A novel procedure is developed to describe and reproduce experimental coherent anti-Stokes Raman scattering (CARS)
data, with particular emphasis on highly congested spectral regions. The approach - exemplified here with high-quality
multiplex CARS data - makes use the maximum entropy method for phase retrieval. The retrieved imaginary part of the
nonlinear susceptibility is shown to be equal to the spontaneous Raman spectrum. The phase retrieval procedure does not
influence the noise contained in the spectra. The conversion of CARS to Raman data permits a quantitative interpretation
of CARS spectra. This novel approach is demonstrated for highly congested multiplex CARS spectra of sucrose, fructose
and glucose. This novel procedures enables extraction of vibrational information from multiplex CARS data without the
use of a priori information of the sample.
A method for noninvasively determining blood oxygenation in individual vessels inside bulk tissue would provide a powerful tool for biomedical research. We explore the potential of coherent anti-Stokes Raman scattering (CARS) spectroscopy to provide this capability. Using the multiplex CARS approach, we measure the vibrational spectrum in hemoglobin solutions as a function of the oxygenation state and observe a clear dependence of the spectral shape on oxygenation. The direct extraction of the Raman line shape from the CARS data using a maximum entropy method phase retrieval algorithm enables quantitative analysis. The CARS spectra associated with intermediate oxygenation saturation levels can be accurately described by a weighted sum of the fully oxygenated and fully deoxygenated spectra. We find that the degree of oxygenation determined from the CARS data agrees well with that determined by optical absorption. As a nonlinear optical technique, CARS inherently provides the 3-D imaging capability and tolerance to scattering necessary for biomedical applications. We discuss the challenges in extending the proof of principle demonstrated to in vivo applications.
We present here predictions for a nonlinear optical response of nanocomposites having a layered structure. In particular, we have studied nanocomposites with alternating thin layers of titanium dioxide and nonlinear optical polymers (PT10 and PDHS). It is demonstrated that both enhancement in magnitude and change in spectral properties of the degenerate third-order nonlinear susceptibility can be tailored in such composites.
We have studied the applicability of optically nonlinear organic polymers (PT10, PDHS) and linear amorphous TiO2 as Maxwell Garnett nanocomposite. According to the simulations it is possible to enhance the nonlinear optical properties of polymers in Maxwell Garnett configuration.
The case of layered two-phase nanocomposites has been numerically studied. Contrary to the earlier studies on the subject, material's susceptibilities are allowed to be complex valued and to change as a function of frequency. Thereby, new effects arise, e.g., the phase of the effective third-order susceptibility (chi) eff(3) of a nanocomposite can have a huge frequency dependent increase near resonances compared to corresponding phase changes of (chi) (3) of the constituent materials. Unfortunately, the phase changes cannot be predicted from the amplitude measurement (chi) eff(3) by the Kramers-Kronig methods, because (chi) eff(3) is, in general, a meromorphic function in a complex frequency plane, and thus, conventional Kramers- Kronig relation does not exist between the amplitude and phase of (chi) eff(3). In this paper another type of phase retrieval procedure, based on the maximum entropy model, is shown to be applicable for (chi) eff(3).
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