  Brillouin spectroscopy is commonly used to study the acoustic properties of
materials. Here we explored its feasibility in studying the photoinduced
isomerization of azobenzene. The isomerization of azobenzene changes the
solution elastic modulus, and Brillouin scattering is sensitive to these
changes. In this study, we experimentally demonstrated the photoswitching of
azobenzene in DMSO using our home-made VIPA-based high-resolution optical
Brillouin spectrometer, and confirmed the results by ultraviolet-visible
spectrophotometry. Remarkable Brillouin frequency shift variations were
quantitatively recorded upon irradiation, and it was found that this method can
indeed be used to monitor the isomerization process in situ. Importantly, our
strategy also allows us to provide the relationship between the ratio of trans-
and cis- azobenzene and the Brillouin frequency shift. This shows that
Brillouin spectroscopy has broad prospects for the characterization of
azobenzene isomerization and other photo responsive materials.
