Switching on and off acoustic waves through metamaterials has potential in noise canceling, underwater detection, and communication. However, traditional acoustic designs are challenging in manipulating underwater acoustic waves when the device thickness is less than wavelength. Here we report an alternative design of an underwater metasurface-based acoustic switcher to achieve this goal. The switching mechanism is revealed by combing acoustic diffraction of grating with mode conversion of double-layer PMMA plates. The device is tuned to control wave transmission by changing grating angle. Furthermore, we experimentally fabricate the metasurface acoustic switcher. The broadband-switching performance is realized to control underwater target detection and to produce binary digital encoding for acoustic waves. The proposed metasurface acoustic switcher offers the advantages of broadband performance and thin structure, which promises the opportunity for designing next-generation broadband-switching devices in underwater acoustic detection and communication.

Capacitive micromachined ultrasound transducers (CMUTs) have broad application prospects in medical imaging, flow monitoring, and nondestructive testing. CMUT arrays are limited by their fabrication process, which seriously restricts their further development and application. In this paper, a vacuum-sealed device for medical applications is introduced, which has the advantages of simple manufacturing process, no static friction, repeatability, and high reliability. The CMUT array suitable for medical imaging frequency band was fabricated by a silicon wafer bonding technology, and the adjacent array devices were isolated by an isolation slot, which was cut through the silicon film. The CMUT device fabricated following this process is a 4 × 16 array with a single element size of 1 mm × 1 mm. Device performance tests were conducted, where the center frequency of the transducer was 3.8 MHz, and the 6 dB fractional bandwidth was 110%. The static capacitance (29.4 pF) and center frequency (3.78 MHz) of each element of the array were tested, and the results revealed that the array has good consistency. Moreover, the transmitting and receiving performance of the transducer was evaluated by acoustic tests, and the receiving sensitivity was −211 dB @ 3 MHz, −213 dB @ 4 MHz. Finally, reflection imaging was performed using the array, which provides certain technical support for the research of two-dimensional CMUT arrays in the field of 3D ultrasound imaging.

Auroral kilometric radiations (AKR) are widely existing superluminous waves in the magnetosphere. Due to rarely available measurements of AKR magnetic field components, we derive the explicit expression of diffusion coefficients in terms of electric field components, which can be applied to various space plasma waves. Using Van Allen Probes data, we calculate diffusion coefficients with different harmonic orders for two AKR events. It is found that the first (higher) harmonic order contributes to diffusion coefficients at small (larger) pitch angles. The effect of higher harmonic orders becomes more significant as the electron energy increases. These results are contrary to the case of chorus wave, in which the first order usually contributes much to wave‐particle interactions. Estimations show that the phase space density of 0.5 MeV electrons can increase ∼10 times within 6 hr by AKR. This study is important for better modeling of AKR‐induced radiation belt electron dynamics.

Non-contact laser ultrasonic detection technology provides an innovative solution for evaluating the internal conditions of lithium-ion batteries (LIBs), offering significant advantages in gas defect assessment and structural defect identification. This study proposes a method for evaluating internal gas defects in LIBs based on a non-contact laser ultrasonic system. The system uses a pulsed laser to generate ultrasonic waves, with a full-optical probe receiving the signals, enabling high-resolution imaging of the internal features of the battery. The study analyzes key ultrasonic characteristics under different laser parameters (energy, pulse width, and focal length) and their correlation with defective regions. Through both time-domain and frequency-domain analysis of the ultrasonic features, the results demonstrate that the signal amplitude attenuation characteristics of ultrasound in media with acoustic impedance mismatches can be used for precise detection and quantitative characterization of gas defect regions within the battery. This non-contact technology offers a promising method for real-time, non-destructive monitoring of the internal condition of lithium-ion batteries, significantly enhancing battery safety and reliability.

Asymmetric acoustic transmission devices with less restrictions on incident waves, i.e. incident direction and wavelength, are an inevitable trend of its future development. In this study, we reveal the mechanism for the subwavelength asymmetric transmission for underwater sound with multiple incident directions in an assembly system, constituting of a cascade solid-fluid superlattice coupled with an acoustic grating (AG) immersed in water. Theoretical analysis and numerical simulations verified that acoustic waves are almost totally reflected for negative incidence with multiple incident directions, while having a high transmission for positive cases. Moreover, our design can work within a remarkable broad bandwidth and possesses a preserved waveform in the subwavelength region. The unique feature is attributed to the extraordinary ultra-wide forbidden transmission of the cascade structure, as well as the diffraction effect of AG at low frequencies. The perfect subwavelength performance of our design suggests its potential practical application in underwater communication and signal transmission.

Recently, rainbow trapping of acoustic/elastic waves in gradient artificial structures has shown promising applications in energy harvesting and frequency separation. However, the novel phenomenon of acoustic superlattices for underwater acoustic waves has not been explored. In this work, we establish the theoretical model of underwater acoustic waves propagating through the gradient-index superlattice (GISL) by using the transfer matrix method. We show that the combined band structures of infinite sub-superlattices and the transmission curves of finite GISL exhibit Bragg forbidden bands that are wider than those of periodic ones, which implies the occurrence of rainbow trapping. On this basis, the effects of gradient factor, period number, and water thickness are discussed, and the tunability of the trapping range is also explained. Furthermore, simulations with Gaussian beam incidence are performed through finite element methods. The results in frequency-domain and time-domain both indicate perfect performances of energy localization and frequency separation for acoustic waves of different frequencies. Our findings show possibilities for the design of underwater acoustic devices, such as acoustic sensors and frequency dividers.