Parallelized Ultrasound-Guiding for Enhanced Light Delivery within Scattering Media

The delivery of light over an extended area within a sample forms the basis of biomedical applications that are as relevant as photoacoustic tomography, fluorescence imaging, and phototherapy techniques. However, light scattering limits the ability of these methods to reach deep regions within biological tissues. As a result, their operational range remains confined to superficial areas of samples, posing a significant barrier to effective optical treatment and diagnosis. Here, we propose an approach to address this issue and enhance light delivery across an extended region inside scattering samples. Our strategy involves using ultrasound to directly modulate the optical properties of the sample, generating refractive index gradients that act as embedded optical waveguides. By employing two perpendicularly oriented piezoelectric plates, several parallel waveguides can be simultaneously formed within the sample, allowing light to be guided over a wide area (3 x 3 mm2 in current experiments). Supported by Monte Carlo simulations, we demonstrate that ultrasound-light-guiding can enhance the intensity of light delivered inside scattering samples with an optical thickness of 2.5 and 12.5 by up to a factor of 700 and 42%, respectively. As a proof-of-concept, we demonstrated the ability of our approach to irradiate nanoparticles located within a scattering sample at light intensities that are not possible without ultrasound.