Browsing by Author "Balci, O."
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Item Open Access 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.Item Open Access Comparison of back and top gating schemes with tunable graphene fractal metasurfaces(American Chemical Society, 2016) Aygar, A. M.; Balci, O.; Cakmakyapan, S.; Kocabas, C.; Caglayan, H.; Özbay, EkmelIn this work, fractal metasurfaces that consist of periodic gold squares on graphene are used to increase light-graphene interaction. We show by simulations and experiments that higher level fractal structures result in higher spectral tunability of resonance wavelength. This is explained by higher field localization for higher level fractal structures. Furthermore, spectral tunability of fractal metasurfaces integrated with graphene is investigated comparing two different schemes for electrostatic gating. Experiment results show that a top-gated device yields more spectral tunability (8% of resonance wavelength) while requiring much smaller gate voltages compared to the back-gated device. © 2016 American Chemical Society.Item Open Access Dynamic tuning of plasmon resonance in the visible using graphene(The Optical Society, 2016) Balci, S.; Balci, O.; Kakenov, N.; Atar, F. B.; Kocabas, C.We report active electrical tuning of plasmon resonance of silver nanoprisms (Ag NPs) in the visible spectrum. Ag NPs are placed in close proximity to graphene which leads to additional tunable loss for the plasmon resonance. The ionic gating of graphene modifies its Fermi level from 0.2 to 1 eV, which then affects the absorption of graphene due to Pauli blocking. Plasmon resonance frequency and linewidth of Ag NPs can be reversibly shifted by 20 and 35 meV, respectively. The coupled graphene-Ag NPs system can be classically described by a damped harmonic oscillator model. Atomic layer deposition allows for controlling the graphene-Ag NP separation with atomic-level precision to optimize coupling between them.Item Open Access Enhanced tunability of V-shaped plasmonic structures using ionic liquid gating and graphene(Elsevier Ltd, 2016) Ozdemir, O.; Aygar, A. M.; Balci, O.; Kocabas, C.; Caglayan, H.; Özbay, EkmelGraphene is a strong candidate for active optoelectronic devices because of its electrostatically tunable optical response. Current substrate back-gating methods are unable to sustain high fields through graphene unless a high gate voltage is applied. In order to solve this problem, ionic liquid gating is used which allows substrate front side gating, thus eliminating the major loss factors such as a dielectric layer and a thick substrate layer. On the other hand, due to its two dimensional nature, graphene interacts weakly with light and this interaction limits its efficiency in optoelectronic devices. However, V-shaped plasmonic antennas can be used to enhance the incident electric field intensity and confine the electric field near graphene thus allowing further interaction with graphene. Combining V-shaped nanoantennas with the tunable response of graphene, the operation wavelength of the devices that utilize V-shaped antennas can be tuned in situ. In the present paper, we demonstrate a graphene-based device with ionic liquid gating and V- shaped plasmonic antennas to both enhance and more effectively tune the total optical response. We are able to tune the transmission response of the device for up to 389 nm by changing the gate voltage by 3.8 V in the mid-infrared regime.Item Open Access Graphene as a Reversible and Spectrally Selective Fluorescence Quencher(Nature Publishing Group, 2016) Salihoglu, O.; Kakenov, N.; Balci, O.; Balci, S.; Kocabas, C.We report reversible and spectrally selective fluorescence quenching of quantum dots (QDs) placed in close proximity to graphene. Controlling interband electronic transitions of graphene via electrostatic gating greatly modifies the fluorescence lifetime and intensity of nearby QDs via blocking of the nonradiative energy transfer between QDs and graphene. Using ionic liquid (IL) based electrolyte gating, we are able to control Fermi energy of graphene in the order of 1 eV, which yields electrically controllable fluorescence quenching of QDs in the visible spectrum. Indeed, our technique enables us to perform voltage controllable spectral selectivity among quantum dots at different emission wavelengths. We anticipate that our technique will provide tunable light-matter interaction and energy transfer that could yield hybrid QDs-graphene based optoelectronic devices with novel functionalities, and additionally, may be useful as a spectroscopic ruler, for example, in bioimaging and biomolecular sensing. We propose that graphene can be used as an electrically tunable and wavelength selective fluorescence quencher. � 2016 The Author(s).Item Open Access Graphene based flexible electrochromic devices(Nature Publishing Group, 2014-10-01) Polat, E. O.; Balci, O.; Kocabas, C.Graphene emerges as a viable material for optoelectronics because of its broad optical response and gate-tunable properties. For practical applications, however, single layer graphene has performance limits due to its small optical absorption defined by fundamental constants. Here, we demonstrated a new class of flexible electrochromic devices using multilayer graphene (MLG) which simultaneously offers all key requirements for practical applications; high-contrast optical modulation over a broad spectrum, good electrical conductivity and mechanical flexibility. Our method relies on electro-modulation of interband transition of MLG via intercalation of ions into the graphene layers. The electrical and optical characterizations reveal the key features of the intercalation process which yields broadband optical modulation up to 55 per cent in the visible and near-infrared. We illustrate the promises of the method by fabricating reflective/transmissive electrochromic devices and multi-pixel display devices. Simplicity of the device architecture and its compatibility with the roll-to-roll fabrication processes, would find wide range of applications including smart windows and display devices. We anticipate that this work provides a significant step in realization of graphene based optoelectronics.Item Open Access Graphene based terahertz phase modulators(IOP Publishing, 2018) Kakenov, N.; Ergoktas, M. S.; Balci, O.; Kocabas, C.Electrical control of amplitude and phase of terahertz radiation (THz) is the key technological challenge for high resolution and noninvasive THz imaging. The lack of active materials and devices hinders the realization of these imaging systems. Here, we demonstrate an efficient terahertz phase and amplitude modulation using electrically tunable graphene devices. Our device structure consists of electrolyte-gated graphene placed at quarter wavelength distance from a reflecting metallic surface. In this geometry, graphene operates as a tunable impedance surface which yields electrically controlled reflection phase. Terahertz time domain reflection spectroscopy reveals the voltage controlled phase modulation of π and the reflection modulation of 50 dB. To show the promises of our approach, we demonstrate a multipixel phase modulator array which operates as a gradient impedance surface.Item Open Access Graphene mode-locked multipass-cavity femtosecond Cr4+:forsterite laser(Optical Society of America, 2013-04-19) Ozharar, S.; Baylam, I.; Cizmeciyan, M. N.; Balci, O.; Pince, E.; Kocabas, C.; Sennaroglu, A.We report, for the first time to our knowledge, the use of graphene as a saturable absorber in an energy-scaled femtosecond Cr4+: forsterite laser. By incorporating a multipass cavity, the repetition rate of the original short resonator was reduced to 4.51 MHz, which resulted in the generation of 100 fs, nearly transform-limited pulses at 1252 nm with a peak power of 53 kW. To the best of our knowledge, this is the highest peak power obtained from a room-temperature, femtosecond Cr4+: forsterite laser mode locked with a graphene saturable absorber. The corresponding pulse energy was 5.3 nJ with only 24 mW of average output power. The saturation fluence and modulation depth of the GSA were measured to be 25 mu J/cm(2) and 0.74%, respectively. The nonlinear effects in the Cr4+: forsterite medium that limit further power scaling were also investigated by using different output couplers. (c) 2013 Optical Society of AmericaItem Open Access Graphene-enabled optoelectronics on paper(American Chemical Society, 2016-06) Polat, E. O.; Uzlu, H. B.; Balci, O.; Kakenov, N.; Kovalska, E.; Kocabas, C.The realization of optoelectronic devices on paper has been an outstanding challenge due to the large surface roughness and incompatible nature of paper with optical materials. Here, we demonstrate a new class of optoelectronic devices on a piece of printing paper using graphene as an electrically reconfigurable optical medium. Our approach relies on electro-modulation of optical properties of multilayer graphene on paper via blocking the interband electronic transitions. The paper based devices yield high optical contrast in the visible spectrum with a fast response. Pattering graphene into multiple pixels, folding paper into three-dimensional shapes or printing colored ink on paper substrates enable us to demonstrate novel optoelectronic devices which cannot be realized with wafer-based techniques.Item Open Access Graphene-gold supercapacitor as a voltage controlled saturable absorber for femtosecond pulse generation(Optical Society of America, 2016-02) Baylam, I.; Balci, O.; Kakenov, N.; Kocabas, C.; Sennaroglu, A.We report, for the first time to the best of our knowledge, use of a graphene-gold supercapacitor as a voltage controlled fast saturable absorber for femtosecond pulse generation. The unique design involving only one graphene electrode lowers the insertion loss of the device, in comparison with capacitor designs with two graphene electrodes. Furthermore, use of the high-dielectric electrolyte allows reversible, adjustable control of the absorption level up to the visible region with low bias voltages of only a few volts (0-2 V). The fast saturable absorber action of the graphene-gold supercapacitor was demonstrated inside a multipass-cavity Cr:forsterite laser to generate nearly transform-limited, sub-100 fs pulses at a pulse repetition rate of 4.51 MHz at 1.24 μm.Item Open Access Graphene-quantum dot hybrid optoelectronics at visible wavelengths(American Chemical Society, 2018) Salihoglu, O.; Kakenov, N.; Balci, O.; Balci, S.; Kocabas, C.With exceptional electronic and gate-tunable optical properties, graphene provides new possibilities for active nanophotonic devices. Requirements of very large carrier density modulation, however, limit the operation of graphene based optical devices in the visible spectrum. Here, we report a unique approach that avoids these limitations and implements graphene into optoelectronic devices working in the visible spectrum. The approach relies on controlling nonradiative energy transfer between colloidal quantum-dots and graphene through gate-voltage induced tuning of the charge density of graphene. We demonstrate a new class of large area optoelectronic devices including fluorescent display and voltage-controlled color-variable devices working in the visible spectrum. We anticipate that the presented technique could provide new practical routes for active control of light-matter interaction at the nanometer scale, which could find new implications ranging from display technologies to quantum optics.Item Open Access A microfluidic based differential plasmon resonance sensor(Elsevier, 2011) Okan, M.; Balci, O.; Kocabas, C.A new type of differential surface plasmon (SPR) sensor integrated with a microfluidic system is presented. The working principle of the microfluidic device is based on hydrodynamic modulation of two laminar streams inside a microchannel to provide periodic changes of the environment on the SPR sensor. The modulated reflectance is then demodulated using a lock-in amplifier. The presented sensor provides sensitivities of index of refraction about 4 × 10-8 RIU together with a 4 orders of magnitude dynamic range. This method demonstrates a sensitive detection scheme which could be used for label-free detection. © 2011 Elsevier B.V. All rights reserved.Item Open Access Monitoring the operation of a graphene transistor in an integrated circuit by XPS(Elsevier BV, 2016) Aydogan, P.; Balci, O.; Kocabas, C.; Süzer, ŞefikOne of the transistors in an integrated circuit fabricated with graphene as the current controlling element, is investigated during its operation, using a chemical tool, XPS. Shifts in the binding energy of C1s are used to map out electrical potential variations, and compute sheet resistance of the graphene layer, as well as the contact resistances between the metal electrodes. Measured shifts depend on lateral positions probed, as well as on polarity and magnitude of the gate-voltage. This non-contact and chemically specific characterization can be pivotal in diagnoses.Item Open Access 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.Item Open Access Probing molecular interactions on carbon nanotube surfaces using surface plasmon resonance sensors(A I P Publishing LLC, 2012) Kakenov, N.; Balci, O.; Balci, S.; Kocabas, C.In this work, we present a method to probe molecular interactions on single-walled carbon nanotube (SWNT) surfaces using a surface plasmon sensor. SWNT networks were synthesized by chemical vapor deposition and transfer-printed on gold surfaces. We studied the excitation of surface plasmon-polaritons on nanotube coated gold surfaces with sub-monolayer, monolayer, and multilayer surface coverage. Integrating the fabricated sensor with a microfluidic device, we were able to obtain binding dynamics of a bovine serum albumin (BSA) protein on SWNT networks with various tube densities. The results reveal the kinetic parameters for nonspecific binding of BSA on SWNT coated surfaces having various tube densities. © 2012 American Institute of Physics.Item Open Access Rapid thermal annealing of graphene-metal contact(A I P Publishing LLC, 2012) Balci, O.; Kocabas, C.High quality graphene-metal contacts are desirable for high-performance graphene based electronics. Process related factors result large variation in the contact resistance. A post-processing method is needed to improve graphene-metal contacts. In this letter, we studied rapid thermal annealing (RTA) of graphene-metal contacts. We present results of a systematic investigation of device scaling before and after RTA for various metals. The results reveal that RTA provides a convenient technique to reduce contact resistance, thus to obtain reproducible device operation. © 2012 American Institute of Physics.Item Open Access Synthesis of graphene on gold(A I P Publishing, 2011) Oznuluer, T.; Pince, E.; Polat, E. O.; Balci, O.; Salihoglu, O.; Kocabas, C.Here we report chemical vapor deposition of graphene on gold surface at ambient pressure. We studied effects of the growth temperature, pressure, and cooling process on the grown graphene layers. The Raman spectroscopy of the samples reveals the essential properties of the graphene grown on gold surface. In order to characterize the electrical properties of the grown graphene layers, we have transferred them on insulating substrates and fabricated field effect transistors. Owing to distinctive properties of gold, the ability to grow graphene layers on gold surface could open new applications of graphene in electrochemistry and spectroscopy. © 2011 American Institute of Physics.Item Open Access Tunable plexcitonic nanoparticles: a model system for studying plasmon-exciton interaction from the weak to the ultrastrong coupling regime(American Chemical Society, 2016) Balci, S.; Kucukoz, B.; Balci, O.; Karatay, A.; Kocabas, C.; Yaglioglu, G.Controlling the number of dye molecules on metallic nanoparticles, which in turn affects the magnitude of Rabi splitting energy, is crucial for obtaining hybrid metal core–organic shell nanoparticles with tunable optical properties in the visible spectrum since the magnitude of the Rabi splitting energy directly determines the strength of the coupling between plasmonic nanoparticles and dye molecules. In this work, we present a new method for the synthesis of plexcitonic nanoparticles, and thus we are able to control the number of dye molecules self-assembled on Ag nanoprisms (Ag NPs) by adjusting the concentration of dye molecules used in the synthesis. Indeed, individual dye molecules self-assemble into J-aggregates on Ag NPs. Thus, in the finite-element simulations and experimental data of the hybrid metal organic nanoparticles, we observed a transition from weak coupling to the ultrastrong coupling regime. Besides, ultrafast energy transfer between plasmonic nanoparticles and excitonic aggregated dye molecules has been extensively studied as a function of Rabi splitting energy. We observe that the lifetime of the polariton states increases with the coupling strength and the upper polaritons are short-lived, whereas the lower polaritons are long-lived. Hybrid metal–organic nanoparticles presented in this study (i) have tunable Rabi splitting energies, (ii) are easy to prepare in large quantities in aqueous medium, (iii) can be uniformly assembled on solid substrates, (iv) have resonance frequencies in the visible spectrum, and (v) have small mode volume, thus making them an excellent model system for studying light–matter interaction at nanoscale dimensions from the weak to ultrastrong coupling regime.Item Open Access Weighing graphene with QCM to monitor interfacial mass changes(American Institute of Physics Inc., 2016) Kakenov, N.; Balci, O.; Salihoglu, O.; Hur, S. H.; Balci, S.; Kocabas, C.In this Letter, we experimentally determined the mass density of graphene using quartz crystal microbalance (QCM) as a mechanical resonator. We developed a transfer printing technique to integrate large area single-layer graphene on QCM. By monitoring the resonant frequency of an oscillating quartz crystal loaded with graphene, we were able to measure the mass density of graphene as ∼118 ng/cm2, which is significantly larger than the ideal graphene (∼76 ng/cm2) mainly due to the presence of wrinkles and organic/inorganic residues on graphene sheets. High sensitivity of the quartz crystal resonator allowed us to determine the number of graphene layers in a particular sample. Additionally, we extended our technique to probe interfacial mass variation during adsorption of biomolecules on graphene surface and plasma-assisted oxidation of graphene.