Non-conventional ultrasound transducer geometry facilitates remote light control within samples

Abstract Light control inside a sample is central in areas as relevant as life sciences and material sciences. Traditionally, this is achieved by using external devices, which can be limited by a short working distance, scattering-limited focusing depth, and other device geometrical restrictions. A widely used solution consists of inserting lenses, optical fibers or mirrors directly inside a sample, as in endoscopy. However, this approach is invasive and can damage the studied medium. Recent advances in acousto-optics have shown promising results in controlling light inside media, but they usually require acoustic cavities, restricting sample geometry or compromising its non-invasiveness. Here, we report on the development of an ultrasound (US) transducer that obviates the use of cavities and allows the external generation of an US lens inside a medium. When combined with pulsed illumination, the US lens can focus light down to a line and scan it across a 1.2 mm region in only 0.8 µ s, while increasing peak intensities by a factor of 10. Additionally, the focused light shows a high robustness against scattering, as demonstrated when scanning a target sample at up to an optical thickness of 6. We anticipate that this work will be a step towards non-invasive light control inside samples.