High harmonic generation in solids attracted great attentions as a new scheme for frequency conversion. We report observation of an extremely efficient terahertz (THz) third- and fifth-harmonic generation in thin films of Cd3As2, a three-dimensional Dirac semimetal with massless electron dispersion, as it is observable with tabletop THz source at room temperature. Our THz pump-THz probe study with subcycle time resolution elucidates that the intraband current of coherently accelerated Dirac electrons is the main source of the THz harmonics as expected theoretically. The results pave the way toward novel devices for ultrafast THz electronics and photonics based on topological semimetals.
Despite the significant advances made in the field of metamaterials and metasurfaces in recent years, many applications of such devices are hampered by the lack of active refractive index tuning. Here, we report on a new class of tunable quantum materials based on 3D topological Dirac semimetals with extremely high electrical and thermal refractive index tuning. Realized optical reflectivity data, performed on thin films of Cd3As2 over a broad range of frequencies demonstrate larger than traditional thermo-optic shifts in III-V semiconductors. Dynamic Fermi level tuning, instigated from the Pauli blocking in the linear Dirac cone, offers large and tunable absorption peak in the mid-infrared region. In contrast to recent efforts in 3D Dirac semimetals which are mostly focused on single crystal Cd3As2, our data based on MBE-grown Cd3As2 can galvanize newfound applications in the field of meta-optics and can enable several applications such as ultra-thin programmable optical devices, photodetectors, and on-chip directional antennas.
Heterostructures and superlattices consisting of a prototype Mott insulator, GdTiO3, and the band insulator SrTiO3, are grown by molecular beam epitaxy and show intrinsic electronic reconstruction, approximately ½ electron per surface unit cell at each GdTiO3/SrTiO3 interface. Insights into charge distribution, the influence of the electrostatic boundary conditions, and strong correlation effects will be presented.
The ability to analyze the chemistry, atomic and electronic structure of interfaces with atomic spatial resolution is afforded by modern scanning transmission electron microscopy techniques. By combining atomic resolution imaging with spectroscopy, structure-property relationships of functional oxide thin films can be established. In this paper, we describe two specific examples where we have applied high-spatial resolution electron energy-loss spectroscopy to dielectric thin films.
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