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Submitted on 1 Jan 1975
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BRILLOUIN SCATTERING IN LIQUID CRYSTALS
M. Čopič, B. Lavrenčič
To cite this version:
M. Čopič, B. Lavrenčič. BRILLOUIN SCATTERING IN LIQUID CRYSTALS. Journal de Physique
Colloques, 1975, 36 (C1), pp.C1-89-C1-90. �10.1051/jphyscol:1975114�. �jpa-00215891�
JOURNAL DE PHYSIQUE Colloque C1, supplkment au no 3, Tome 36, Mars 1975, page C1-89
Classification Physics Abstracts
7.130
BRILLOUIN SCATTERING IN LIQUID CRYSTALS
M. COPIC and B. B. L A V R E N ~ I ~
Institute J.-Stefan, University of Ljubljana, 61001 Ljubljana, Yougoslavie
RhumB. - On determine ici le spectre Brillouin du MBBA dans sa phase isotrope. On n'observe aucune dispersion au point de transition.
Abstract. - The Brillouin spectrum of the isotropic phase of MBBA was investigated. No dispersion was found at the transition point.
1. Introduction. - Light scattering is a very power- ful tool to investigate the low-wavevector part of the fluctuation spectrum. However, rather few investiga- tions of the Brillouin component of scattered radiation in liquid crystals have been made [I, 2, 31. It appears that the reason for this is serious experimental diffi- culties, which are due to the relatively strong central component that tends to mask the Brillouin peaks.
In ultrasonic experiments some dispersion of ultra- sound velocity was found up to frequencies of about 15 MHz [4, 51. Also, dispersion of Brillouin shifts was observed in a cholesteric liquid crystal [2]. We investi- gated the influence of the isotropic-nematic transition in MBBA upon the Brillouin spectrum.
2. Experimental. - The measurements have been done in the isotropic phase only and even in the iso- tropic phase the observation was rather difficult. Since in the vicinity of the transition the central component is more than thousand times stronger than the Brillouin part of the spectrum, we had to increase the contrast of our instrument by using a triple-pass piezoelectrically scanned Fabry-PCrot interferometer [6]. With such system the alignment and stability problems are consi- derably greater than with the usual single pass arran- gement, and also the signal intensity is much reduced, which was one of our main problems. Our light source was He-Ne laser which gave typically about 5 mW single frequency power on the sample. The sample was placed in a temperature stabilized index-matching cell with temperature setting accuracy about 0.1 OC and stability about 0.01 OC. The scattering angle was 900.
The Fabry-Pkrot free spectral range was 0.73 cm-' with a long term finesse of about 40. The signal was detected with a photon-counting photomultiplier and stored in a multichannel analyser. Typical accumulation time was about 15 min. The transition point of our sample of MBBA was originally 44.1
O C ,but owing to the dete- rioration of the sample it decreased after every heating cycle and so had to be determined on every experi- mental run. The lowest transition point on cooling was 41.0 OC.
MBBA
FIG. 1. - The polarized (W) and depolarized (VH) Brillouin spectrum of MBBA in isotropic phase.
3. Results. - A typical spectrum obtained is shown on figure 1. There is no observable Brillouin component in the depolarized spectrum, whereas in the polarized spectrum the Brillouin peaks are visible, but much weaker than the central component. The asymmetry of the spectrum is purely instrumental. Instrumental line- width was about half the Brillouin linewidth. The
MBBA
FIG. 2. - The observed Brillouin shift of MBBA in the isotropic phase as a function of temperature.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975114
C1-90 M. ~ O P I C AND B. B. LAVRENCIC Brillouin shift near the transition temperature is about
0.2 cm- and is quite constant (Fig. 2). At temperatures more than 150 above transition point the shift does decrease slightly, as one would expect in an ordinary liquid. 30 OC above transition, that is at about 72 OC, the shift is 0.17 cm-I. We cannot determine the trend of the Brillouin linewidth since the scatter of the results is too large. The Brillouin linewidths are, however, considerably above instrumental linewidths. We have also tried to estimate the Landau-Placzek ratio - that is the ratio of Rayleigh to Brillouin intensities (Fig. 3).
The results are, however, very undependable due to their critical dependence upon instrument alignment.
But a marked increase near the transition point is unmistakable.
From the present measurements it appears that there is no appreciable coupling between orientational fluctuations and density waves at frequencies in the GHz range, which agrees with some observations on
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