Optical wavefront shaping has emerged as a powerful tool for manipulating light in strongly scattering media. It enables diffraction-limited focusing and imaging at depths where conventional ...microscopy techniques fail. However, to date, most examples of wavefront shaping have relied on direct access to the targets or implanted probes, and the challenge is to apply it non-invasively inside complex samples. Recently, ultrasonic-tagging techniques have been utilized successfully, but these allow only small acoustically tagged volumes to be addressed at each measurement. Here, we introduce an approach that allows the non-invasive measurement of an optical transmission matrix over a large volume, inside complex samples, using a standard photoacoustic imaging set-up. We demonstrate the use of this matrix for detecting, localizing and selectively focusing light on absorbing targets through diffusive samples, as well as for extracting the scattering medium properties. Combining the transmission-matrix approach with the advantages of photoacoustic imaging opens a path towards deep-tissue imaging and light delivery utilizing endogenous optical contrast.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The hippocampus is thought to initiate systems-wide mnemonic processes through the reactivation of previously acquired spatial and episodic memory traces, which can recruit the entorhinal cortex as a ...first stage of memory redistribution to other brain areas. Hippocampal reactivation occurs during sharp wave–ripples, in which synchronous network firing encodes sequences of places. We investigated the coordination of this replay by recording assembly activity simultaneously in the CA1 region of the hippocampus and superficial layers of the medial entorhinal cortex. We found that entorhinal cell assemblies can replay trajectories independently of the hippocampus and sharp wave–ripples. This suggests that the hippocampus is not the sole initiator of spatial and episodic memory trace reactivation. Memory systems involved in these processes may include nonhierarchical, parallel components.
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BFBNIB, NMLJ, NUK, ODKLJ, PNG, SAZU, UL, UM, UPUK
We introduce a method to experimentally measure the monochromatic transmission matrix of a complex medium in optics. This method is based on a spatial phase modulator together with a full-field ...interferometric measurement on a camera. We determine the transmission matrix of a thick random scattering sample. We show that this matrix exhibits statistical properties in good agreement with random matrix theory and allows light focusing and imaging through the random medium. This method might give important insight into the mesoscopic properties of a complex medium.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UM
We report experimental evidence and correction of defocus in full-field optical coherence tomography of biological samples owing to mismatch of the refractive index of biological tissues and water. ...Via a metric based on the image quality, we demonstrate that we are able to compensate this index-induced defocus and to recover a sharp image in depth.
We present a new technique that produces en face tomographic images with a 10-micros acquisition time per image. The setup consists of an interference microscope with stroboscopic illumination ...provided by a xenon arc flash lamp (10-micros flashes at 15 Hz). The tomographic images are obtained from two phase-opposed interferometric images recorded simultaneously by two synchronized CCD cameras. Transverse resolution better than 1.0 microm is achieved by use of high-numerical-aperture microscope objectives. The short coherence length of the source yields an axial resolution of 0.9 microm. 3 x 3 pixel binning leads to a detection sensitivity of 71 dB. Our system is suitable for various applications, particularly in biology for in vivo cellular-level imaging.
Imaging objects embedded within highly scattering media by coupling light and ultrasounds (US) is a challenging approach. In deed, US enable direct access to the spatial localization, though ...resolution can be poor along their axis (cm). Up to now, several configurations have been studied, giving a millimetric axial resolution by applying to the US a microsecond pulse regime, as is the case with conventional echography. We introduce a new approach called Acousto-Optical Coherence Tomography (AOCT), enabling us to get a millimetric resolution with continuous US and light beams by applying random phase jumps on US and light. An experimental demonstration is performed with a self-adaptive holographic setup containing a photorefractive GaAs bulk crystal and a single large area photodetector.
We present a new detection scheme for acousto-optic tomography based on pulsed-wave ultrasound and illumination combined with heterodyne parallel speckle detection. This setup can perform ...tomographies inside several-centimeter-thick scattering samples. Test experiments confirm the suitability of this method for performing tomographies inside various types of optically scattering media, including liquids.
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