One-to-many group communication is common in modern data centers running cloud and web-based applications, or High Performance Computing (HPC) applications. In this paper, the authors propose FlyCast, an architecture using the physical layer of Free-Space Optics (FSO) to accelerate multicast communication. FlyCast leverages off-the-shelf devices (e.g. switchable mirror and beam splitter) to physically split the FSO beam to multi receivers on demand, which enables to build dynamical multicast trees in physical layer and accelerates multicast communications. They demonstrate the feasibility of FlyCast through their theoretical analysis and the proof-of-concept prototype.