Browsing by Subject "Terahertz"

Now showing 1 - 6 of 6

Open Access
Frequency selective surfaces for terahertz applications
(2015) Ramzan, Mehrab
This thesis presents Terahertz (THz) Frequency selective surfaces (FSS) that can be realized using standard microfabrication techniques. These FSS structures are designed for frequencies around 0.8 THz, which is a crucial operating frequency in security and medical imaging. Using THz waves for such applications, multilayer frequency selective surfaces are preferred due to their wide at band response, lower dependency to angle of incidence, and low loss. The implementation of such structures requires very thin layers of substrates and membranes in order to improve the performance in THz regime. In order to alleviate the di culty in the implementation of multilayer structures, a fabrication process is proposed where a 100 m-thick glass membrane is formed through HF etching of a 500 m-thick glass wafer. Using this fabrication process, three separate designs consisting of single-layer FSS are investigated using high frequency structural simulator (HFSS). The rst design, consists of a circular ring slot in a square metallic structure on top of a 100 m-thick Pyrex glass membrane with 95% transmission bandwidth of approximately 0.042 THz , which remains nearly constant till 30o angle of incidence. The second design consists of a tripole structure on top of a 100 m-thick Pyrex glass membrane with nearly 95% transmission bandwidth of 0.015 THz, which remains nearly constant till 30o angle of incidence. The third structure consists of a triangular ring slot in a square metal on top of a 100 m-thick Pyrex glass membrane with 95% transmission bandwidth of 0.015 THz, which remains nearly constant upto 20o angle of incidence. These designs show that the re ections from samples can be reduced compared to the conventional sample holders used in THz spectroscopy applications; by using single layer FSS structures manufactured through a relatively simple fabrication process.
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.
Open Access
Optically implemented broadband blueshift switch in the terahertz regime
(American Physical Society, 2011-01-18) Shen, N. H.; Massaouti, M.; Gokkavas, M.; Manceau J. M.; Özbay, Ekmel; Kafesaki, M.; Koschny, T.; Tzortzakis, S.; Soukoulis, C. M.
We experimentally demonstrate, for the first time, an optically implemented blueshift tunable metamaterial in the terahertz (THz) regime. The design implies two potential resonance states, and the photoconductive semiconductor (silicon) settled in the critical region plays the role of intermediary for switching the resonator from mode 1 to mode 2. The observed tuning range of the fabricated device is as high as 26% (from 0.76 THz to 0.96 THz) through optical control to silicon. The realization of broadband blueshift tunable metamaterial offers opportunities for achieving switchable metamaterials with simultaneous redshift and blueshift tunability and cascade tunable devices. Our experimental approach is compatible with semiconductor technologies and can be used for other applications in the THz regime.
Open Access
A performance-enhanced planar Schottky diode for Terahertz applications: an electromagnetic modeling approach
(Cambridge University Press, 2017) Ghobadi, Amir; Khan, Talha Masood; Celik, Ozan Onur; Biyikli, Necmi; Okyay, Ali Kemal; Topalli, Kagan
In this paper, we present the electromagnetic modeling of a performance-enhanced planar Schottky diode for applications in terahertz (THz) frequencies. We provide a systematic simulation approach for analyzing our Schottky diode based on finite element method and lumped equivalent circuit parameter extraction. Afterward, we use the developed model to investigate the effect of design parameters of the Schottky diode on parasitic capacitive and resistive elements. Based on this model, device design has been improved by deep-trench formation in the substrate and using a closed-loop junction to reduce the amount of parasitic capacitance and spreading resistance, respectively. The results indicate that cut-off frequency can be improved from 4.1 to 14.1 THz. Finally, a scaled version of the diode is designed, fabricated, and well characterized to verify the validity of this modeling approach.
Open Access
Plasmonic enhanced terahertz time-domain spectroscopy system for identification of common explosives
(SPIE, 2017) Demirağ, Yiğit; Bütün, Bayram; Özbay, Ekmel
In this study, we present a classification algorithm for terahertz time-domain spectroscopy systems (THz-TDS) that can be trained to identify most commonly used explosives (C4, HMX, RDX, PETN, TNT, composition-B and blackpowder) and some non-explosive samples (lactose, sucrose, PABA). Our procedure can be used in any THz-TDS system that detects either transmission or reflection spectra at room conditions. After preprocessing the signal in low THz regime (0.1-3 THz), our algorithm takes advantages of a latent space transformation based on principle component analysis in order to classify explosives with low false alarm rate. © 2017 SPIE.
Open Access
Terahertz Bandpass Frequency Selective Surfaces on Glass Substrates Using a Wet Micromachining Process
(Springer New York LLC, 2017) Ramzan, Mehrab; Khan, Talha Masood; Bolat, Sami; Nebioglu, Mehmet Ali; Altan, Hakan; Okyay, Ali Kemal; Topallı, Kağan
This paper presents terahertz (THz) frequency selective surfaces (FSS) implemented on glass substrate using standard microfabrication techniques. These FSS structures are designed for frequencies around 0.8 THz. A fabrication process is proposed where a 100-μm-thick glass substrate is formed through the HF etching of a standard 500-μm-thick low cost glass wafer. Using this fabrication process, three separate robust designs consisting of single-layer FSS are investigated using high-frequency structural simulator (HFSS). Based on the simulation results, the first design consists of a circular ring slot in a square metallic structure on top of a 100-μm-thick Pyrex glass substrate with 70% transmission bandwidth of approximately 0.07 THz, which remains nearly constant till 30° angle of incidence. The second design consists of a tripole structure on top of a 100-μm-thick Pyrex glass substrate with 65% transmission bandwidth of 0.035 THz, which remains nearly constant till 30° angle of incidence. The third structure consists of a triangular ring slot in a square metal on top of a 100-μm-thick Pyrex glass substrate with 70% transmission bandwidth of 0.051 THz, which remains nearly constant up to 20° angle of incidence. These designs show that the reflections from samples can be reduced compared to the conventional sample holders used in THz spectroscopy applications, by using single layer FSS structures manufactured through a relatively simple fabrication process. Practically, these structures are achieved on a fabricated 285-μm-thick glass substrate. Taking into account the losses and discrepancies in the substrate thickness, the measured results are in good agreement with the electromagnetic simulations. © 2017, Springer Science+Business Media New York.