Investigation of depth selectivity of polarization gating for tissue characterization.

Polarization gating has been widely used to selectively probe the structure of superficial biological tissue. However, the penetration depth selectivity of polarization gating has not been well understood. Using polarized light Monte Carlo simulations, we investigated how the optical properties of a scattering medium and light collection geometry affect the penetration depth of polarization gating. We show that, for a wide range of optical properties, polarization gating enables attaining a very shallow penetration depth, which is on the order of the mean free path length. Furthermore, we discuss the mechanisms responsible for this surprisingly short depth of penetration of polarization gating. We show that polarization-gated signal is generated primarily by photons emerging from the surface of the medium within a few mean free path lengths from the point of incidence.