Understanding the near-field photoacoustic spatiotemporal profile from nanostructures

Understanding the mechanism of photoacoustic generation at the nanoscale is key to developing more efficient photoacoustic devices and agents. Unlike the far-field photoacoustic effect that has been well employed in imaging, the near-field profile leads to a complex wave-tissue interaction but is understudied. Here we show that the spatiotemporal profile of the near-field photoacoustic waves can be shaped by laser pulses, anisotropy, and the spatial arrangement of nanostructure(s). Using a gold nanorod as an example, we discovered that the near-field photoacoustic amplitude in the short axis is ∼75 % stronger than the long axis, and the anisotropic spatial distribution converges to an isotropic spherical wave at ∼50 nm away from the nanorod's surface. We further extend the model to asymmetric gold nanostructures by arranging isotropic nanoparticles anisotropically with broken symmetry to achieve a precisely controlled near-field photoacoustic "focus" largely within an acoustic wavelength.