ELSEVIER Physica B 219&220 (1996) 556-558 Superheterodyne Brillouin spectroscopy using frequency-tunable lasers Hajime Tanaka*, Tsuyoshi Sonehara Institute of Industrial Science, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106, Japan Abstract We have succeeded in measuring dynamic light scattering spectra (Rayleigh line and Brillouin doublet) of simple liquids in an ultrawide frequency range (0-3 GHz) by applying a new optical superheterodyne method using frequency- tunable lasers. Its frequency resolution (~ 300 kHz) is determined by the short-time coherency between the two lasers. It is a great advantage of our method over the conventional optical beating methods that the speed of the photodetector never limits the upper frequency bound. This is realized by the continuous tunability of the laser frequency. The method has a high potential for studying the dynamics in a wide class of condensed matter. Dynamic light scattering is one of the most powerful experimental means to study the dynamics of condensed matters including liquids, solids, polymers, and meso- phases, without giving any pertubation to samples [1]. The experimental methods to obtain the spectrum of the scattered lights can be classified into the following two types (see Fig. 1): (i) optical filter methods [2] and (ii) optical mixing or beating methods [3,4]. Filter methods are used to study relatively rapid dynamic pro- cesses (~c < 1 ns, re: the characteristic time). The typical filter method for Brillouin scattering is the spectral de- composition of the scattered light by a Fabry-Perot interferometer [2]. On the other hand, the optical mixing method [3, 4] which is the optical analog of the beating techniques developed in radio-frequency specroscopy, has been used for slower processes (zc < 1 ns) [6-8]. The development of this light beating spectroscopy [3,4] including the heterodyne and self-beating approaches [ 1 ] has been a key technique for the measurement of the * Corresponding author. frequency shifts of scattered lights. There have so far been many efforts to improve the frequency resolution of Bril- louin scattering measurement based on both of the above methods, as discussed below. A Fabry-Perot interferometer has been most com- monly used for studying Brillouin scattering since the first observation of Brillouin doublet [2] by this tech- nique. This method is suitable to study the thermal phonon whose frequency is more than several GHz. For studying the phonon of lower frequency, however, the resolution is sometimes not enough. This is an intrinsic difficulty of the optical filter method. The scanning fin- earity and the determination of the absolute frequency cause another serious problem of the Fabry-Perot inter- ferometer [10]. The optical heterodyne method generally has a much higher frequency resolution than the optical filter method [5]. This is primarily because in the optical mixing the beating signal has a carrier frequency of the difference between the excitation and local lights. Thus a much less relative accuracy is required for the optical beating tech- nique to obtain a certain frequency resolution than for 0921-4526/96/$15.00 ~) 1996 Elsevier Science B.V. All rights reserved SSDI 0921-4526(95)00810-1