Imaging techniques with subdiffraction-limited spatial resolutions are highly desired for a deeper understanding of subcellular systems. Optical imaging enables high resolution under 200 nm while diattenuation, multiple scattering, and depolarization limit the visible light penetration depth to merely 2 mm. Ultrasound images achieve two orders of magnitude lower resolutions but can penetrate two orders of magnitude deeper into a medium than optical images. This work combines the strengths of optical and acoustic imaging techniques through AuNP-based metasurfaces utilizing the photoacoustic effect. Our novel imaging technique can simultaneously achieve high resolution and deep penetration depths without any destruction of media. Moreover, the application to photoacoustic structured illumination microscopy would lead to new avenues in super-resolution imaging.<br/>Optical metasurfaces are two-dimensional arrays of subwavelength scatterers that enable versatile wave manipulation, including modification of the wavefront, polarization distribution, intensity distribution, or spectral response. A metasurface's effective refractive index and spectral characteristics depend on material composition, intrinsic and extrinsic resonances, structure size, and ambient conditions. The rational design of the scatterer and its array makes versatile control of the metasurface interaction with the incident and generated waves possible.<br/>Here, a nanophotonic metasurface composed of AuNPs was designed for efficient and tunable dynamic wavefront shaping by merging the physical principles of nanoscale light and sound. AuNPs exhibit the photoacoustic effect, surface plasmon resonances, and strong tunable optical absorption. Additionally, absorptive properties of AuNPs at visible frequencies increase by orders of magnitude. The metasurface design was optimized using full-field numerical simulations for maximum thermal dissipation and efficiency in photoacoustic wave generation. The dynamic acoustic wave propagation resulting from varying optical excitation and the resulting acoustic wavefront was also simulated.<br/>The optimized metasurface is active and dynamic as the acoustic wavefront can be diversified in a multitude of patterns by varying incident light polarization. This suggests a miniaturized on-chip platform for photoacoustic structured illumination microscopy with facile implementation, including super-resolution biochemical imaging deep in tissue.

Super-resolution, deep-penetration, and non-destructive photoacoustic imaging with gold nanoparticle (AuNP) based wavefront shaping

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