The use of wavefront shaping to compensate for scattering has brought a renewed interest as a potential solution to imaging through scattering walls. A key to the practicality of any imaging through scattering technique is the capability to focus light without direct access behind the scattering wall. Here we address this problem using photoacoustic feedback for wavefront optimization. By combining the spatially non-uniform sensitivity of the ultrasound transducer to the generated photoacoustic waves with an evolutionary competition among optical modes, the speckle field develops a single, high intensity focus significantly smaller than the acoustic focus used for feedback. Notably, this method is not limited by the size of the absorber to form a sub-acoustic optical focus. We demonstrate imaging behind a scattering medium using two different imaging modalities with up to ten times improvement in signal-to-noise ratio and five to six times sub-acoustic resolution.

Liquid perfluorocarbon (PFC) droplets incorporating optical absorbers can be vaporized through photothermal heating using a pulsed laser source. Here, we report on the effect of droplet core material on the optical fluence required to produce droplet vaporization. We fabricate gold nanoparticle templated microbubbles filled with various PFC gases (C 3 F 8 , C 4 F 10, and C 5 F 12 ) and apply pressure to condense them into droplets. The core material is found to have a strong effect on the threshold optical fluence, with lower boiling point droplets allowing for vaporization at lower laser fluence. The impact of droplet size on vaporization threshold is discussed, as well as a proposed mechanism for the relatively broad distribution of vaporization thresholds observed within a droplet population with the same core material. We propose that the control of optical vaporization threshold enabled by engineering the droplet core may find application in contrast enhanced photoacoustic imaging and therapy.

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We demonstrate enhanced three-dimensional photoacoustic imaging behind a scattering material by increasing the fluence in the ultrasound transducer focus. We enhance the optical intensity using wavefront shaping before the scatterer. The photoacoustic signal induced by an object placed behind the scattering medium serves as feedback to optimize the wavefront, enabling one order of magnitude enhancement of the photoacoustic amplitude. Using the enhanced optical intensity, we scan the object in two-dimensions before post-processing of the data to reconstruct the image. The temporal profile of the photoacoustic signal provides the information used to reconstruct the third dimension.

We report on the optical excitation and detection of resonant microbubble oscillations. Optically absorbing nanoparticles were attached to the shell of a lipid-encapsulated microbubble, allowing for optical pulsing to photothermally drive the microbubble into resonance. A modified optical microscope was used to track the bubble wall radius as a function of time using light scattering. The microbubble response from a nanosecond laser pulse was measured, and the eigenfrequency and vibrational amplitude were determined and compared to theory. The ability to optically drive microbubble oscillations may have applications in basic studies of bubble dynamics and biomedical imaging and therapy.

Lipid monolayer rheology plays an important role in a variety of interfacial phenomena, the physics of biological membranes, and the dynamic response of acoustic bubbles and drops. We show here measurements of lipid monolayer elasticity and viscosity for very small strains at megahertz frequency. Individual plasmonic microbubbles of 2-6 μm radius were photothermally activated with a short laser pulse, and the subsequent nanometer-scale radial oscillations during ring-down were monitored by optical scatter. This method provided average dynamic response measurements of single microbubbles. Each microbubble was modeled as an underdamped, linear oscillator to determine the damping ratio and eigenfrequency, and thus the lipid monolayer viscosity and elasticity. Our non-isothermal measurement technique revealed viscoelastic trends for different lipid shell compositions. We observed a significant increase in surface elasticity with lipid acyl chain length for 16 to 20 carbons, and this effect was explained by an intermolecular forces model that accounts for lipid composition, packing and hydration. The surface viscosity was found to be equivalent for these lipid shells. We also observed an anomalous decrease in elasticity and increase in viscosity when increasing the acyl chain length from 20 to 22 carbons. These results illustrate the use of a novel nondestructive optical technique to investigate lipid monolayer rheology in new regimes of frequency and strain, possibly elucidating phase behavior, as well as how the dynamic response of a microbubble can be tuned by the lipid intermolecular forces.

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