  As a kind of wavefront with helical phase, orbital angular momentum (OAM)
shows the great potential to enhance the security results of wireless
communications due to its unique orthogonality and central hollow
electromagnetic wave structure. Therefore, in this paper we propose the
reconfigurable-intelligent-surface (RIS) assisted OAM scheme, where RIS is
deployed to weaken the information acquisition at eavesdroppers by adjusting
the OAM beams pointed to the eavesdropper and artificial noise (AN) is applied
to interfere with the eavesdropper, thus significantly increasing the secrecy
rates of short-range secure communications. Aiming at obtaining the maximum
secrecy rate, we develop the Riemannian manifold conjugate gradient (RMCG)
based alternative optimization (AO) algorithm to assign much power to low-order
OAM-modes and optimize the OAM beams direction with the programmable RIS, thus
respectively enhancing and weakening the received signal strength at the
legitimate receiver and the eavesdropper. Numerical results show that our
proposed scheme outperforms the existing works in terms of the secrecy rate and
the eavesdropper's bit error rate.
