In a topological crystalline insulator such as Pb<sub>1-x</sub>Sn<sub>x</sub>Se, massless Dirac states emerge at an interface with a trivial insulator. We demonstrate the great versatility of Pb<sub>1-x</sub>Sn<sub>x</sub>Se electronic properties, which makes it a highly promising material to control the massless Dirac states emerging from topological properties. Using magnetooptical transmission spectroscopy on high quality molecular beam epitaxy grown Pb<sub>1-x</sub>Sn<sub>x</sub>Se, we probe the variation of its bulk energy gap versus chemical composition, temperature, and strain. The determination of its bulk electronic properties will be of critical relevance to design heterostructures. A magneto-optical study on PbSnSe/PbEuSe superlattices will allow us to characterize the topological interface states occurring at each interface, as well as their tunability versus temperature. The engineering of these massless states is shown to be a promising route to achieve photo detection and photoemission in the terahertz range.