Ultra-broadband achromatic acoustic metasurfaces with customized dispersions

Recently, National Science Review publishes the latest study led by Prof. Daining Fang (Beijing Institute of Technology), Prof. Li Cheng (The Hong Kong Polytechnic University), Prof. Yue-Sheng Wang (Tianjin University) and conducted by Dr. Hao-Wen Dong (Beijing Institute of Technology), Dr. Chen Shen (Rowan University), Dr. Sheng-Dong Zhao (Qingdao University), and cooperated with Prof. Chuanzeng Zhang (University of Siegen), Prof. Steven A. Cummer (Duke University), Prof. Hairong Zheng (Shenzhen Institutes of Advanced Technology), Dr. Weibao Qiu (Shenzhen Institutes of Advanced Technology). Combing the wave mechanics with the advanced structure technology, this international team proposed an inverse-design methodology for achieving the customized dispersions and constructed the achromatic acoustic metasurfaces successfully, thus realizing multiple functionalities of high-efficiency and ultra-broadband acoustic directional energy transmission, energy convergence and ultrasound particle levitation. They further revealed the synergistic mechanism of ultra-broadband achromatic characteristic. The study can provide the theory guidance and structure foundation for the realizations of ultra-broadband acoustic metamaterials and meta-devices.

In recent years, as a class of subwavelength artificial surface structures for manipulating the phase, amplitude and propagation mode of acoustic waves, acoustic metasurfaes possess the extraordinary ability of shaping wavefronts and can lead to the high-efficiency and even controllable acoustic absorption, reflection and transmission properties, showing the broadband application prospects in the fields of communication, medical testing, aerospace and national defense engineering, etc. However, most of acoustic metasurfaces were faced with the issues of narrow bands and dispersions of functionalities. “Although the tunable and coding approaches can enlarge the bandwidth to a certain extent, the system still suffered from the obvious dispersions, low reliability, high complexity and fabricating cost, etc. Besides, the tunable strategy of metasurfaces is highly frequency dependent. Despite the possibility of guaranteeing the functionality at individual frequencies, it can hardly accommodate broadband incident wave packet containing simultaneously multiple frequencies. Therefore, it is urgent to implement the passive, ultra-broadband and frequency-independent acoustic metasurfaces” Hao-Wen Dong says.

To address the aforementioned challenges, this team proposed came up with an inverse-design framework for ultra-broadband achromatic metasurfaces (Fig. 1). In theory, if the reflective/refractive wavefronts are identical, the metasurfaces can be regarded as achromatic ones (Fig. 1a). In order to realize the steered, focusing and bottle beams within the broadband range (Fig. 1b), metasurfaces have to support the linear non-dispersive, nonlinear non-dispersive and dispersive features, respectively (Fig. 1c). Therefore, for the particular dispersion, rigorous phase distribution and high transmission (Fig. 1d), all metasurface elements have to simultaneously satisfy the prescribed effective indices, relative group delays and relative group delay dispersions. To compatibly take these properties into consideration, this team built a topology-optimization “phase-efficiency-dispersion” model of metasurfaces and develop an inverse-design methodology for ultra-broadband, achromatic and high-efficiency metasurfaces (Fig. 1e).

Researchers firstly designed the acoustic metasurface (Fig. 2a) comprising several asymmetrical local cavities and curved channels which can possess the constant effective index, high transmission (Fig. 2b, 2c). Accordingly, the metasurface can bring about the abnormal high-efficiency acoustic directional transmission with the nearly fixed refraction angle (Fig. 2d, 2e). Besides, researchers constructed similar asymmetric metasurfaces (Fig. 3a) to capture the constant indices within the range of [1000 Hz, 4000 Hz] (Fig. 3b, 3c) and thus realizing the acoustic focusing with the nearly same focal planes and high transmission (>80%) (Fig. 3d, 3e). Furthermore, this team designed an achromatic metasurface having the non-dispersive and nonlinear dispersive features within [16.5 kHz, 66 kHz] (Fig. 4a-4c). Due to the bottle beams with identical levitation location, the metasurface can make a polyethylene ball suspension in air (Fig. 4f, Video-a, Video-b). Compared with the existing ultrasound levitation technologies, the levitation technology based on an achromatic metasurface can have the advantages of stability, ultra-broadband nature and single-sided manipulation. It was worth noting that all inverse-designed metasurface elements support the internal resonances (Fig. 5a-5c), bi-anisotropy (Fig. 5d-5f) and multiple scattering (Fig. 5g-5i). This indicated that the multiple scattering can contribute to achieve the ultra-broadband and achromatic properties. And the multiple scattering can also be deemed as a new degree of freedom of designing novel metsasurfaces. Consequently, the ultra-broadband achromatic and high-efficiency functionalities were benefited from the synergetic mechanisms of internal resonances, bi-anisotropy and multiple scattering.

This published study demonstrated the feasibility of realizing the achromatic acoustic metasurfaces with the arbitrary dispersions through the inverse-design method. The proposed methodology and elaborate metasurfaces are promising in the fields of passive, ultra-broadband and multi-functional metamaterials, and can offer the effective strategies for broadband, high-efficiency acoustic energy radiation, noise shielding, energy harvesting, diverse particle manipulation and transport, etc. “Although the research objects are acoustic metasurfaces, the developed inverse-design methodology here also can be extended to the fields of elastic/electromagnetic metamaterials” Hao-Wen Dong says.

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See the article:

Achromatic metasurfaces by dispersion customization for ultra-broadband acoustic beam engineering

https://doi.org/10.1093/nsr/nwac030

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