Subwavelength-resolution near-field Raman spectroscopy

The resolution capabilities of near-field Raman spectroscopy based on a giant enhancement of the electric field near a nanosized metal probe are studied. As a test sample, bundles of single-walled carbon nanotubes deposited on glass substrates are used. It is shown that this method ensures a subwavelength spatial resolution of about 50 nm and demonstrates a Raman scattering enhancement of the order of 10⁴.     

Resolution enhancement of composite spectra using wavelet-based derivative spectrometry

An approach based on the using of the continuous wavelet transform (CWT) in derivative spectrometry (DS) is considered. Within the framework of the approach we develop a numerical differentiation algorithm with continuous wavelets for improving resolution of composite spectra. The wavelet-based derivative spectrometry (WDS) method results in best contrast in differential curves compared to the conventional derivative spectrometry method. A main advantage is that, as opposed to DS, WDS gives stable estimations of derivative in the wavelet domain without using the regularization. A wavelet shape and the information redundancy are of the greatest importance when the continuous wavelet transform is used. As an appropriate wavelet we offer to utilize the nth derivative of a component with a priori known shape. The energy distribution into scales allows one to determine a unique wavelet projection and in that way to avoid the information redundancy. A comparative study of WDS and DS with the statistical regularization method (SRM) is made; in particular, limits of applicability of these are given. Examples of the application of both DS and WDS for improving resolution of synthetic composite bands and real-world composite ones coming from molecular spectroscopy are given.     

Stimulated Raman Scattering in Metal-Dielectric Nanocomposites with Spectrally Degenerate Dielectric Constant

JETP Letters - The effects of enhancement of stimulated Raman scattering in titanium oxynitride (TiON) nanofilms are considered. The mechanism of giant amplification of a Stokes wave is based on,...     

Band shape determination with robust estimator based on continuous wavelet transform

The paper focuses on an alternative approach that allows one to identify overlapping band shapes with the help of the continuous wavelet transform (CWT). We show that less number of special points for determining a band shape is required unlike the fractional derivative spectrometry method [S.S. Kharintsev, M.Kh. Salakhov, Spectrochim. Acta Part A, 60 (2004) 2125]. Besides, the CWT-based derivative spectrometry can be successfully utilized in a case of complex spectra corrupted with a white and/or high-frequency noise. The power of this method is illustrated on model examples and experimental spectra of 1,2-diphenylethane in crystalline and melted phase.     

Study of Formation Mechanisms of Photo-Induced Dichroism in Azo-Containing Polymer Films

Photoinduced dichroism (PD) in azo-containing polymers can result from chromophore orientation and/or cis-isomer accumulation. The PD formation mechanism of azochrommophores DO3 (DisperseOrange 3) doped in polymethyl methacrylate (PMMA) at a temperature varying from –20 to +120 °C is studied by UV-visible spectroscopy. It is shown that the main PD formation mechanism at T < 0°C and T > 60°C is cis-isomer accumulation and trans-isomer photoorientation, respectively.     

Amplification of Stimulated Raman Scattering in Media with a Near-Zero Refractive Index

JETP Letters - Materials with a near-zero refractive index open up new possibilities for enhancing nonlinear optical interactions. This lowers the threshold for nonlinear generation, reduces the...     

A simple method to extract spectral parameters using fractional derivative spectrometry

The nonlinear fitting method, based on the ordinary least squares approach, is one of several methods that have been applied to fit experimental data into well-known profiles and to estimate their spectral parameters. Besides linearization measurement errors, the main drawback of this approach is the high variance of the spectral parameters to be estimated. This is due to the overlapping of individual components, which leads to ambiguous fitting. In this paper, we propose a simple mathematical tool in terms of a fractional derivative (FD) to determine the overlapping band spectral parameters. This is possible because of several positive effects of FD connected with the behavior of its zero-crossing and maximal amplitude. For acquiring a stable and unbiased FD estimate, we utilize the statistical regularization method and the regularized iterative algorithm when a priori constraints on a sought derivative are available. Along with the well-known distributions such as Lorentzian, Gaussian and their linear combinations, the Tsallis distribution is used as a model to correctly assign overlapping bands. To demonstrate the power of the method, we estimate unresolved band spectral parameters of synthetic and experimental infra-red spectra.