  Intelligent reflecting surfaces (IRS) can improve the physical layer security
(PLS) by providing a controllable wireless environment. In this paper, we
propose a novel PLS technique with the help of IRS implemented by an
intelligent mirror array for the visible light communication (VLC) system.
First, for the IRS aided VLC system containing an access point (AP), a
legitimate user and an eavesdropper, the IRS channel gain and a lower bound of
the achievable secrecy rate are derived. Further, to enhance the IRS channel
gain of the legitimate user while restricting the IRS channel gain of the
eavesdropper, we formulate an achievable secrecy rate maximization problem for
the proposed IRS-aided PLS technique to find the optimal orientations of
mirrors. Since the sensitivity of mirrors' orientations on the IRS channel gain
makes the optimization problem hard to solve, we transform the original problem
into a reflected spot position optimization problem and solve it by a particle
swarm optimization (PSO) algorithm. Our simulation results show that secrecy
performance can be significantly improved by adding an IRS in a VLC system.
