The influence of a static electric field on a non-polar molecule has been studied by means of multiplex coherent anti-Stokes Raman scattering (M-CARS). A parallel measurement of electric field induced second harmonic generation (EFISHG) has also been led. Both techniques suggest a reorientation of the molecule due to the presence of an electric field. This phenomenon can be used to increase the chemical selectivity and the signal to non-resonant background ratio, namely, the sensitivity of the M-CARS spectroscopy.
Particle size analyzers based on laser scattering commonly make use of light diffraction and scattering around the particle considered in its medium. For particle size below 50 μm, Fraunhofer theory must be abandoned in favor of Mie model, which requires to know the complex refractive index of both the particle and the medium. In this paper, we demonstrate that particle size characterization can be realized by measuring the macroscopic Raman spectral response of the whole set of particles excited by a laser beam. We use a home-made setup based on coherent anti-Stokes Raman scattering (CARS) and having a 0.36 cm-1 spectral resolution, in which the laser source is a dual-output infrared nanosecond supercontinuum source (1064 nm monochromatic pump wave, 1100-1640 nm broadband Stokes wave). The samples are latex beads in water with different diameters (20 nm, 50 nm, 100 nm, 5 μm). The C-H stretching line around 3050 cm-1 is studied. For this vibration, we study the variation of both the CARS central frequency and linewidth as a function of the particles size. A quasi linear increase of the linewidth with the inverse of the diameter is measured. A difference of 15 cm-1 is obtained between beads with diameters of 5 μm and 20 nm respectively. The physical phenomena at the origin of this difference are discussed, especially considering the contributions of the center and of the boundaries of the object to the global Raman response.
We demonstrate all-normal dispersion supercontinuum generation in the 1080 nm-1600 nm range by propagating subnanosecond pulses in a high numerical aperture standard optical fiber. The extreme saturation of the Raman gain provides a flat spectrum in the considered range, making this broadband source particularly suitable for coherent Raman spectroscopy. This unusual regime of supercontinuum generation (Raman gain saturation regime) is investigated through an experimental spectrotemporal study. The possibility of operating spectrometer-free time-coded coherent Raman spectroscopy is introduced.