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ALT'23 The 30th International Conference on Advanced Laser Technologies
LD-O-14
Enhancement of Raman scattering efficiency in suspensions of
submicron particles
O.I. Sokolovskaya, L.A. Golovan
Faculty of Physics, Lomonosov Moscow State University, 1/2 Leninskie Gory 119991 Moscow, Russia
oi.sokolovskaja@physics. msu. ru
Elastic light scattering in a disordered medium may result in an increase in the volume of light-matter interaction compared to a homogeneous medium [1]. As a result, an increase in the efficiency of various optical processes, e.g., Raman scattering (RS) can be observed. Therefore, it is necessary to establish scatterer parameters allowing RS efficiency to be enhanced and find its maximal possible increase due to elastic light scattering.
In this work, we have carried out both numerical simulation and experimental study of the light propagation and RS in scattering media. Suspensions of rutile particles with diameters of 350 and 500 nm and gallium phosphide particles with diameter of 3 ^m in dimethyl sulfoxide (DMSO), which is Raman active liquid, were considered as a scattering medium. The volume fraction of particles in suspensions ranged from 10-4 to 10-1. To record the dynamics of the radiation scattered by suspensions, the optical heterodyning method [2] employing laser pulses with a duration of 80 fs (wavelength 1250 nm) was used. Raman spectra were obtained with excitation wavelengths of 1064 and 532 nm.
Numerical simulation of light propagation in suspensions by the Monte Carlo method [3] showed that the dependence of the RS signal from DMSO on scatterer volume fraction is non-monotonic with the maximal 8-fold possible increase in the backscattered RS signal efficiency (see Fig. 1) When GaP powder is added to DMSO, the mean photon path length and the RS signal decreased significantly with an increase in the scatterer volume fraction due to light absorption in them.
10-5 10-4 10-3 10-2 10-1 Rutile volume fraction
Fig. 1. Backscattered Raman signal enhancemen in rutile suspensions for excitation wavelength of 1064 nm, simulation data.
The results of numerical simulation of the temporal dynamics of laser pulses propagation in suspensions are in good agreement with experimental data for the particle volume fraction not higher than 0.01. For denser suspensions, experimental data indicate non-diffusion light propagation caused probably by scattered waves interference. Experiments on RS in rutile particle suspensions demonstrate maximum 3-fold increase in the RS signal achieved at excitation wavelength of 1064 nm with a scatterer volume fraction of 0.006. A nonmonotonic dependence of the RS signal on the scatterer volume fraction is shown: the monotonic increase of RS signal up to maximum, then decline and subsequent signal increase are observed. The found dependence is consistent with the experimentally observed increase in the photon dwelling time for relatively low scatterer volume fractions and possible appearance of coherent effects for higher scatterer concentrations.
[1] B.H. Hokr et al. Bright emission from a random Raman laser, Nature Communications, 5, 4356 (2014).
[2] K.P. Bestem'yanov, V.M. Gordienko, A.A. Ivanov, A.N. Konovalov, A.A. Podshivalov, Optical heterodyning study of the propagation dynamics of IR femtosecond laser pulses in a strongly scattering porous medium, Quantum Electronics, 34, 666 (2004).
[3] L. Wang, S.L. Jacques, L. Zheng, MCML—Monte Carlo modeling of light transport in multi-layered tissues, Computer Methods and Programs in Biomedicine, 47, 131-146 (1995).
