  Visible light is a proper spectrum for secure wireless communications because
of its high directivity and impermeability in indoor scenarios. However, if an
eavesdropper is located very close to a legitimate receiver, secure
communications become highly risky. In this paper, to further increase the
level of security of visible light communication (VLC) and increase its
resilience against to malicious attacks, we propose to capitalize on the
recently synthesized gold nanoparticles (GNPs) with chiroptical properties for
circularly polarized light resulting the phase retardation that interacts with
the linear polarizer angle. GNP plates made by judiciously stacking many GNPs
perform as physical secret keys. Transmitters send both the intended symbol and
artificial noise to exploit the channel variation effect by the GNP plates,
which is highly effective when an eavesdropper is closely located to the
legitimate receiver. A new VLC channel model is first developed by representing
the effect of GNP plates and linear polarizers in the circular polarization
domain. Based on the new channel model, the angles of linear polarizers at the
transmitters and legitimate receiver are optimized considering the effect of
GNP plates to increase the secrecy rate in wiretapping scenarios. Simulations
verify that when the transmitters are equipped with GNP plates, even if the
eavesdropper is located right next to the legitimate receiver, insightful
results on the physical layer security metrics are gained as follows: 1) the
secrecy rate is significantly improved and 2) the symbol error rate gap between
the legitimate receiver and eavesdropper becomes much larger due to the
chiroptical properties of GNP plates.
