY FROM AN ANALYTICAL TO A PREPARATIVE TOOL

Light intensities of up to 10(13) W/cm2 can be generated by focusing light, particularly laser pulses, into a microscope. Such power densities can be used to cut, perforate, or fuse microscopic objects with submicrometre accuracy. Suitable light sources for such a 'microbeam' are nitrogen lasers with a working wavelength of 337 nm, frequency-multiplied Neodym YAG lasers (266 or 355 nm) or excimer lasers (308 nm). In combination with dye lasers, tunable microbeams covering the wavelength range from the ultraviolet to the infra-red can be constructed. Such laser microbeams can be used to modify microchip substrates. Micro-injection of materials into biological cells or fusion of selected cell pairs under total microscopic control is also possible. Using the same equipment, elongated biological objects can be microdissected with submicrometre precision, for example in attempts to isolate DNA from a specific region of the human genome. In addition to the use of high-power pulsed lasers, the light of a continuous-wave infra-red laser can be used for the transport of microscopic objects. There, light pressure and the inhomogeneity of the electric field in a light pulse are used to trap microscopic objects in the focused laser beam, using the beam as ultrafine nonmechanical tweezers. Unlike mechanical microtools the optical trap is gentle and absolutely sterile. A combined laser microbeam and optical trap (a microbeam-trap) converts the light microscope, which is usually regarded as an analytical instrument, into a universal preparative instrument that allows micromanipulation of microscopic objects without mechanical contact. In contrast to any other micromanipulator, the microbeam-trap can work in the depth of an object without opening it.