Kneipp Janina, Kneipp Harald, Kneipp Katrin
Wellman Center for Photomedicine, Harvard Medical School, Boston, MA 02114, USA.
Proc Natl Acad Sci U S A. 2006 Nov 14;103(46):17149-53. doi: 10.1073/pnas.0608262103. Epub 2006 Nov 6.
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.
双光子激发在光谱学和显微镜学领域正迅速引起越来越多的关注并具有重要意义。在此，我们介绍一种新方法，该方法提出了适用于单光子和双光子激发的通用光学标记，以及双光子激发的超灵敏、无损化学探测。潜在的光谱效应是两个光子在称为超拉曼散射（HRS）的振动量子态上的非相干非弹性散射。如果HRS发生在金和银纳米结构的局部光学场中，这种相当微弱的效应可以大大增强。这种所谓的表面增强超拉曼散射（SEHRS）是表面增强拉曼散射（SERS）的双光子类似物。SEHRS提供与通过SERS获得的信息互补的结构敏感振动信息。SEHRS将双光子光谱学的优势与振动光谱学的结构信息以及基于等离子体激元的光谱学的高灵敏度和纳米级局部限制相结合。我们推断SEHRS的有效双光子截面约为10^(-46)至10^(-45) cm4·s，类似于或高于双光子荧光的最佳“作用”截面（双光子吸收截面与荧光量子产率的乘积），并且我们展示了在用金纳米颗粒孵育后的生物结构（如单细胞）上的HRS。
