A
laser scanning
microscope produces molecular excitation in a target material by simultaneous absorption of three or more photons to thereby provide intrinsic three-dimensional resolution. Fluorophores having
single photon absorption in the short (
ultraviolet or visible)
wavelength range are excited by a beam of strongly focused subpicosecond pulses of
laser light of relatively long (red or
infrared)
wavelength range. The fluorophores absorb at about one third, one fourth or even smaller fraction of the
laser wavelength to produce fluorescent images of living cells and other microscopic objects. The fluorescent emission from the fluorophores increases cubicly, quarticly or even
higher power law with the excitation intensity so that by focusing the
laser light,
fluorescence as well as
photobleaching are confined to the vicinity of the focal plane. This feature provides
depth of field resolution comparable to that produced by
confocal laser scanning microscopes, and in addition reduces
photobleaching and
phototoxicity. Scanning of the laser beam by a
laser scanning microscope, allows construction of images by collecting multi-
photon excited
fluorescence from each point in the scanned object while still satisfying the requirement for very high excitation intensity obtained by focusing the laser beam and by
pulse time compressing the beam. The focused pulses also provide three-dimensional
spatially resolved photochemistry which is particularly useful in photolytic release of caged
effector molecules, marking a recording medium or in
laser ablation or
microsurgery. This invention refers explicitly to extensions of two-
photon excitation where more than two photons are absorbed per excitation in this nonlinear
microscopy.