  Three-dimensional (3D) semimetals have been predicted and demonstrated to
have a wide variety of interesting properties associated with its linear energy
dispersion. In analogy to two-dimensional (2D) Dirac semimetals, such as
graphene, Cd3As2, a 3D semimetal, has shown ultra-high mobility, large Fermi
velocity, and has been hypothesized to support plasmons at terahertz
frequencies. In this work, we demonstrate synthesis of high-quality large-area
Cd3As2 thin-films through thermal evaporation as well as the experimental
realization of plasmonic structures consisting of periodic arrays of Cd3As2
stripes. These arrays exhibit sharp resonances at terahertz frequencies with
associated quality-factors (Q) as high as ~ 3.7. Such spectrally-narrow
resonances can be understood on the basis of a large kinetic-inductance,
resulting from a long momentum scattering time, which in our films can approach
~1 ps at room-temperature. Moreover, we demonstrate an ultrafast tunable
response through excitation of photo-induced carriers in optical pump /
terahertz probe experiments. Our results evidence that the intrinsic 3D nature
of Cd3As2 provides for a very robust platform for terahertz plasmonic
applications. Overall, our observations pave a way for the development of
myriad terahertz (opto) electronic devices based on Cd3As2 and other 3D Dirac
semimetals, benefiting from strong coupling of terahertz radiation, ultrafast
transient response, magneto-plasmon properties, etc. Moreover, the long
momentum scattering time, thus large kinetic inductance in Cd3As2, also holds
enormous potential for the re-design of passive elements such as inductors and
hence can have a profound impact in the field of RF integrated circuits.
