Abstract-Video-speed Graphene Modulator Arrays for Terahertz Imaging Applications

Yury Malevich, M. Said Ergoktas, Gokhan Bakan, Pietro Steiner, Coskun Kocabas

https://arxiv.org/abs/2006.01056

Electrically tuneable high mobility charges on graphene yield an efficient electro-optical platform to control and manipulate terahertz (THz) waves. Real-world applications require a multiplex THz device with efficient modulation over a large active area. The trade-off between the efficient gating and switching speed, however, hinders the realization of these applications. Here, we demonstrate a large-format 256-pixel THz modulator which provides high-frame-rate reconfigurable transmission patterns. The time-domain and frequency-domain THz characterizations of graphene devices reveal the relaxation pathways of gate-induced charges and ion packing at graphene-electrolyte interface. The fundamental understanding of these limiting factors enables us to break the trade-off permitting switching frequencies up to 1 kHz. To show the promises of these devices, we demonstrate a single-pixel THz camera which allows spatial and spectroscopic imaging of large-area objects without any moving components. These results provide a significant advancement towards the achievement of non-invasive THz imaging systems using graphene-based platforms.

Abstract-Graphene Based Terahertz Phase Modulators

Nurbek Kakenov, M Said Ergoktas, Osman Balci, Coskun Kocabas

https://arxiv.org/abs/1804.08279

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 an 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 {\pi} 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.

Abstract-Observation of Gate-Tunable Coherent Perfect Absorption of Terahertz Radiation in Graphene

Nurbek KakenovOsman BalciTaylan TakanVedat Ali OzkanHakan AltanCoskun Kocabas



https://figshare.com/articles/Observation_of_Gate-Tunable_Coherent_Perfect_Absorption_of_Terahertz_Radiation_in_Graphene/3497729

Abstract-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.

Abstract-Terahertz modulation using a bandpass filter combined with a graphene supercapacitor

Taylan Takan, Mehmet A. Nebioglu, Hakan Altan

Middle East Technical Univ. (Turkey)

Nurbek Kakenov, Osman Balci, Coskun Kocabas

Bilkent Univ. (Turkey)

Proc. SPIE 9651, Millimetre Wave and Terahertz Sensors and Technology VIII, 96510F (October 21, 2015); doi:10.1117/12.2195140

http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=2466622

Graphene is proving to be an efficient medium for the control of mm-wave/THz radiation. Its electrical and dielectric properties allows it to be incorporated into various existing device architectures. One such application is in the modulation of the amplitude of the propagating THz radiation. Due to its electrical properties this interaction is typically broadband in nature. To make this frequency selective we propose the use of metamaterials or frequency selective surfaces. Generally, these structures perform the frequency filtering by modifying the propagation of the input wave with respect to changing structural parameters of the device itself. By fabricating a frequency selective surface based on a periodic circular hole array on an aluminum sheet we show that the transmission of a narrow band of THz radiation can be modulated when the sheet is combined with a highly efficient graphene based supercapacitor device. The modulation depth of the device was 15% in the frequency region of interest. The simple structure of the device coupled with the obtained performance shows that graphene based devices have great potential for the development of THz technologies.

 © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Abstract-Broadband terahertz modulators using self-gated graphene capacitors

Nurbek Kakenov, Osman Balci, Emre O. Polat, Hakan Altan, and Coskun Kocabas
https://www.osapublishing.org/josab/abstract.cfm?uri=josab-32-9-1861

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.

© 2015 Optical Society of America

Full Article  |  PDF Article