Browsing by Author "Topalli, K."

Now showing 1 - 7 of 7

Open Access
All-silicon ultra-broadband infrared light absorbers
(Nature Publishing Group, 2016) Gorgulu, K.; Gok, A.; Yilmaz, M.; Topalli, K.; Blylkll, N.; Okyay, Ali Kemal
Absorbing infrared radiation efficiently is important for critical applications such as thermal imaging and infrared spectroscopy. Common infrared absorbing materials are not standard in Si VLSI technology. We demonstrate ultra-broadband mid-infrared absorbers based purely on silicon. Broadband absorption is achieved by the combined effects of free carrier absorption, and vibrational and plasmonic absorption resonances. The absorbers, consisting of periodically arranged silicon gratings, can be fabricated using standard optical lithography and deep reactive ion etching techniques, allowing for cost-effective and wafer-scale fabrication of micro-structures. Absorption wavebands in excess of 15 micrometers (5-20 μm) are demonstrated with more than 90% average absorptivity. The structures also exhibit broadband absorption performance even at large angles of incidence (θ = 50°), and independent of polarization.
Open Access
A Miniaturized Patch Antenna by Using a CSRR Loading Plane
(Hindawi Publishing Corporation, 2015) Ramzan, M.; Topalli, K.
This paper presents a design methodology for the implementation of a miniaturized square patch antenna and its circuit model for 5.15 GHz ISM band. The miniaturization is achieved by employing concentric complementary split ring resonator (CSRR) structures in between the patch and ground plane. The results are compared with the traditional square patch antenna in terms of area, bandwidth, and efficiency. The area is reduced with a ratio of 1/4 with respect to the traditional patch. The miniaturized square patch antenna has an efficiency, bandwidth, and reflection coefficient of 78%, 0.4%, and -16 dB, respectively. The measurement and circuit modeling results show a good agreement with the full-wave electromagnetic simulations. © 2015 Mehrab Ramzan and Kagan Topalli.
Open Access
Nanosecond pulsed laser ablated sub-10 nm silicon nanoparticles for improving photovoltaic conversion efficiency of commercial solar cells
(Institute of Physics Publishing Ltd., 2017) Rasouli, H. R.; Ghobadi, A.; Ghobadi, T. G. U.; Ates, H.; Topalli, K.; Okyay, Ali Kemal
In this paper, we demonstrate the enhancement of photovoltaic (PV) solar cell efficiency using luminescent silicon nanoparticles (Si-NPs). Sub-10 nm Si-NPs are synthesized via pulsed laser ablation technique. These ultra-small Si nanoparticles exhibit photoluminescence (PL) character tics at 425 and 517 nm upon excitation by ultra-violet (UV) light. Therefore, they can act as secondary light sources that convert high energetic photons to ones at visible range. This down-shifting property can be a promising approach to enhance PV performance of the solar cell, regardless of its type. As proof-of-concept, polycrystalline commercial solar cells with an efficiency of ca 10% are coated with these luminescent Si-NPs. The nanoparticle-decorated solar cells exhibit up to 1.64% increase in the external quantum efficiency with respect to the uncoated reference cells. According to spectral photo-responsivity characterizations, the efficiency enhancement is stronger in wavelengths below 550 nm. As expected, this is attributed to down-shifting via Si-NPs, which is verified by their PL characteristics. The results presented here can serve as a beacon for future performance enhanced devices in a wide range of applications based on Si-NPs including PVs and LED applications.
Open Access
On-chip characterization of THz Schottky diodes using non-contact probes
(IEEE Computer Society, 2016) Khan, T. M.; Ghobadi, A.; Celik, O.; Caglayan, C.; Bıyıklı, Necmi; Okyay, Ali Kemal; Topalli, K.; Sertel, K.
We present non-contact characterization of GaAs Schottky contacts in the 140-220 GHz band. The non-contact probing technique utilizes planar on-chip antennas that are monolithically integrated with the coplanar waveguide environment housing the Schottky diode under test. The diode contact is fabricated through a 6 mask lithographic process with a 5 μm deep-trench under the contact to minimize parasitics and extend operation into the THz band. A quasi-optical link between the VNA ports and on-chip probe antennas enables efficient signal coupling into the test device. The non-contact probe station is calibrated using on-chip quick-offset-short method and the effectiveness of this approach is demonstrated for integrated diodes for under various bias conditions.
Open Access
Optical characterization of high and low resistive silicon samples suitable for reconfigurable antenna design
(Wiley Periodicals, Inc., 2018) Ali, A.; Topalli, K.; Ramzan, M.; Khan, Talha Masood; Altıntaş, Ayhan; Colantonio, P.
Highly resistive (HR) silicon (Si) can behave as a switch when illuminated by optical source of suitable wavelength. Different reconfigurable passive structures, such as filters, waveguides, and antennas, can be constructed using such silicon switches. This letter presents experimental characterization of high and low resistive (HR & LR) silicon for switching application. In the experiment, HR Si is modeled on a switched transmission line by halogen lamp and a laser source. The experiment of utilizing halogen lamp for Si switch characterization is cost‐effective and can assist engineers in designing reconfigurable antennas. In future, this experiment could be utilized in designing novel reconfigurable antennas.
Open Access
Optically reconfigurable planar monopole antenna for cognitive radio application
(Wiley Periodicals, Inc., 2019) Ali, A.; Topalli, K.; Ramzan, M.; Alibakhshikenari, M.; Khan, Talha Masood; Altıntaş, Ayhan; Colantonio, P.
Frequency reconfigurable antenna is one of the important elements needed for cognitive radio application. Such antenna can be designed using highly resistive (HR) silicon (Si) operating as an optical switch. This letter presents a novel frequency reconfigurable planar monopole antenna suitable for cognitive radio application. The antenna is designed using HR Si working as an optical switch. The main idea behind the design of antenna is the redistribution of surface current on the antenna while changing the state of Si switches optically from high resistance to low resistance. The antenna is highly compact and uses only two switches for multiband reconfiguration. It is switchable on 1.9 GHz, 2.75 GHz, 3.7 GHz, 4.1 GHz, 4.6 GHz, 4.8 GHz, and 7.6 to 11 GHz frequency bands. Simulated and measured results are presented for the antenna. To the best of authors knowledge, this is the first multiband optically reconfigurable planar monopole antenna.
Open Access
Reconfigurable nested ring-split ring transmitarray unit cell employing the element rotation method by microfluidics
(Institute of Electrical and Electronics Engineers, 2015) Erdil, E.; Topalli, K.; Esmaeilzad, N. S.; Zorlu, O.; Kulah, H.; Aydin, C. O.
A continuously tunable, circularly polarized X-band microfluidic transmitarray unit cell employing the element rotation method is designed and fabricated. The unit cell comprises a double layer nested ring-split ring structure realized as microfluidic channels embedded in Polydimethylsiloxane (PDMS) using soft lithography techniques. Conductive regions of the rings are formed by injecting a liquid metal (an alloy of Ga, In, and Sn), whereas the split region is air. Movement of the liquid metal together with the split around the ring provides 360° linear phase shift range in the transmitted field through the unit cell. A circularly polarized unit cell is designed to operate at 8.8 GHz, satisfying the necessary phase shifting conditions provided by the element rotation method. Unit cell prototypes are fabricated and the proposed concept is verified by the measurements using waveguide simulator method, within the frequency range of 8-10 GHz. The agreement between the simulation and measurement results is satisfactory, illustrating the viability of the approach to be used in reconfigurable antennas and antenna arrays.