Transmitted photon intensity through biological tissues within various time windows.

The intensity of the early-arriving snake portion of 100-fs ultrashort laser pulses transmitted through biological tissue of increasing thickness was measured by a streak camera. The snake photon intensity within the first arrival time interval Deltat was found to decrease exponentially with tissue thickness (z) as I(Deltat) = I(0)A exp[-b(Deltat)z/l(t)], where I(0) is the incident laser pulse intensity, l(t) is the transport mean free path of the medium, and the parameters b and A depend on the time interval Deltat. This result shows that the intensity of snake photons decays significantly more slowly than that of ballistic photons as tissue thickness increases.