Browsing by Author "Kakenov, Nurbek"

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Open Access
Broadband THz modulators based on multilayer graphene on PVC
(IEEE, 2016) Kaya, E.; Kakenov, Nurbek; Kocabaş, Coşkun; Altan, H.; Esentürk, O.
In this study we present the direct terahertz time-domain spectroscopic measurement of CVD-grown multilayer graphene (MLG) on PVC substrate with an electrically tunable Fermi level. In a configuration consisting MLG and injected organic dopant, the transmitted intensity loss of terahertz radiation was observed with an applied voltage between 0 and 3.5 V. We showed that MLG on PVC devices provided approximately 100 % modulation between 0.2 and 1.5 THz at preferentially low operation voltage of ca. 3V. The observed modulation bandwidth in terahertz frequencies appears to be instrument limited.
Open Access
Compressive sensing imaging with a graphene modulator at THz frequency in transmission mode
(IEEE, 2016) Özkan, V. A.; Takan, T.; Kakenov, Nurbek; Kocabaş, Coşkun; Altan, H.
In this study we demonstrate compressive sensing imaging with a unique graphene based optoelectronic device which allows us to modulate the THz field through an array of columns or rows distributed throughout its face.
Open Access
Controlling phase of microwaves with active graphene surfaces
(American Institute of Physics Inc., 2017) Balcı, Osman; Kakenov, Nurbek; Kocabaş, Coşkun
In this letter, we report a method to control the reflection phase of microwaves using electrically tunable graphene devices. The device consists of mutually gated large-area graphene layers placed at a quarter-wave distance from a metallic surface. This device structure yields electrically tunable resonance absorbance and step-like phase shift around the resonance frequency when the impedance of graphene matches with the free space impedance. Electrostatic control of charge density on graphene yields unprecedented ability to control both intensity (>50 dB) and phase (∼π) of the reflected electromagnetic waves with voltage. Furthermore, using the asymmetry of the doping at opposite polarity of the bias voltages, we showed bidirectional phase control with the applied voltage.
Open Access
Electrically controlled terahertz spatial light modulators with graphene arrays
(IEEE, 2016) Kakenov, Nurbek; Takan, T.; Özkan, V. A.; Balcı, Osman; Polat, Emre Ozan; Altan, H.; Kocabaş, Coşkun
Gate-tunable high-mobility electrons on atomically thin graphene layers provide a unique opportunity to control electromagnetic waves in a very broad spectrum. In this paper, we describe an electrically-controlled multipixel terahertz light modulators. The spatial light modulator is fabricated using two large-area graphene layers grown by chemical vapor deposition and transferred on THz transparent and flexible substrates. Room temperature ionic liquid, inserted between the graphene, provides mutual gating between the graphene layers. We used passive matrix addressing to control local charge density thus the THz transmittance. With this device configuration, we were able to obtain 5×5 arrays of graphene modulator with 65% modulation between 0.1 to 1.5 THz.
Open Access
Electrically switchable metadevices via graphene
(American Association for the Advancement of Science, 2018) Balcı, Osman; Kakenov, Nurbek; Karademir, E.; Balcı, S.; Çakmakyapan, S.; Polat, E. O.; Cağlayan, H.; Özbay, Ekmel; Kocabaş, Coşkun
Metamaterials bring subwavelength resonating structures together to overcome the limitations of conventional materials. The realization of active metadevices has been an outstanding challenge that requires electrically reconfigurable components operating over a broad spectrum with a wide dynamic range. However, the existing capability of metamaterials is not sufficient to realize this goal. By integrating passive metamaterials with active graphene devices, we demonstrate a new class of electrically controlled active metadevices working in microwave frequencies. The fabricated active metadevices enable efficient control of both amplitude (>50 dB) and phase (>90°) of electromagnetic waves. In this hybrid system, graphene operates as a tunable Drude metal that controls the radiation of the passive metamaterials. Furthermore, by integrating individually addressable arrays of metadevices, we demonstrate a new class of spatially varying digital metasurfaces where the local dielectric constant can be reconfigured with applied bias voltages. In addition, we reconfigure resonance frequency of split-ring resonators without changing its amplitude by damping one of the two coupled metasurfaces via graphene. Our approach is general enough to implement various metamaterial systems that could yield new applications ranging from electrically switchable cloaking devices to adaptive camouflage systems. Copyright
Open Access
Femtosecond pulse generation from a Ti3+: Sapphire laser near 800 nm with voltage reconfigurable graphene saturable absorbers
(OSA - The Optical Society, 2017) Baylam, Işınsu; Özharar, Sarper; Kakenov, Nurbek; Kocabaş, Coşkun; Sennaroglu, Alphan
We experimentally show that a voltage-controlled graphene-gold supercapacitor saturable absorber (VCG-gold-SA) can be operated as a fast saturable absorber with adjustable linear absorption at wavelengths as low as 795 nm. This was made possible by the use of a novel supercapacitor architecture, consisting of a high-dielectric electrolyte sandwiched between a graphene and a gold electrode. The high-dielectric electrolyte allowed continuous, reversible adjustment of the Fermi level and, hence, the optical loss of the VCG-gold-SA up to the visible wavelengths at low bias voltages of the order of a few volts (0-2 V). The fast saturable absorber action of the VCG-gold-SA and the bias-dependent reduction of its loss were successfully demonstrated inside a femtosecond Ti3+:sapphire laser operating near 800 nm. Dispersion compensation was employed by using dispersion control mirrors and a prism pair. At a bias voltage of 1.2 V, the laser operated with improved power performance in comparison with that at zero bias, and the VCG-gold-SA initiated the generation of nearly transform-limited pulses as short as 48 fs at a pulse repetition rate of 131.7 MHz near 830 nm. To the best of our knowledge, this represents the shortest wavelength where a VCG-gold-SA has been employed as a mode locker with adjustable loss. © 2017 Optical Society of America.
Open Access
Graphene mode-locked diode-pumped Cr:LiSAF laser at 857 nm
(IEEE, 2015) Canbaz, F.; Kakenov, Nurbek; Kocabaş, Coşkun; Demirbaş, U.; Sennaroglu, A.
The Cr:LiSAF gain medium, belonging to the class of Cr:colquiirite lasers, possesses a broad gain bandwidth suitable for the generation of femtosecond pulses near 850 nm [1]. Because the nonlinear refractive index of these media is typically low, Kerr lens mode locking alone does not provide sufficient modulation for stable mode locking. To increase the effective modulation depth for mode-locked operation, gain matched output couplers [2], semiconductor saturable absorber mirrors (SESAM) [3], and single-walled carbon nanotubes (SWCNT) [4] have been used in previous studies. One drawback of SESAMs and SWCNTs is the narrow operation bandwidth which limits the pulse widths as well as the mode-locked tuning range. An attractive alternative involves use of the graphene saturable absorber (GSA) which provides constant absorption over a very broad wavelength range due to the zero band-gap energy [5]. However, one challenge remains in the case of Cr.colquiirite lasers since the relatively low optical gain may not be sufficient to overcome the small signal loss of the GSA (around 5% per round trip), especially in low-power systems. In previous studies, GSA has been used to generate mode-locked pulses from bulk solid-state lasers between 800 and 2500 nm [6, 7].
Open Access
Graphene-Based Adaptive Thermal Camouflage
(American Chemical Society, 2018) Salihoğlu, Ömer; Uzlu, H. B.; Yakar, Ozan; Aas, Shahnaz; Balci, Osman; Kakenov, Nurbek; Balci, S.; Olcum, S.; Süzer, Şefik; Kocabas, Coşkun
In nature, adaptive coloration has been effectively utilized for concealment and signaling. Various biological mechanisms have evolved to tune the reflectivity for visible and ultraviolet light. These examples inspire many artificial systems for mimicking adaptive coloration to match the visual appearance to their surroundings. Thermal camouflage, however, has been an outstanding challenge which requires an ability to control the emitted thermal radiation from the surface. Here we report a new class of active thermal surfaces capable of efficient real-time electrical-control of thermal emission over the full infrared (IR) spectrum without changing the temperature of the surface. Our approach relies on electro-modulation of IR absorptivity and emissivity of multilayer graphene via reversible intercalation of nonvolatile ionic liquids. The demonstrated devices are light (30 g/m2), thin (<50 μm), and ultraflexible, which can conformably coat their environment. In addition, by combining active thermal surfaces with a feedback mechanism, we demonstrate realization of an adaptive thermal camouflage system which can reconfigure its thermal appearance and blend itself with the varying thermal background in a few seconds. Furthermore, we show that these devices can disguise hot objects as cold and cold ones as hot in a thermal imaging system. We anticipate that, the electrical control of thermal radiation would impact on a variety of new technologies ranging from adaptive IR optics to heat management for outer space applications.
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
Graphene-gold supercapacitor as a voltage-controlled saturable absorber for femtosecond pulse generation
(OSA, 2015) Baylam, I.; Balcı, Osman; Kakenov, Nurbek; Kocabaş, Coşkun; Sennaroğlu, A.
We report, for the first time to our knowledge, a voltage-controlled graphene-gold supercapacitor saturable absorber, as a modulator with adjustable insertion loss for low-gain mode-locked lasers. Nearly transform-limited, 80-fs pulses were generated near 1240 nm.
Open Access
Investigation of the ultrafast response and saturable absorption of voltage-controlled graphene
(OSA, 2018) Baylam, I.; Çizmeciyan, M. N.; Kakenov, Nurbek; Kocabaş, Coşkun; Sennaroğlu, A.
Ultrafast pump-probe measurements show that at a bias voltage of 1V, voltage reconfigurable graphene supercapacitors can operate as fast saturable absorbers with adjustable insertion loss over an ultrabroad spectral range from 630 to 1100 nm.
Open Access
Modulation behaviors, conductivities, and carrier dynamics of single and multilayer graphenes
(IEEE Computer Society, 2019) Kaya, E.; Kakenov, Nurbek; Kocabaş, Coşkun; Altan, H.; Esentürk, O.
Time domain and time resolved terahertz studies of single- and multi-layer graphene (SLG and MLG) samples and modulator devices will be presented. A high performance up to 100% of modulators were observed with the devices even at very low voltages. High modulation depth over such a broad spectrum and simple device structure brings significant importance toward application of this type of device in THz and related technologies. In addition, conductivities of SLG and MLG devices were also investigated and a change in behavior was observed as the layer thickness increased. The charge carriers dynamics of the samples with pulp fluence and color was also highly interesting.
Open Access
Probing interfacial processes on carbon nanotubes and graphene surfaces
(2012) Kakenov, Nurbek
The surface of low-dimensional carbon (carbon nanotubes and graphene) has unique electronic properties due to the delocalized p-orbitals. Very high carrier mobility with nanoscale dimension make carbon nanotubes and graphene promising candidates for high performance electronics. Besides electronic properties, the delocalized orbitals have a strong tendency to adsorb aromatic molecules via p-electronic interactions. The strong non-covalent interactions between the graphitic surface and organic molecules provide a unique template for supramolecular chemistry and sensing applications. A comprehensive understanding of these forces at atomic and molecular level still remains a challenge. In this thesis, we have used carbon nanotube networks and graphene as model systems to understand molecular interactions on carbon surface. We have developed processes to integrate these model materials with sensitive and surface specific sensors, such as surface plasmon sensor and quartz crystal microbalance. In the first part of the thesis, we integrated surface plasmon resonance (SPR) sensors with networks of single-walled carbon nanotubes to study interactions between SWNT and organic molecules. In the second part, we probe interfacial processes on graphene surface by mass detection. We anticipate that the developed methods could provide a sensitive means of detecting fundamental interaction on carbon surfaces.
Open Access
Synthesis of graphene on ultra-smooth copper foils for large area flexible electronics
(IEEE, 2015) Polat, E. O.; Balcı, Osman; Kakenov, Nurbek; Kocabaş, Coşkun; Dahiya, R.
This work demonstrates the synthesis of high quality, single layer graphene on commercially available ultra-smooth copper foils. The presented method will result in improved scalability of graphene based electronic and optical devices. Our approach is compatible with roll-to-roll printing as well as transfer printing of graphene layers on to a broad range of substrates including flexible and ultra-thin polymers. We propose that using commercially available ultra-smooth coppers provides scalable approach with the reduced variation of transport properties sourced from local graphene quality.
Open Access
Topological engineering of terahertz light using electrically tunable exceptional point singularities
(American Association for the Advancement of Science (AAAS), 2022) Ergoktas, M. Said; Soleymani, Sina; Kakenov, Nurbek; Wang, Kaiyuan; Smith, Thomas B.; Bakan, Gokhan; Balci, Sinan; Principi, Alessandro; Novoselov, Kostya S.; Ozdemir, Sahin K.; Kocabas, Coskun
The topological structure associated with the branch point singularity around an exceptional point (EP) can provide tools for controlling the propagation of light. Through use of graphene-based devices, we demonstrate the emergence of EPs in an electrically controlled interaction between light and a collection of organic molecules in the terahertz regime at room temperature. We show that the intensity and phase of terahertz pulses can be controlled by a gate voltage, which drives the device across the EP. Our electrically tunable system allows reconstruction of the Riemann surface associated with the complex energy landscape and provides topological control of light by tuning the loss imbalance and frequency detuning of interacting modes. Our approach provides a platform for developing topological optoelectronics and studying the manifestations of EP physics in light–matter interactions.
Open Access
Ultrafast spectroscopy of voltage reconfigurable graphene saturable absorbers in the visible and near infrared
(IOP, 2019-04-23) Baylam, I.; Kakenov, Nurbek; Kocabaş, Coşkun; Sennaroğlu, A.; Çizmeciyan, M. N.
We describe a detailed experimental investigation of the ultrafast nonlinear response of a voltagecontrolled graphene-gold saturable absorber (VCG-gold-SA) by employing femtosecond pump probe spectroscopy. Visible and near-infrared continuum probe pulses covering the spectral range from 500nm to 1600nm were used. In the experiments, the saturation fluence, modulation depth, ultrafast relaxation times, and the saturable absorption bandwidth of the VCG-gold-SA were measured as a function of the applied bias. We observed both saturable absorption and multi-photon absorption regimes as the applied bias voltage was varied between 0 and 2 V. Measurements indicate that under bias voltages in the range of 0–2 V, it should be possible to adjust the insertion loss of the VCG-gold-SA and at the same time, maintain a sufficient amount of modulation depth as well as an attainable level of saturation fluence over an ultrabroad spectral bandwidth. In particular, at the bias voltage of 1 V, the VCG-gold-SA exhibited fast saturable absorber behavior with adjustable insertion loss from 630nm to 1100nm. These results clearly demonstrate that the VCG-gold-SA can operate as a versatile mode locker for femtosecond pulse generation from lasers operating in the visible and near-infrared wavelengths.