Cd3As2,as a three-dimensional (3D) topological Dirac semimetal,has attracted wide attention due to its unique physical properties originating from the 3D massless Dirac fermions.While many efforts have been devoted to the exploration of novel physical phenomena such as chiral anomaly and phase transitions by using bulk crystals,the development of high-quality and large-scale thin films becomes necessary for practical electronic and optical applications.Here,we report our recent progress in developing single-crystalline thin films with improved quality and their optical devices includingCd3As2-based heterojunctions and ultrafast optical switches.We find that a post-annealing process can significantly enhance the crystallinity of Cd3As2 in both intrinsic and Zn-doped thin films.With excellent characteristics of high mobility and linear band dispersion,Cd3As2 exhibits a good optical response in the visible-to-mid-infrared range due to an advantageous optical absorption,which is reminiscent of 3D graphene.It also behaves as an excellent saturable absorber in the mid-infrared regime.Through the delicate doping process in this material system,it may further open up the long-sought parameter space crucial for the development of compact and high-performance mid-infrared ultrafast sources.