Superheterodyne Brillouin spectroscopy using frequency-tunable lasers

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We have succeeded in measuring dynamic light scattering spectra, so-called Rayleigh Brillouin Tripplet of simple liquid in an ultrawide frequency from 0 to 3GHz by applying a new optical superheterodyne method using frequency tunable lasers. Its frequency resolution (-300kHz) 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.Optical heterodyne method generally has a much higher frequency resolution than the optical filter method. 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 technique to obtain a certain frequency resolution than for the opitical filter method.