Directionality of the Photoacoustic Effect during Laser Synthesis and Processing of Colloids

Photoacoustic detection technology combines deep penetration with high contrast, making it highly attractive for advanced imaging applications. The spatial directivity of photoacoustic signals plays a pivotal role in determining imaging resolution, signal-to-noise ratio, and effective penetration depth. Despite its importance, the formation mechanism of this directivity and its relationship with signal intensity remain insufficiently understood. To address this gap, we developed a three-dimensional multiangle photoacoustic signal acquisition system to systematically investigate the spatial distribution of photoacoustic signals generated by silver and titanium dioxide micro/nanoparticles under pulsed laser irradiation. In parallel, finite-difference time-domain simulations were performed to comprehensively analyze the effects of light polarization, material properties, and particle size on photoacoustic signal intensity and frequency spectra. Our results reveal that linearly polarized light produces a symmetric double-lobe optical field distribution, leading to strongly directional photoacoustic emission, whereas circularly polarized light generates a uniform ring-shaped optical field and correspondingly weaker signal directivity. The optical absorption mechanism of the material and the particle size jointly modulate the optical field distribution, thereby governing both the spatial directivity and frequency characteristics of the photoacoustic signals. Moreover, during liquid-phase laser melting synthesis, anisotropic optical fields and acoustic pressure distributions can dominate product evolution, driving the formation of nonspherical morphologies. Based on these spatial distribution characteristics, we further propose a novel method for distinguishing photoacoustic signals generated by micro/nanoparticles from those originating in the surrounding liquid medium. Overall, this work clarifies the intrinsic coupling between optical and acoustic fields underlying photoacoustic signal directivity, deepens the physical understanding of the photoacoustic effect, and provides a new strategy for the controllable synthesis of colloidal micro/nanoparticles.