Enhancing the detectability of ballistic photons travelling through highly scattering media by frequency-doubling their far-field pattern

The quality of an image of an object immersed in a scattering medium is generally degraded because light signals in such a medium split into a coherent (ballistic) and an incoherent components, of which only the former one brings information about the object. We made experiments in which the part of a Nd laser pulse (l mm spot size, either 1.2 ps at 1.055 μm or 19 ps at 1.064 μm) emerging undeviated from a cell (1-cm path) containing micellar suspensions of Intralipid (Pharmacia, Italy) at different concentrations was cross-correlated with the incident pulse in a non-collinearly phase-matched BaB₂O₄ (BBO) type I crystal. We obtained cross-correlated SH pulses of duration virtually identical to that of the auto-correlated incident pulses. The absence of signal at greater delays, at which the diffuse component is expected, suggests that only ballistic photons contribute to the detected SH signal