Terahertz technology promises myriad applications including imaging, spectroscopy and communications. However, one major bottleneck at present for advancing this field is the lack of efficient devices to manipulate the terahertz electromagnetic waves. Here we demonstrate that exceptionally efficient broadband modulation of terahertz waves at room temperature can be realized using graphene with extremely low intrinsic signal attenuation. We experimentally achieved more than 2.5 times superior modulation than prior broadband intensity modulators, which is also the first demonstrated graphene-based device enabled solely by intraband transitions. The unique advantages of graphene in comparison to conventional semiconductors are the ease of integration and the extraordinary transport properties of holes, which are as good as those of electrons owing to the symmetric conical band structure of graphene. Given recent progress in graphene-based terahertz emitters and detectors, graphene may offer some interesting solutions for terahertz technologies.
We investigate high-field transport in graphene nanoribbons (GNRs) on SiO(2), up to breakdown. The maximum current density is limited by self-heating, but can reach >3 mA/μm for GNRs ~15 nm wide. Comparison with larger, micron-sized graphene devices reveals that narrow GNRs benefit from 3D heat spreading into the SiO(2), which enables their higher current density. GNRs also benefit from lateral heat flow to the contacts in short devices (<~0.3 μm), which allows extraction of a median GNR thermal conductivity (TC), ~80 W m(-1)K(-1) at 20 °C across our samples, dominated by phonons. The TC of GNRs is an order of magnitude lower than that of micron-sized graphene on SiO(2), suggesting strong roles of edge and defect scattering, and the importance of thermal dissipation in small GNR devices.
Transient absorption microscopy was employed to image charge carrier dynamics in epitaxial multilayer graphene. The carrier cooling exhibited a biexponential decay that showed a significant dependence on carrier density. The fast and slow relaxation times were assigned to coupling between electrons and optical phonon modes and the hot phonon effect, respectively. The limiting value of the slow relaxation time at high pump intensity reflects the lifetime of the optical phonons. Significant spatial heterogeneity in the dynamics was observed due to differences in coupling between graphene layers and the substrate.
Graphene nanoribbons ͑GNRs͒ were fabricated by metal mask lithography and plasma etching. GNRs with width ϳ20 nm show field-effect conductance modulation of ϳ12 at room temperature and Ͼ10 6 at 4.2 K. Conductance quantization due to quantum confinement in low field transport was observed. Landauer formula was utilized to fit the experimental data and excellent agreement was obtained. The extracted subband energy separation was found to deviate from the predicted values of perfect armchair GNRs. Transmission probability is much smaller than unity due to scattering by GNR edge/bulk disorder and impurities, indicating a mean free path ϳ40 nm. High field family I-Vs exhibited current saturation tendency and current density as high as 2 A/mm has been measured at low temperature.
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