Signal filtering aims to extract the target component from overall signals. Typically, filter in the frequency domain, or spectral separation, helps to suppress interference and distinguish signal components from different frequencies, which is of great significance in many applications such as sensors, multiplexers, filters and other multi-functional devices. However, rather than the readily optical spectral separation benefiting from the intrinsic dispersion of natural materials, frequency separation in acoustic is challenging due to the negligible dispersion in natural materials over a wide frequency range, yet imperative for acoustic signal processing and biomedical science.

Here, we numerically design and experimentally realize the frequency separation and perception for underwater multitone ultrasound, or dubbed frequency distillation in our work. It is achieved by a dispersive reflector, which spatially splits the ultrasound wave of different frequencies superimposed in the incident beam into different reflection directions. The precise frequency distillation with strong robustness is validated by the evidences of high distilled accuracy rate (over 95%), highly distinguishable spectral resolution (within 5%) and broad effective frequency range (over 0.85 octaves), even in the presence of defects or alterations in the configuration. Moreover, compared to the previous spectral separation devices based on rainbow trapping where the sound is localized inside the specific positions of the structure, our scheme allows the distilled wave propagating outside, which facilitates the post-processing of signals. These pronounced properties of the underwater ultrasound dispersive reflector for frequency distillation and perception are promising for the integrated and chip-scale devices in acoustic communication, signal processing, biomedical sensing and imaging.

The relevant research was published in Applied Physics Letters on October 7, 2021, with the title of"Frequency distillation with dispersive reflector for multitone ultrasound perception". The doctoral student Jiajie He is the first author, and the Young Associate Researcher Xue Jiang and Professor Dean Ta are the corresponding authors. This work was supported by the National Natural Science Foundation of China, the Shanghai Committee of Science and Technology, the Program of Shanghai Academic Research Leader, etc.（Jiajie He, Xue Jiang*, Hualiang Zhao, Dean Ta*, and Weiqi Wang, Applied Physics Letters, 119, 144102, 2021）