Superheterodyne Brillouin spectroscopy using frequency-tunable lasers

我们采用一种新型的光学超外差方法，利用频率可调激光器，成功地测量了0 ~ 3GHz超宽频率范围内简单液体的动态光散射光谱，即所谓的瑞利布里渊三重态。它的频率分辨率（-300kHz）由两个激光器之间的短时间相干决定。与传统的光学加热方法相比，我们的方法有一个很大的优点，即光电探测器的速度不受频率上限的限制。这是通过激光频率的连续可调谐来实现的。该方法在广泛的凝聚态物质动力学研究中具有很高的潜力。光外差法通常具有比滤光片法高得多的频率分辨率。这主要是因为在光学混频中，跳动信号的载波频率与激发光和局部光之间的差有关。因此，与滤光片法相比，光学加热法获得一定频率分辨率所需的相对精度要低得多。

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.