On the use of evanescent plane waves for low-frequency energy transmission across material interfaces

Hybrid Metasurfaces for Perfect Transmission and Customized Manipulation of Sound Across Water-Air Interface

Extreme impedance mismatch causes sound insulation at water-air interfaces, limiting numerous cross-media applications such as ocean-air wireless acoustic communication. Although quarter-wave impedance transformers can improve transmission, they are not readily available for acoustics and are restricted by the fixed phase shift at full transmission. Here, this limitation is broken through impedance-matched hybrid metasurfaces assisted by topology optimization. Sound transmission enhancement and phase modulation across the water-air interface are achieved independently. Compared to the bare water-air interface, it is experimentally observed that the average transmitted amplitude through an impedance-matched metasurface at the peak frequency is enhanced by ≈25.9 dB, close to the limit of the perfect transmission 30 dB. And nearly 42 dB amplitude enhancement is measured by the hybrid metasurfaces with axial focusing function. Various customized vortex beams are experimentally realized to promote applications in ocean-air communication. The physical mechanisms of sound transmission enhancement for broadband and wide-angle incidences are revealed. The proposed concept has potential applications in efficient transmission and free communication across dissimilar media.

Lotus Metasurface for Wide-Angle Intermediate-Frequency Water-Air Acoustic Transmission

Only 0.1% of the acoustic energy can transmit across the water-air interface because of the huge acoustic impedance mismatch. Enhancing acoustic transmission across the water-air interface is of great significance for sonar communications and sensing. However, due to the interface instability and subwavelength characteristics of acoustic metamaterials, wide-angle intermediate-frequency (10 kHz-100 kHz) water-air acoustic transmission remains a great challenge. Here, we demonstrate that the lotus leaf is a natural low-cost acoustic transmission metasurface, namely, the lotus acoustic metasurface (LAM). Experiments demonstrate the LAM can enhance the acoustic transmission across the water-air interface, with an energy transmission coefficient of about 40% at 28 kHz. Furthermore, by fabricating artificial LAMs, the operating frequencies can be flexibly adjusted. Also, the LAM allows a wide-angle water-to-air acoustic transmission. It will enable various promising applications, such as detecting and imaging underwater objects from the air, communicating between ocean and atmosphere, reducing ocean noises, etc.

Tunable Fluid-Type Metasurface for Wide-Angle and Multifrequency Water-Air Acoustic Transmission

Efficient acoustic communication across the water-air interface remains a great challenge owing to the extreme acoustic impedance mismatch. Few present acoustic metamaterials can be constructed on the free air-water interface for enhancing the acoustic transmission because of the interface instability. Previous strategies overcoming this difficulty were limited in practical usage, as well as the wide-angle and multifrequency acoustic transmission. Here, we report a simple and practical way to obtain the wide-angle and multifrequency water-air acoustic transmission with a tunable fluid-type acoustic metasurface (FAM). The FAM has a transmission enhancement of acoustic energy over 200 times, with a thickness less than the wavelength in water by three orders of magnitude. The FAM can work at an almost arbitrary water-to-air incident angle, and the operating frequencies can be flexibly adjusted. Multifrequency transmissions can be obtained with multilayer FAMs. In experiments, the FAM is demonstrated to be stable enough for practical applications and has the transmission enhancement of over 20 dB for wide frequencies. The transmission enhancement of music signal across the water-air interface was performed to demonstrate the applications in acoustic communications. The FAM will benefit various applications in hydroacoustics and oceanography.

Bounded inhomogeneous wave profiles for increased surface wave excitation efficiency at fluid-solid interfaces

Though the ultrasonic excitation of surface waves in solids is generally realized through the use of a contact transducer, remote excitation would enable standoff testing in applications such as the nondestructive evaluation of structures. With respect to the optimal incident wave profile, bounded inhomogeneous waves, which include an exponentially decaying term, have been shown to improve the surface wave excitation efficiency as compared to Gaussian and square waves. The purpose of this work is to investigate the effect of varying the incident wave spatial decay rate, as applied to both lossless fluid-solid interfaces and to solids with viscoelastic losses included. The Fourier method is used to decompose the incident profile and subsequently compute the reflected wave profile. It is shown that inhomogeneous plane wave theory predicts, to a close approximation, the location of the minimum in the local reflection coefficient with respect to the decay rate for bounded incident waves. Moreover, plane wave theory gives a reasonable indication of the decay rate that maximizes the surface wave excitation efficiency.