Pulsed lasers operating in the 2-5 {\mu}m band are important for a wide range
of applications in sensing, spectroscopy, imaging and communications. Despite
recent advances with mid-infrared gain media, the lack of a capable pulse
generation mechanism, i.e. a passive optical switch, remains a significant
technological challenge. Here we show that mid-infrared optical response of
Dirac states in crystalline Cd3As2, a three-dimensional topological Dirac
semimetal (TDS), constitutes an ideal ultrafast optical switching mechanism for
the 2-5 {\mu}m range. Significantly, fundamental aspects of the photocarrier
processes, such as relaxation time scales, are found to be flexibly controlled
through element doping, a feature crucial for the development of convenient
mid-infrared ultrafast sources. Although various exotic physical phenomena have
been uncovered in three-dimensional TDS systems, our findings show for the
first time that this emerging class of quantum materials can be harnessed to
fill a long known gap in the field of photonics.Comment: 17 pages, 3 figure
