Browsing by Subject "Electrolyte gating"

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Open Access
Graphene-based electrically tunable terahertz optoelectronics
(2016-09) Kakenov, Nurbek
Advances in terahertz (THz) research and technology, has bridged the gap between radio-frequency electronics and optics. More efficient control of THz waves would highly benefit noninvasive, high-resolution imaging and ultra-fast wireless communications. However, lack of active materials in THz, hinders the realization of these technologies. Graphene, 2d-crystal of carbon atoms, is a promising candidate for reconfigurable THz optoelectronics due to its unique electronic band structure which yields gate-tunable optical response. Here, we studied gate-tunable optical properties of graphene in THz frequencies. Using time-domain and continuous wave THz spectroscopy techniques, tunable Drude response of graphene is investigated at very high doping levels with Fermi energies up to 1 eV. Our results show that, transport scattering time decreases significantly with doping. Unlike conventional semiconductors, we observed nearly perfect electron-hole symmetry even at very high doping levels. In the second part, we implemented using these unique tunable properties for novel THz optoelectronic devices such as THz intensity modulators and THz spatial light modulators. These devices are based on various designs of mutually gated capacitive structures consisting of ionic liquid electrolyte sandwiched between graphene and metallic electrodes. Low insertion losses (<2 dB), high modulation depth (>50 %) over a broad spectrum (0.1-2 THz), and the simplicity of the device structure are the key attributes of graphene based THz devices. Furthermore, with the optimized device architectures, gate tunable coherent perfect absorption is observed in THz which yields modulation depth of nearly 100 %. The approaches developed in this work surpass the challenges of generating high carrier densities on graphene, and introduce low-loss devices with practical fabrication methods which we believe can lead to more responsive and sophisticated optoelectronic devices.
Open Access
Investigation of optical residual absorption in graphene
(2018-02) Eftekhari, Zeinab
Graphene, a 2-dimensional crystal of carbon, can absorb 2.3% of light over a very broad spectrum. Doped graphene, however has a gap in optical absorption due to the Pauli blocking principle. For doped graphene, the interband optical transitions with energy less than 2EF are not allowed, therefore the consequent optical absorption is expected to fall down to zero for energies below 2EF threshold. In this thesis, we investigated the optical residual absorption of graphene in Pauli-blocked region. Optical absorption of the monolayer graphene transferred on transparent substrates was analyzed via optical spectroscopy. We used electrostatic and chemical doping methods to shift Fermi energy of graphene. The observed residual absorption of 0.5% which is due to chemical impurities reduced slightly by increasing doping level.