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.
In a topological crystalline insulator such as Pb1-xSnxSe, massless Dirac states emerge at an interface with a trivial insulator. We demonstrate the great versatility of Pb1-xSnxSe 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 Pb1-xSnxSe, 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.
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