Browsing by Subject "Large-area graphene"

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    Broadband terahertz modulators using self-gated graphene capacitors
    (Optical Society of America, 2015) Kakenov, N.; Balci, O.; Polat, E. O.; Altan, H.; Kocabas, C.
    We demonstrate a terahertz intensity modulator using a graphene supercapacitor which consists of two large-area graphene electrodes and an electrolyte medium. The mutual electrolyte gating between the graphene electrodes provides very efficient electrostatic doping with Fermi energies of 1 eV and a charge density of 8 × 1013 cm-2. We show that the graphene supercapacitor yields more than 50% modulation between 0.1 and 1.4 THz with operation voltages less than 3 V. The low insertion losses, high modulation depth over a broad spectrum, and the simplicity of the device structure are the key attributes of graphene supercapacitors for THz applications.
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    Graphene-based optical modulators
    (World Scientific Publishing Co. Pte. Ltd., 2017) Balci S.; Kocabas, C.
    In this chapter, we summarize the recent progress on graphene based optical modulators. Ability to control density of high mobility electrons on large area graphene surface enables realization of new type of electrooptical modulators in optoelectronics. Due to the low electronic density of states, accumulation of charges on graphene significantly shifts the Fermi energy up to 1 eV giving rise to profound optical effects in the infrared and visible spectra. On the other hand, graphene operates as a tunable Drude metal in long wavelengths such as THz and microwave. This unique broadband activity of graphene has stimulated a great deal of interest in graphene community due to its potential use in new optoelectronic devices. After discussing the electrically tunable optical properties of graphene, we highlight the key achievements in the field.
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    Observation of gate-tunable coherent perfect absorption of terahertz radiation in graphene
    (American Chemical Society, 2016) Kakenov, N.; Balci, O.; Takan, T.; Ozkan, V. A.; Altan, H.; Kocabas, C.
    We report experimental observation of electrically tunable coherent perfect absorption (CPA) of terahertz (THz) radiation in graphene. We develop a reflection-type tunable THz cavity formed by a large-area graphene layer, a metallic reflective electrode, and an electrolytic medium in between. Ionic gating in the THz cavity allows us to tune the Fermi energy of graphene up to 1 eV and to achieve a critical coupling condition at 2.8 THz with absorption of 99%. With the enhanced THz absorption, we were able to measure the Fermi energy dependence of the transport scattering time of highly doped graphene. Furthermore, we demonstrate flexible active THz surfaces that yield large modulation in the THz reflectivity with low insertion losses. We anticipate that the gate-tunable CPA will lead to efficient active THz optoelectronics applications.