  Topological crystalline insulators -- topological insulators whose properties
are guaranteed by crystalline symmetry -- can potentially provide a promising
platform for terahertz optoelectronic devices, as their properties can be tuned
on demand when layered in heterostructures. We perform the first optical-pump
terahertz-probe spectroscopy of topological crystalline insulators, using them
to study the dynamics of Pb1-xSnxSe as a function of temperature. At low
temperatures, excitation of Dirac fermions leads to an increase in terahertz
transmission; from this negative photoconductivity, the intrasubband relaxation
rate of 6 ps is extracted. At high temperatures where only massive fermions
exist, the free-carrier losses induced by the pump reduce the terahertz
transmission for the duration of the 27 ps interband lifetime. Both effects are
present at temperatures near the topological-to-trivial transition. Our
experimental observations provide critical details for potential applications
of Pb1-xSnxSe and provide a direct measurement of the topological character of
Pb1-xSnxSe heterostructures.
