Focusing light through turbid media using wavefront shaping generally requires a noninvasive guide star to provide feedback on the focusing process. Here we report a photoacoustic guide star mechanism suitable for wavefront shaping through a scattering wall that is based on the fluctuations in the photoacoustic signals generated in a micro-vessel filled with flowing absorbers. The standard deviation of photoacoustic signals generated from random distributions of particles is dependent on the illumination volume and increases nonlinearly as the illumination volume is decreased. We harness this effect to guide wavefront shaping using the standard deviation of the photoacoustic response as the feedback signal. We further demonstrate sub-acoustic resolution optical focusing through a diffuser with a genetic algorithm optimization routine.

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In this work, we report that not only do gold nanorods coated with hydrophobically modified mesoporous silica shells enhance the photoacoustic (PA) signal over unmodified mesoporous silica-coated gold nanorods but also that the relationship between the PA amplitude and the input laser fluence is strongly nonlinear. Mesoporous silica shells of ∼14 nm thickness with ∼3 nm pores were grown on gold nanorods showing nearinfrared absorption. The silica shell was rendered hydrophobic with addition of dodecyltrichlorosilane and then resuspended in aqueous media with a lipid monolayer. Analysis of the PA signal revealed not only an enhancement of the PA signal compared to mesoporous silica-coated gold nanorods at lower laser fluences but also a nonlinear relationship between the PA signal and the laser fluence. We attribute each effect to the entrapment of solvent vapor in the mesopores: the vapor has both a larger expansion coefficient and a larger thermal resistance than silica that enhances conversion to acoustic energy, and the hydrophobic porous surface is able to promote a phase transition at the surface, leading to a nonlinear PA response even at fluences as low as 5 mJ cm −2 . At 21 mJ cm −2 , the highest laser fluence tested, the PA enhancement was >12-fold over mesoporous silica-coated gold nanorods.

Vaporizable endoskeletal droplets are solid hydrocarbons in liquid fluorocarbon droplets in which melting of the hydrocarbon phase leads to the vaporization of the fluorocarbon phase. In prior work, vaporization of the endoskeletal droplets was achieved thermally by heating the surrounding aqueous medium. In this work, we introduce a near-infrared (NIR) optically absorbing naphthalocyanine dye (zinc 2,11,20,29-tetra-tert-butyl-2,3-naphthalocynanine) into the solid hydrocarbon (eicosane, n-C 20 H 42 ) core of liquid fluorocarbon (C 5 F 12 ) drops suspended in an aqueous medium. Droplets with a uniform diameter of 11.7 ± 0.7 μm were formed using a flow-focusing microfluidic device. The solid hydrocarbon formed a crumpled spherical structure within the liquid fluorocarbon droplet. The photoactivation behavior of these dye-containing endoskeletal droplets was investigated using NIR laser irradiation. When exposed to a pulsed laser of 720 nm wavelength, the dyecontaining droplets vaporized at an average laser fluence of 65 mJ/cm 2 , whereas blank droplets without the dye did not vaporize at any fluence up to 100 mJ/cm 2 . Furthermore, dye-loaded droplets with a smaller, polydisperse size distribution were prepared using a simple shaking method and studied in a flow phantom for their photoacoustic signal and ultrasound contrast imaging. These results demonstrate that dye-containing endoskeletal droplets can be made to vaporize by externally applied optical energy. Such droplets may be useful for a variety of photoacoustic applications for sensing, imaging, and therapy.

A photoacoustic contrast mechanism is presented based on the photoacoustic fluctuations induced by microbubbles flowing inside a micro vessel filled with a continuous absorber. It is demonstrated that the standard deviation of a homogeneous absorber mixed with microbubbles increases non-linearly as the microbubble concentration and microbubble size is increased. This effect is then utilized to perform photoacoustic fluctuation imaging with increased visibility and contrast of a blood flow phantom.