  The hybrid relay-reflecting intelligent surface (HR-RIS) has been recently
introduced as an efficient solution to overcome the double path loss and
limited beamforming diversity of the conventional fully passive reflecting
surface. This motivates us to investigate the application of the HR-RIS in
improving the secrecy capacity of millimeter wave
multiple-input-multiple-output (MIMO) systems with the presence of
multi-antenna eavesdropper. The joint optimization of the transmit beamformer
and the relay-reflecting coefficients at RIS is tackled via alternating
optimization. In the proposed solution, a closed-form expression for the
optimal transmit beamformer at the transmitter is derived, and a metaheuristic
solution based on particle swarm optimization is proposed to optimize the
active and passive elements at the HR-RIS. The simulation results verify that
under various scenarios, the HR-RIS provides significant improvement in the
secrecy capacity with respect to the conventional passive reflecting surface.
