CONTINUOUS-WAVE EXCITATION 2-PHOTON FLUORESCENCE MICROSCOPY EXEMPLIFIED WITH THE 647-NM ARKR LASER LINE

We report on efficient two-photon fluorescence imaging in beam scannin g microscopy by exciting UV dyes at the 647-nm line of a continuous wa ve ArKr mixed gas laser. For a numerical aperture of 1.4 (oil), we use d an illumination power of up to 210mW at the sample. High-resolution images were obtained for DAPI-labelled cell nuclei within 4-60 s. Our method is a simple two-photon alternative to UV confocal imaging with the potential of becoming a very useful feature of laser scanning micr oscopy.