  Moving towards $6^{\text{th}}$ generation (6G), backhaul networks require
significant improvements to support new use-cases with restricted joint
capacity and availability requirements. In this paper, we investigate the
potentials and challenges of joint sub-teraHertz (sub-THz) and free space
optical (FSO), in short sub-THz-FSO, multi-hop networks as a candidate
technology for future backhaul communications. As we show, with a proper
deployment, sub-THz-FSO networks have the potential to provide high-rate
reliable backhauling, while there are multiple practical challenges to be
address before they can be used in large-scale.
