Surprisingly slow light effects can improve the possibility to carry out optical imaging inside tissue. How this can happen is now explained (see figure). Laser light at frequency vL is entering tissue (grey colour) from the left. It is scattered by the tissue and exit in essentially any direction (black arrows). Due to scattering in the tissue, light exiting carry no spatial information. Ultrasound (frequency vS) however propagates linearly and is not scattered by tissue. The solid black contour shows the position of an ultrasound pulse when laser light enters the tissue. Parts of the laser light passing through the ultrasound pulse will be shifted in frequency and obtain the new frequency vL+vS (red arrows). The frequency shifted light therefore carries information about the absorption at the position of the ultrasound pulse. Thus if we can select only the frequency-shifted light we obtain spatially resolved tissue absorption information. However, only an exceedingly small fraction of the light will pass through the ultrasound pulse so the frequency shifted light is hidden in a huge background of non-frequency-shifted light. A spectral filter can absorb the non-shifted light while transmitting the frequency-shifted light. The action of such a filter is indicated in the rightmost part of the figure. The thick green line indicates that the carrier peak (non-frequency-shifted light, black line peak) is absorbed, while the weak side-band (frequency-shifted light, red line peak) is transmitted without losses. This is also indicated to the right of the spectral filter (blue). Most of the arrows after the filter are red. Unfortunately some of the carrier light will leak through and can actually still completely mask the weak side-band signal. However, the spectral filter is done out of a slow light material which can slow down the side-band signal by more than a factor 10000 (while not affecting the speed of the carrier signal). Thereby strong prompt carrier light is eliminated by a blocking time gate, which then at a later time unblocks the delayed (red) side-band signal, carrying information about the tissue properties at the ultrasound pulse position.
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