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1. Broadband subwavelength imaging of flexural elastic waves in flat phononic crystal lenses

   https://doi.org/10.1038/s41598-023-34314-5  

   Danawe, Hrishikesh ; Tol, Serife ( May 2023 , Scientific Reports)

   Subwavelength imaging of elastic/acoustic waves using phononic crystals (PCs) is limited to a narrow frequency range via the two existing mechanisms that utilize either the intense Bragg scattering in the first phonon band or negative effective properties (left-handed material) in the second (or higher) phonon band. In the first phonon band, the imaging phenomenon can only exist at frequencies closer to the first Bragg band gap where the equal frequency contours (EFCs) are convex. Whereas, for the left-handed materials, the subwavelength imaging is restricted to a narrow frequency region where wave vectors in PC and background material are close to each other, which is essential for single-point image formation. In this work, we propose a PC lens for broadband subwavelength imaging of flexural waves in plates exploiting the second phonon band and the anisotropy of a PC lattice for the first time. Using a square lattice design with square-shaped EFCs, we enable the group velocity vector to always be perpendicular to the lens interface irrespective of the frequency and incidence angle; thus, resulting in a broadband imaging capability. We numerically and experimentally demonstrate subwavelength imaging using this concept over a significantly broadband frequency range.

    

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2. Hyperbolic material enhanced scattering nanoscopy for label-free super-resolution imaging

   https://doi.org/10.1038/s41467-022-34553-6  

   Lee, Yeon Ui ; Li, Shilong ; Wisna, G. Bimananda M. ; Zhao, Junxiang ; Zeng, Yuan ; Tao, Andrea R. ; Liu, Zhaowei ( November 2022 , Nature Communications)

   Fluorescence super-resolution microscopy has, over the last two decades, been extensively developed to access deep-subwavelength nanoscales optically. Label-free super-resolution technologies however have only achieved a slight improvement compared to the diffraction limit. In this context, we demonstrate a label-free imaging method, i.e., hyperbolic material enhanced scattering (HMES) nanoscopy, which breaks the diffraction limit by tailoring the light-matter interaction between the specimens and a hyperbolic material substrate. By exciting the highly confined evanescent hyperbolic polariton modes with dark-field detection, HMES nanoscopy successfully shows a high-contrast scattering image with a spatial resolution around 80 nm. Considering the wavelength at 532 nm and detection optics with a 0.6 numerical aperture (NA) objective lens, this value represents a 5.5-fold resolution improvement beyond the diffraction limit. HMES provides capabilities for super-resolution imaging where fluorescence is not available or challenging to apply.

    

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3. Sub-wavelength lateral detection of tissue-approximating masses using an ultrasonic metamaterial lens

   https://doi.org/10.1038/s41467-020-19591-2  

   Walker, Ezekiel L. ; Jin, Yuqi ; Reyes, Delfino ; Neogi, Arup ( December 2020 , Nature Communications)

   null (Ed.)

   Abstract Practically applied techniques for ultrasonic biomedical imaging employ delay-and-sum (DAS) beamforming which can resolve two objects down to 2.1 λ within the acoustic Fresnel zone. Here, we demonstrate a phononic metamaterial lens (ML) for detection of laterally subwavelength object features in tissue-like phantoms beyond the phononic crystal evanescent zone and Fresnel zone of the emitter. The ML produces metamaterial collimation that spreads 8x less than the emitting transducer. Utilizing collimation, 3.6x greater lateral resolution beyond the Fresnel zone limit was achieved. Both hard objects and tissue approximating masses were examined in gelatin tissue phantoms near the Fresnel zone limit. Lateral dimensions and separation were resolved down to 0.50 λ for hard objects, with tissue approximating masses slightly higher at 0.73 λ . The work represents the application of a metamaterial for spatial characterization, and subwavelength resolution in a biosystem beyond the Fresnel zone limit. 

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4. Enhanced Resolution of the Subducting Plate Interface in Central Alaska From Autocorrelation of Local Earthquake Coda

   https://doi.org/10.1029/2018JB016167  

   Kim, Doyeon ; Keranen, Katie M. ; Abers, Geoffrey A. ; Brown, Larry D. ( February 2019 , Journal of Geophysical Research: Solid Earth)

   Subducting plates below 10‐km depth are primarily imaged using phases from teleseismic earthquakes at frequencies dominantly below 1 Hz, resulting in low‐resolution images compared to fault zone thickness. Here we image the plate boundary zone in Alaska using scattered body wave arrivals in local earthquake coda to produce a higher‐resolution image of the slab. An autocorrelation method successfully extracts coherent arrivals from the local earthquake signals. Our autocorrelation results image interfaces associated with the subducting oceanic plate at higher resolution than our teleseismic receiver functions, with increased coherence and sharper boundaries. Our results provide one of the first coherent structural images of the seismogenic zone using scattered local body waves. Amplitudes suggest that seismic wave speed decreases with increasing depth within the low‐velocity zone, supporting lithologic rather than purely overpressure models for the zone in our region. Similar methodologies using dense stations could provide higher‐resolution images to characterize crustal and uppermost mantle boundaries globally.

    

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5. Electroelastic metasurface with resonant piezoelectric shunts for tunable wavefront control

   https://doi.org/10.1088/1361-6463/acbd5f  

   Lin, Z. ; Tol, S. ( March 2023 , Journal of Physics D: Applied Physics)

   In this paper, we design a tunable phase-modulated metasurface composed of periodically distributed piezoelectric patches with resonant-type shunt circuits. The electroelastic metasurface can control the wavefront of the lowest antisymmetric mode Lamb wave (A₀mode) in a small footprint due to its subwavelength features. The fully coupled electromechanical model is established to study the transmission characteristics of the metasurface unit and validated through numerical and experimental studies. Based on the analysis of the metasurface unit, we first explore the performance of electroelastic metasurface with single-resonant shunts and then extend its capability with multi-resonant shunts. By only tuning the electric loads in the shunt circuits, we utilize the proposed metasurface to accomplish wave deflection and wave focusing ofA₀mode Lamb waves at different angles and focal points, respectively. Numerical simulations show that the metasurface with single-resonant shunts can deflect the wavefront of 5 kHz and 6 kHz flexural waves by desired angles with less than$2\mathrm{\%}$deviation. In addition, it can be tuned to achieve nearly three times displacement amplification at the designed focal point for a wide range of angles from$-{75}^{\circ }$to 75^∘. Furthermore, with multi-resonant shunts, the piezoelectric-based metasurface can accomplish anomalous wave control over flexural waves at multiple frequencies (i.e. simultaneously at 5 kHz and 10 kHz), developing new potentials toward a broad range of engineering applications such as demultiplexing various frequency components or guiding and focusing them at different positions.

    

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