Relevant Length Scales in Brillouin Imaging of Biomaterials: The Interplay between Phonons Propagation and Light Focalization

Recent advances in photonics technologies pushed optical microscopy toward new horizons in materials characterization. In this framework, Brillouin microscopy emerged as an innovative method to provide images of materials with mechanical contrast without any physical contact, but exploiting the light-matter interaction. Brillouin imaging holds great promise: To allow mechanical analysis inside soft and heterogeneous materials, addressing the pivotal role played by viscoelastic properties in the physiology and pathology of living tissues and cells. Nevertheless, extending the approach of Brillouin imaging to characterize elastic heterogeneities of micro- A nd nanostructured samples is especially challenging, and it poses a critical question about the actual spatial resolution reachable in the mechanical characterization. We focus this critical review on the key quantities that define the spatial resolution in the Brillouin scattering process, and we highlight that not only the optical focalization of the light, but also the acoustic excitations present in the material influence the information collected from a sample by Brillouin imaging. Referring to the body of knowledge gained in the field of material science, we review new results and recently obtained progresses in the more unexplored context of life science. In future developments, a comprehensive strategy to tackle both the acoustic and the optical aspects of the measurement will be required to maximize the efficacy of the technique.