Browsing by Author "Eftekhari, Zeinab"
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Item Open Access Investigation of optical residual absorption in graphene(Bilkent University, 2018-02) Eftekhari, ZeinabGraphene, 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.Item Open Access Lithography-free disordered metal–insulator–metal nanoantennas for colorimetric sensing(OSA - The Optical Society, 2020) Eftekhari, Zeinab; Ghobadi, Amir; Özbay, EkmelThe colorimetric detection of bio-agent targets has attracted considerable attention in nanosensor designs. This platform provides an easy to use, real-time, and rapid sensing approach, as the color change can be easily distinguished by the naked eye. In this Letter, we propose a large scale compatible fabrication route to realize colorimetric optical nanosensors with a novel configuration. For this purpose, we design and fabricate a tightly packed disordered arrangement of Fabry–Perot based metal–insulator–metal nanoantennas with a resonance frequency at visible light wavelengths. In this design, the adsorbed bio-agent changes the effective refractive index of the cavity, and this causes a shift in the resonance wavelength. The experimental data show that the proposed design can have sensitivity values >70nm/refractive>70nm/refractive index unit. Unlike other optical sensing schemes that rely mainly on hot spot formation and field enhancement, this design has a large active area with relatively uniform patterns that make it a promising approach for low-level and reliable bio-detection.Item Open Access Strong light emission from a defective hexagonal boron nitride monolayer coupled to near-touching random plasmonic nanounits(Optica, 2021-04) Eftekhari, Zeinab; Ghobadi, Amir; Soydan, Mahmut Can; Yıldırım, Deniz Umut; Cinel, Neval Ayşegül; Özbay, EkmelIn this Letter, we demonstrate strong light emission from defective hexagonal boron nitride (hBN) defect centers upon their coupling with disorder near-touching plasmonic units. Based on numerical simulations and characterization results, the plasmonic design at thin layer thicknesses of 20 nm can provide above 2 orders of magnitude enhance ment in photoluminescence (PL) spectra. Moreover, this plasmonic platform shortens the luminescence lifetime of the emitters. The proposed design can be easily extended to other plasmonic-emitter combinations where strong light–matter interaction can be achieved using large-scale compatible routes.