title: Two-photon vibrational spectroscopy for biosciences based on surface-enhanced hyper-Raman scattering.
abstract: Two-photon excitation is gaining rapidly in interest and significance in spectroscopy and microscopy. Here we introduce a new approach that suggests versatile optical labels suitable for both one- and two-photon excitation and also two-photon-excited ultrasensitive, nondestructive chemical probing. The underlying spectroscopic effect is the incoherent inelastic scattering of two photons on the vibrational quantum states called hyper-Raman scattering (HRS). The rather weak effect can be strengthened greatly if HRS takes place in the local optical fields of gold and silver nanostructures. This so-called surface-enhanced HRS (SEHRS) is the two-photon analogue to surface-enhanced Raman scattering (SERS). SEHRS provides structurally sensitive vibrational information complementary to those obtained by SERS. SEHRS combines the advantages of two-photon spectroscopy with the structural information of vibrational spectroscopy and the high-sensitivity and nanometer-scale local confinement of plasmonics-based spectroscopy. We infer effective two-photon cross-sections for SEHRS on the order of 10(-46) to 10(-45) cm4 x s, similar to or higher than the best "action" cross-sections (product of the two-photon absorption cross-section and fluorescence quantum yield) for two-photon fluorescence, and we demonstrate HRS on biological structures such as single cells after incubation with gold nanoparticles.
authors.affiliation: Wellman Center for Photomedicine, Harvard Medical School, Boston, MA 02114, USA. kneipp@usa.net
authors.forename: Janina
authors.initials: J
authors.lastname: Kneipp
authors.forename: Harald
authors.initials: H
authors.lastname: Kneipp
authors.forename: Katrin
authors.initials: K
authors.lastname: Kneipp
mesh_terms: D000818:Animals; D020811:Biomedical Technology / Q000379:methods*; D002460:Cell Line; D051379:Mice; D049329:Nanostructures; D017785:Photons; D013059:Spectrum Analysis, Raman / Q000379:methods*; D014732:Vibration