title: Hyper-Rayleigh and hyper-Raman scattering background of liquid water in two-photon excited fluorescence detection.
abstract: The detection of two-photon excited fluorescence is practically free from background caused by linear scattering because two-photon excited fluorescence occurs at a much shorter wavelength region than the excitation light. This property can be used to achieve ultrasensitive fluorescence detection. However, similar to linear scattering in the detection of one-photon excited fluorescence, the question arises whether background caused by non-linear scattering may limit the detection sensitivity of two-photon excited fluorescence. In this work, quantitative comparisons between two-photon induced scattering of liquid water and two-photon excited dye fluorescence in a standard epifluorescence geometry show that the relative scattering background is typically reduced by orders of magnitude in two-photon excitation as compared to single-photon excitation with confocal detection. Hyper-Rayleigh and hyper-Raman (3400 cm-1) cross sections of liquid water have been measured to be 8 x 10(-62) and 7 x 10(-63) cm4.s/photon, respectively, at 840 nm incident wavelength, with absolute values calibrated with respect to the known two-photon fluorescence excitation cross section of fluorescein.
authors.affiliation: Cornell University, Ithaca, New York 14853, USA.
authors.forename: C
authors.initials: C
authors.lastname: Xu
authors.forename: J B
authors.initials: JB
authors.lastname: Shear
authors.forename: W W
authors.initials: WW
authors.lastname: Webb
mesh_terms: D005453:Fluorescence; D007834:Lasers; D017785:Photons; D012542:Scattering, Radiation; D013059:Spectrum Analysis, Raman; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water