A new method of scattering-angle scanning for optical beating light scattering spectroscopy

Heterodyne light scattering spectroscopy is one of the powerful methods used to study the dynamics of density and concentration fluctuations in simple and complex fluids. In conventional optical beating (heterodyne) techniques, however, the beam configuration must be readjusted each time the scattering angle is changed, so as to get the maximum efficiency of the optical mixing of the scattered and the local light. This readjustment process usually takes a long time, and furthermore, leads to the loss of information on the angular dependence of the scattering intensity, which is extremely sensitive to the optical configuration. Here, we report a new method of scattering-angle scanning that is free from the aforementioned problems, which is particularly suitable for optical beating (heterodyne) light scattering spectroscopy. In our method, the maximum optical mixing efficiency between the scattered and the local light can always be maintained amidst change in the scattering angle. Its validity is demonstrated for Rayleigh scattering. Our method also enables us to determine the absolute scattering angle with a very high accuracy, which is particularly important for Brillouin scattering measurements.