One of the more surprising applications of femtosecond lasers is the imaging of sensitive biological structures, which appear to tolerate the onslaught of peak power densities of more that 1011 W/cm2, better than the 105 W/cm2 in a confocal or even the mere 500 W/cm2 of conventional widefield illumination. Ultrashort-pulse lasers thus open a new way for fluorescence microscopy in living tissues, reducing the severe photodamage and photobleaching often associated with such experiments.The instrument that performs this feat is called a multi-photon laser scanning microscope and was invented in 1989 by Denk, Strickler, and Webb. It basically consists of a modified laser scanning microscope with a femtosecond (typically a Ti:Sapphire laser) mode-locked laser as the excitation light source…