Fig. 3: Scenario layout and the temporal response in the polyethylene foam.

a The scenario layout of the target and the polyethylene foam. Taking the target letter ‘T’ as an example, it is located at 8 cm in the foam and the total thickness of the foam is 15 cm. The object depth is defined as the distance between the object and the front surface of the media. b The temporal response at one of the scanning points is captured by the ultrafast detector in the scenario shown in (a). Since the foam is an isotropic scattering medium where photon scatters at all angles with equal probability, the signal photons reflected by the target are highly coupled with background photons reflected by the scattering media.