Abnormal anti-Stokes Raman emission as a coherent anti-Stokes Raman scattering-like process in disordered media
Ioan Baltog, Mihaela A. Baibarac, Ion Smaranda +1 more
In this paper, we demonstrate that, by continuous single beam excitation, one can generate an abnormal anti-Stokes Raman emission (AASRE) whose properties are similar to a coherent anti-Stokes Raman scattering (CARS). The effect has been observed in materials which possess intrinsically nonlinear properties (LiNbO3 and CdS), which have the electric susceptibility of third order different from zero, χ(3) ≠ 0, as well as in materials that become nonlinear under resonant optical excitation. In the latter case, we used poly-3,4-ethylendioxythiophene (PEDOT) in its undoped state deposited electrochemically on Au support. Raman studies corroborated with images of optical microscopy demonstrate that the production of AASRE is conditioned by the existence of a particular morphology of the sample able to ensure efficient transport of the light inside the sample through a multiple light scattering mechanism. In this context, it was found that LiNbO3 and CdS in powder form as well as the PEDOT films layered on a rough Au substrate are suitable morphological forms. We explain AASRE as resulting from a wave-mixing mechanism of the incident laser light ωl with a Stokes-shifted Raman light ωS produced by a spontaneous Raman light scattering process, both strongly scattered inside the sample. As a CARS process, AASRE is conditioned by the achievement of phase-matching requirements, which makes the difference between the wave vectors of mixing light close to zero, Δk =/2kl − kS − kCARS/≈ 0. In condensed media, the small dispersion of the refractive index makes Δk ≈ 0 so that the formation of a favourable phase-matching geometry may be accomplished even at a crossing angle θ of travelling scattered light ωl and ωS. For tightly focused beams, the requirement of phase matching relaxes; it is no longer sensitive to the Raman shift, so that a wide intense anti-Stokes Raman spectrum is observed at an angle larger than the Stokes Raman spectrum.
Journal of Physics B Atomic Molecular and Optical Physics•Apr 19, 2011•1 citation