Optical scattering by biological tissue limits the resolution of deep optical imaging. Beyond a few hundred microns, the resolution is on the order of the imaging depth for techniques such as diffuse optical tomography. Exploiting light and sound interaction provides a means to obtain images of optical properties deep inside tissue with the resolution of ultrasound imaging, typically a few hundred microns, at depths up to a few centimeters. Both the photoacoustic interaction (generation of sound through light absorption) and the acousto-optic interaction (modulation of light propagation by an ultrasound field) have been investigated towards this goal over the past two decades. In this work, the authors exploit the acousto-optic interaction, by which photons that propagate through a focused ultrasound field may undergo a frequency shift given by the ultrasound frequency. By selectively detecting these so called "tagged-photons", one obtains optical information from the ultrasound focal zone, i.e. with the ultrasound resolution, a technique often referred to as ultrasound-modulated optical tomography. Here, the authors exploit single-shot off-axis holography to detect tagged photons and optimize the signal-to-noise ratio while limiting the measurement time: with this approach, a 1D profile of an absorbing object was obtained in vitro through a piece of human skull. While encouraging results have been obtained over the past two decades in soft tissue with acousto-optic imaging, this work extends the potential applications of the approach to the field of deep optical brain imaging through the human skull.

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