Escalated Deep-Subwavelength Acoustic Imaging with Field Enhancement Inside a Metalens

Super-resolution acoustic imaging with state-of-the-art spatial resolution (λ/50), with λ being the wavelength, is showcased with a holey-structured metalens. However, the imaging mechanism under unity transmission based on Fabry-Perot resonances means the metalens fundamentally suffers from narrow bandwidth and limited deep-subwavelength contrast, and therefore further advancement of deep-subwavelength imaging has been stalled. Here we break the barriers for deep-subwavelength acoustic imaging comprehensively in spatial resolution, resolving contrast, and working bandwidth, by exploiting field enhancement inside the metalens. A microscopic model is established to theoretically reveal the underlying physics for escalated deep-subwavelength acoustic imaging. For a proof-of-concept, the imaging performance of the proposed method is numerically proven and experimentally demonstrated. Specifically, a breakthrough resolution below λ/100 is achieved while resolving contrast is improved by at least 6.5 times and working bandwidth is broadened to approximately 25% of the operating frequency. Furthermore, pulsed acoustic imaging on the deep-subwavelength scale is showcased, which is an important step towards the practical application of the ultrahigh-resolution acoustic imaging technique. We believe the work presented here may greatly benefit a variety of fields in acoustics, such as visualizing subcutaneous structures in medical diagnosis and characterizing subsurface flaws in industrial nondestructive evaluation.

Figure 1

Illustration of escalated deep-subwavelength acoustic imaging. The schematic metalens consists of an array of deep-subwavelength waveguides perforated in a rigid block with a thickness of h. The inset schematically illustrates the field enhancement inside the deep-subwavelength waveguide. k_x is the tangential wave vector of the evanescent waves.

Figure 2

Pressure field enhancement inside the metalens. (a) and (b) The intensity distributions of the simulated pressure field along the x direction by varying the edge-to-edge distance between two slits in a thin film. The slit width of the source object and the lattice constant of the metalens in the simulation are λ/100 and λ/200, respectively. The image planes are located (a) 0.01λ away from the output surface and (b) at the middle plane of the metalens. (c) The fundamental transmission and reflection coefficients between a deep-subwavelength cavity and free space. The evanescent wave is coupled and mode converted into propagating waves inside the waveguide.

Figure 3

The schematic of the experimental setup for acoustic field measurements. The inset shows the basic principle and configuration for optical measurement of an acoustic wave inside the deep-subwavelength waveguide.

Figure 4

Broadband deep-subwavelength imaging of defective object. (a) and (b) The normalized intensity maps of (a) simulated and (b) measured pressure fields inside the metalens. The white dashed rectangle in (a) shows the measurement area in the experiments. The inset in (a) above the image schematically shows the imaged object. (c) The pressure amplitude profiles along the horizontal (x) direction at different imaging planes. The solid lines represent the simulated profiles at the output surface (blue), internal plane (red), and reference plane (black), respectively. The reference imaging plane is 0.01λ away from the output surface. The measured profile at the internal plane (green dashed symbol) is averaged by 10 measurements and mapped to the simulated pressure field for ease of comparison. (d) and (e) The normalized amplitude profiles of simulated and measured pressure fields at the internal plane at (d) 2250 Hz and (e) 2900 Hz. The simulated and measured results are normalized to their respective maximum values. The measured results are averaged over 10 measurements.

Figure 5

Deep-subwavelength imaging of complex objects with a pulse wave. (a) The target image of Nanyang Technological University logo. The letters “NTU” are perforated in an acrylic plate. Scale bar, 5 mm. (b) The schematic illustration of the imaging process for pixel-to-pixel sampling. Antireflective glass is used to enhance the optical transmission and seal the waveguides. For better visualization, the objects (metalens, sample, and optical beam) are not to scale. (c) The time traces of the pulse waves along the white dashed line in (a). The pulse waves are recorded at the middle plane of the metalens. (d)–(f) The simulated and measured images at different frequencies: (d) 2250 Hz, (e) 2580 Hz, and (f) 2900 Hz. The simulated and measured pressure fields are obtained at the middle plane of the metalens. The intensity is normalized at the respective frequency. White bars correspond to a length of 5 mm.