  This letter focuses on the secure transmission for an intelligent reflecting
surface (IRS)-assisted millimeter-wave (mmWave) and terahertz (THz) systems, in
which a base station (BS) communicates with its destination via an IRS, in the
presence of a passive eavesdropper. To maximize the system secrecy rate, the
transmit beamforming at the BS and the reflecting matrix at the IRS are jointly
optimized with transmit power and discrete phase-shift constraints. It is first
proved that the beamforming design is independent of the phase shift design
under the rank-one channel assumption. The formulated non-convex problem is
then converted into two subproblems, which are solved alternatively.
Specifically, the closed-form solution of transmit beamforming at the BS is
derived, and the semidefinite programming (SDP)-based method and element-wise
block coordinate descent (BCD)-based method are proposed to design the
reflecting matrix. The complexity of our proposed methods is analyzed
theoretically. Simulation results reveal that the proposed IRS-assisted secure
strategy can significantly boost the secrecy rate performance, regardless of
eavesdropper's locations (near or blocking the confidential beam).
