Direct extraction of Raman line-shapes from congested CARS spectra.Vartiainen, E.M.; Rinia, H.A.; Müller, M.; Bonn, M. Published in: Optics Express DOI:10.1364/OE.14.003622 Link to publication Citation for published version (APA):Vartiainen, E. M., Rinia, H. A., Müller, M., & Bonn, M. (2006). Direct extraction of Raman line-shapes from congested CARS spectra. Optics Express, 14(8), 3622-3630. DOI: 10.1364/OE.14.003622 General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. Abstract:We show that Raman line-shapes can be extracted directly from congested coherent anti-Stokes Raman scattering (CARS) spectra, by using a numerical method to retrieve the phase-information hidden in measured CARS spectra. The proposed method utilizes the maximum entropy (ME) model to fit the CARS spectra and to further extract the imaginary part of the Raman susceptibility providing the Raman line-shape similar to the spontaneous Raman scattering spectrum. It circumvents the challenges arising with experimentally determining the real and imaginary parts of the susceptibility independently. Another important advantage of this method is that no a priori information regarding the vibrational resonances is required in the analysis. This permits, for the first time, the quantitative analysis of CARS spectra and microscopy images without any knowledge of e.g. sample composition or Raman response. ©2006 Optical Society of AmericaOCIS codes: (300.6230) Spectroscopy, coherent anti-Stokes Raman scattering; (000.3860) General, mathematical methods in physics.References and links
We report a numerical algorithm, the maximum entropy method (MEM), to obtain the absolute phase of the sum frequency signal from vibrational sum frequency generation (VSFG) spectra, without the need for phase-sensitive measurements. From the phase of the VSFG susceptibility, we can determine the molecular orientation, i.e., whether molecular groups are pointing “up” or “down”, with respect to the interface. Furthermore, with previous knowledge of the nonresonant phase, the real and imaginary parts of second-order susceptibility can also be determined. The phase retrieval algorithm is successfully applied to spectra obtained from three distinct samples: (1) water vibrations of the SiO2−water interface, (2) methyl vibrations of a dodecanol monolayer on water, and (3) methyl vibrations of self-assembled dodecanethiol monolayers on a gold substrate. These results demonstrate that the approach is applicable to a wide range of spectra, with varying resonance widths and nonresonant background levels. For the SiO2−water interface at high pH, we find that the water molecules are oriented with their hydrogen atoms toward the surface, and we show that the procedure demonstrated here provides information on the interfacial vibrations that cannot be obtained from a multiresonance fit. For surfactant monolayers, we find, as expected, that the methyl groups point away from the substrate. Possible complications and limitations in determining the phase spectrum of the nonlinear susceptibility using MEM are also discussed.
The ability to observe samples qualitatively at the microscopic scale has greatly enhanced our understanding of the physical and biological world throughout the 400 year history of microscopic imaging, but there are relatively few techniques that can truly claim the ability to quantify the local concentration and composition of a sample. We review coherent anti-Stokes Raman scattering (CARS) as a quantitative, chemically specific, and label-free microscopy. We discuss the complicating influence of the nonresonant response on the CARS signal and the various experimental and mathematical approaches that can be adopted to extract quantitative information from CARS. We also review the uses to which CARS has been employed as a quantitative microscopy to solve challenges in material and biological science.
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