<p>Terahertz (THz) frequency band and free-space optical (FSO) communication can be proven potential solutions to the spectrum scarcity challenges in the emerging applications in the 6G. In this direction, a mixed THz-FSO relaying system is proposed in this paper. It is assumed that both THz and FSO links encounter fading effects and pointing errors, where the THz link is modeled using <em>α-μ</em> distribution while Málaga (<em>M</em>) distribution is used to characterize the FSO link. Particularly, we conduct the physical layer security (PLS) analysis and derive the secure outage probability into closed-form for two scenarios: 1) THz eavesdropping attack and 2) FSO eavesdropping attack. The effect of THz and FSO links parameters on the secrecy performance are then further investigated in detail. The results show that the security attacks on FSO links are more severe than those on THz links. Interestingly, the improved THz channel conditions adversely affect the security of the proposed mixed THz-FSO system, whereas better FSO channel conditions provide enhanced secrecy.</p>

<p>Terahertz (THz) frequency band and free-space optical (FSO) communication can be proven potential solutions to the spectrum scarcity challenges in the emerging applications in the 6G. In this direction, a mixed THz-FSO relaying system is proposed in this paper. It is assumed that both THz and FSO links encounter fading effects and pointing errors, where the THz link is modeled using <em>α-μ</em> distribution while Málaga (<em>M</em>) distribution is used to characterize the FSO link. Particularly, we conduct the physical layer security (PLS) analysis and derive the secure outage probability into closed-form for two scenarios: 1) THz eavesdropping attack and 2) FSO eavesdropping attack. The effect of THz and FSO links parameters on the secrecy performance are then further investigated in detail. The results show that the security attacks on FSO links are more severe than those on THz links. Interestingly, the improved THz channel conditions adversely affect the security of the proposed mixed THz-FSO system, whereas better FSO channel conditions provide enhanced secrecy.</p>