In this paper we discuss graphene-dielectric integrated terahertz metasurfaces as an alternative approach to graphene-metal metamaterials for providing effective control of electromagnetic beam propagation at terahertz wavelengths. Our structures consist of a passive lossless dielectric pattern and a reconfigurable graphene sheet whose terahertz optical conductivity could be actively tuned chemically, electrically, or optically. In particular, we investigate dielectric patterns consisting of pillar-and circular hole arrays and show that via optimizing the geometric dimensions in these patterns it is possible to attain almost complete terahertz absorption at an arbitrary frequency of interest. Furthermore, via either (i) controlling the thickness of a lowindex dielectric spacer located between the dielectric pattern and the graphene sheet, or (ii) choosing a material with an appropriate index of refraction when constructing the dielectric pattern, it is possible to control the sensitivity of the overall structure to variations in the graphene sheet conductivity. Full-wave electromagnetic simulations are supported by proof-ofprinciple experiments on structures fabricated via patterning of a silicon-on-insulator wafer followed by graphene transfer and chemical doping. Overall, the proposed approach can lead to the construction of efficient tunable terahertz absorbers; however, other tailored electromagnetic responses might be also possible via the selection of appropriate dielectric patterns.
We study the two-dimensional electron gas at the interface of NdTiO3 and SrTiO3 to reveal its nanoscale transport properties. At electron densities approaching 1015 cm−2, our terahertz spectroscopy data show conductivity levels that are up to six times larger than those extracted from DC electrical measurements. Moreover, the largest conductivity enhancements are observed in samples intentionally grown with larger defect densities. This is a signature of electron transport over the characteristic length-scales typically probed by electrical measurements being significantly affected by scattering by structural defects introduced during growth, and, a trait of a much larger electron mobility at the nanoscale.
We describe studies on the nanoscale transport dynamics of carriers in strained AlN/GaN/AlN quantum wells: an electron-hole bilayer charge system with large difference in transport properties between the two charge layers. From electronic band diagram analysis, the presence of spatially separated two-dimensional electron and hole charge layers is predicted at opposite interfaces.Since these charge layers exhibit distinct spectral signatures at terahertz frequencies, a combination of terahertz and far-infrared spectroscopy enables us to extract (a) individual contributions to the total conductivity, as well as (b) effective scattering rates for charge-carriers in each layer.Furthermore, by comparing direct-current and terahertz extracted conductivity levels, we are able to determine the extent to which structural defects affect charge transport. Our results evidence that (i) a non-unity Hall-factor and (ii) the considerable contribution of holes to the overall conductivity, lead to a lower apparent mobility in Hall-effect measurements. Overall, our work demonstrates that terahertz spectroscopy is a suitable technique for the study of bilayer charge systems with large differences in transport properties between layers, such as quantum wells in III-Nitride semiconductors.
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