Wang Wei, Hu Chengbo, Ni Jincheng, Ding Yujiang, Weng Jingkai, Liang Bin, Qiu Cheng-Wei, Cheng Jian-Chun
Collaborative Innovation Center of Advanced Microstructures and Key Laboratory of Modern Acoustics, MOE Institute of Acoustics, Department of Physics, Nanjing University, Nanjing, 210093, China.
Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore, 117583, Singapore.
Adv Sci (Weinh). 2022 Nov;9(33):e2203482. doi: 10.1002/advs.202203482. Epub 2022 Oct 17.
Despite the significance for wave physics and potential applications, high-efficiency frequency conversion of low-frequency waves cannot be achieved with conventional nonlinearity-based mechanisms with poor mode purity, conversion efficiency, and real-time reconfigurability of the generated harmonic waves in both optics and acoustics. Rotational Doppler effect provides an intuitive paradigm to shifting the frequency in a linear system which, however, needs a spiral-phase change upon the wave propagation. Here a rotating passive linear vortex metasurface is numerically and experimentally presented with close-to-unity mode purity (>93%) and high conversion efficiency (>65%) in audible sound frequency as low as 3000 Hz. The topological charge of the transmitted sound is almost immune from the rotational speed and transmissivity, demonstrating the mechanical robustness and stability in adjusting the high-performance frequency conversion in situ. These features enable the researchers to cascade multiple vortex metasurfaces to further enlarge and diversify the extent of sound frequency conversion, which are experimentally verified. This strategy takes a step further toward the freewheeling sound manipulation at acoustic frequency domain, and may have far-researching impacts in various acoustic communications, signal processing, and contactless detection.
尽管对波动物理学和潜在应用具有重要意义，但基于传统非线性机制无法实现低频波的高效频率转换，因为在光学和声学中，所产生谐波的模式纯度、转换效率和实时可重构性都很差。旋转多普勒效应为线性系统中的频率转换提供了一种直观的范式，然而，这需要在波传播时发生螺旋相位变化。在此，通过数值模拟和实验展示了一种旋转无源线性涡旋超表面，在低至3000 Hz的可听声频率下，具有接近100%的模式纯度（>93%）和高转换效率（>65%）。透射声的拓扑电荷几乎不受转速和透射率的影响，证明了在原位调整高性能频率转换时的机械鲁棒性和稳定性。这些特性使研究人员能够级联多个涡旋超表面，以进一步扩大和多样化声频转换的范围，这已通过实验验证。该策略朝着在声频域进行自由的声音操纵又迈进了一步，并且可能在各种声学通信、信号处理和非接触检测中产生深远影响。
