CC BY
4LD-I-17
LASER DIAGNOSTICS AND SPECTROSCOPY
Elastic scattering controls Raman scattering efficiency in suspension
L.A. Golovan1', O.I. Sokolovskaya1, V.V. Yakovlev2
1-Lomonosov Moscow State University, Faculty of Physics, Moscow 119991, Russia 2- Texas A&M University, College Station, Texas 77843, USA E-mail: [email protected]
Elastic light scattering is a well-studied phenomenon intrinsic to random media, which often gives them completely new properties. In random media this phenomenon can lead to increase in the volume of light-matter interaction in comparison with homogeneous medium, which can result in variation of efficiency of different optical processes, e.g., Raman scattering. The question is whether it is possible to increase spontaneous Raman scattering efficiency by utilizing only scattering properties of medium itself or by adding scatterers in a controlled manner?
In this paper, effect of elastic light scattering on dwell time and efficiency of spontaneous Raman scattering in dimethyl sulphoxide (DMSO) was studied for powders of rutile and gallium phosphide with micrometer-sized particles suspended in DMSO both in experiment and by means of Monte Carlo simulation.
The employed medium consisted of spherical particles of rutile (diameter 0.5 ^m) and of gallium phosphide (diameter 3 ^m) suspended in DMSO. Mean photon dwell time in suspensions was meausred by means of optical heterodyning with the help of 80 fs pulses at wavelength of 1250 nm, repetition rate 80 MHz. Raman measurements were carried out with excitation by cw radiation and picosecond pulses at wavelengths of 1064 and 532 nm, correspondingly.
Both in experiment and in simulation we revealed effect of light elastic scattering on efficiency of Raman scattering in studied suspensions. Applicability the Monte Carlo simulation for description of the light propagation in the suspensions is supported by good agreement of its results and photon femtosecond backscattering dynamics measured in the experiment. In particular, both in experiment and in simulations it was found that increase of the total reflection with rising scatterer volume fraction is corresponded to fall of the photon dwell time. Combination of both these factors results in nonmonotonic dependence of the mean photon pathlength and Raman scattering efficiency on the scatterer volume fraction. Numerical simulations predicted maximal increase of the Raman signal in rutile suspension in DMSO in backscattered geometry up to 5 to 7 times depending on wavelength, whereas experiments demonstrated increase in 3 to 4 times in comparison to pure DMSO. The dependences of the Raman signal on the scatterer volume fraction obtained both in simulations and in experiment are in reasonable agreement. However, despite strong light scattering in gallium phosphide powder suspension weak light absorption in it prevents the Raman signal from its significant enhancement.
Thus, we demonstrated possibility to increase Raman scattering efficiency due to multiple scattering in suspension of transparent microparticles and necessary conditions of this effect.
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