100 years of Brillouin scattering: Historical and future perspectives

The Year 2022 marks 100 years since Leon Brillouin predicted and theoretically described the interaction of optical waves with acoustic waves in a medium. Accordingly, this resonant multi-wave interaction is referred to as Brillouin scattering. Today, Brillouin scattering has found a multitude of applications, ranging from microscopy of biological tissue, remote sensing over many kilometers, and signal processing in compact photonic integrated circuits smaller than the size of a thumbnail. What allows Brillouin scattering to be harnessed over such different length scales and research domains are its unique underlying properties, namely, its narrow linewidth in the MHz range, a frequency shift in the GHz range, large frequency selective gain or loss, frequency tunability, and optical reconfigurability. Brillouin scattering is also a ubiquitous effect that can be observed in many different media, such as freely propagating in gases and liquids, as well as over long lengths of low-loss optical glass fibers or short semiconductor waveguides. A recent trend of Brillouin research focuses on micro-structured waveguides and integrated photonic platforms. The reduction in the size of waveguides allows tailoring the overlap between the optical and acoustic waves and promises many novel applications in a compact footprint. In this review article, we give an overview of the evolution and development of the field of Brillouin scattering over the last one hundred years toward current lines of active research. We provide the reader with a perspective of recent trends and challenges that demand further research efforts and give an outlook toward the future of this exciting and diverse research field.