Ağır kuarkonya tayflarının kuark modeli ile incelenmesi.

Conventional Heavy Quarkonium systems, Charmonium and Bottomonium, are believed to be composed of a heavy quark and anti-quark pair. These systems are investigated by di erent methods resulting from di erent approaches to Quantum Chromodynamics (QCD), such as Lattice QCD, E ective Theories and Sum Rules. In this thesis we study the spectrum of Charmonium and Bottomonium using a non-relativistic Quark Model. Assuming one gluon exchange for the short distances and a linear confining potential for long distances we derive Breit-Fermi interaction Hamiltonian and calculate the spectra arising from this Hamiltonian. Also we calculate the partial widths of E1 and M1 radiative decays.M.S. - Master of Scienc

Görüntüleme uygulamaları için özgün terahertz modülasyon ve algılama yöntemleri.

Terahertz radiation, due to its unique properties offers a plethora of possibilities of applications in various domains including security & defense, communications, material characterization, product inspection and medical imaging. In all these fields, cost-effective, high-resolution, and high-speed terahertz imaging is a greatly desired technology. However due the inherent challenges associated with terahertz detection, building a terahertz imaging system that satisfies all these requirements necessitates the development of novel detectors, components, and imaging methods. In this thesis, we present the work done to advance terahertz detectors, modulators and imaging methods, towards the aim of developing a cost-effective, high-resolution, and high-speed terahertz imaging system. For the cost-effective detection of terahertz radiation, we investigate the use of neon indicator lamps as glow discharge detectors and achieve response comparable to commercially available high-cost detectors. Using a Schottky diode multiplied source and lock-in techniques behavior of glow discharge detectors up to 90 kHz modulation is investigated in the range from 0.260 THz to 0.380 THz. Besides, polarization sensitivity and transmittance of these detectors are also measured in this range. Overall, results obtained from these investigations suggest up to a twofold increase in detected signal as the modulation frequency increases from 1 kHz to 90 kHz. Furthermore, the measurements show that internal geometry of these detectors influences their interaction with terahertz radiation significantly and certain types of indicator lamps perform much better. For spatial modulation of terahertz waves, devices utilizing graphene and vanadium dioxide are characterized. Also, the coupling of metamaterials and frequency selective surfaces to these devices are studied. We present the experimental results for a graphene-based tunable THz cavity that provides %98 modulation at 0.370 THz. Also, a graphene-based spatial light modulator, capable of enabling broadband modulation of THz waves with voltage controlled patterns is investigated. Besides, we achieved frequency selectivity in the spatial light modulator by coupling it to frequency selective surfaces and metamaterials. Furthermore, a vanadium dioxide based metamaterial is shown experimentally to have frequency selectivity and temperature controlled transmissivity. Finally, we give experimental results of four different terahertz imaging systems which were set up to see objects hidden behind and inside walls and other visibly opaque obstructions. We show more than two times increase in speed compared to conventional pixel scanning methods, through the use of compressed sensing. Besides, we present a novel superimposed spatial light modulator that enables up to ten times reduction in size and increase in speed. Furthermore, by using the graphene-based spatial light modulator, we demonstrate an electrically controlled terahertz imaging system.Ph.D. - Doctoral Progra

Low-cost bandpass filter for terahertz applications

Compared to the visible and infrared regions of the electromagnetic spectrum, development of optical components in the terahertz region has progressed slowly even though the feature size of structures makes many of them easily attainable by many conventional manufacturing techniques. Of these, the performance of components with machined metal surfaces, such as filters, typically has suffered from the errors and inconsistencies in the manufacturing which has led many to manufacture these using more expensive deposition and processing tools. Here we show that by using a novel, high power Yb:doped pulsed nanosecond fiber laser system with exceptional beam quality, aluminum metal surfaces can be machined with high precision leading to a high quality band pass filter working in the terahertz frequency range. The filter was laser machined on a 0.05 mm thick aluminum substrate over a 1 cm area where the unit cell in the pattern had an equilateral triangular geometry in which the base length between the holes and the diameters were machined with better than 4 % accuracy. The produced structures are modeled by utilizing the obtained structural parameters in a waveguide configuration and then characterized by the existing home-built time-domain terahertz spectrometers in our laboratories. The results show a near 100 % power transmission at the desired terahertz frequency range which suggests that these manufacturing techniques can be used to produce low-cost THz filters

Biyolojik Malzemelerin T-Işınları İle Karekterizasyonu

Araştırmacılar için terahertz (THz) dalga boylarına dayanan teknolojiler ve uygulamalar fizik alanındaki birçok olayı aydınlatabilmesi yönünden benzeri olmayan bir kazanç sağlamaktadır. Bu çeşitli uygulamalar arasında en iyi sonuç alınan yöntemler atmalı THz spektroskop sistemlerinin geliştirilmesi ve bunların farklı malzemelerdeki uygulamaları olmuştur. Bu proje çalışmasında son bir yıldır Ankara Üniversitesi Diş hekimliği Fakültesi öğretim üyesi Doç. Dr. Kıvanç Kamburoğlu ile başlattığımız T-ışınları ile diş ölçüm çalışmalarını geliştirmek için diş üzerinden yansıma konumunda çalışacak bir optik sistem geliştirmeyi hedefliyoruz. Bu çalışmalar, sağlıklı ve sağlıksız dişlerin yanı sıra diş tedavisinde kullanılan implant veya protezler üzerinde ölçüm yeteneğinin kazanılmasına ve elde edilen THz darbeleri sayesinde incelenen diş yapısının üç boyuttaki analizine odaklanacaktır. Geliştirmeyi hedeflediğimiz ölçüm teknikleri ek olarak ODTÜ biyoloji bölümü Prof. Dr. Feride Severcan ile devam eden THz-biyolojik yapıların etkileşimlerini araştıran çalışmalarımızı destekleyecektir. Burada, polietilen tozlar ile “pellet” şeklinde hazırlanan biyolojik örneklerini yansıma konumunda ölçerek geçiş ölçümleri ile yapılan çalışmalarımızı doğrulamayı hedefliyoruz

Zamana Dayalı Terahertz Ölçüm Sistemi ile Deri Dokuların İncelenmesi

Araştırmacılar için terahertz (THz) dalga boylarına dayanan teknolojiler ve uygulamalar fizik alanındaki birçok olayı aydınlatabilmesi yönünden benzeri olmayan bir kazanç sağlamaktadır. Bu çeşitli uygulamalar arasında son zamanlarda hız kazanan bir konu,sağlık sektörüne yönelik çalışmalar için atmalı THz spektroskop sistemlerinin geliştirilmesidir.Yakın tarihteki literatürde özellikle deri dokuların üzerinde çalışmalar yapıldığı gözlemlenmektedir. THz dalga boyları deri içerisindeki su miktarına hassas olduğundan bu tip sistemlerin kullanımı sonucu dokunun sağlığı hakkında tıbbi çalışanlar daha iyi bilgilendirebilinir. Örneğin, THz ışınları kullanarak derideki yanık durumunun derecesini, bez altından bile, ölçülebileceği yönünde bulgular görülmektedir. Son yıl içerisinde GATA’daki ilgili kişiler (Plastik ve Rekonstrüktif Cerrahi Anabilim Dalı) ile başlattığımız çalışmalar sonucu yanık fare derilerindeki su miktarın etkisini araştırmak için kurduğumuz terahertz sistemler ile literatüre benzer sonuçlar elde ettik. Şu anda mevcut sistemimiz ile sadece geçiş ölçümleri gerçekleştirebildiğimizden sınırlı (şekil, kalınlık olarak) deri örnekleri inceleyebildik. Projede mevcut sistemlerimizi geliştirerek yansıma konumunda ölçüm almak ve ölçüm tekniklerin sınırlarını belirlemeyi hedefliyoruz

Bariyer Arkası Terahertz Görüntüleme Sistemi

ODTÜ Fizik THz Araştırma Laboratuarı’nda farklı görüntüleme sistemleri kullanılarak, farklı THz ve THz altı görüntüler elde edilmektedir. Bunlardan birincisinde Schottky diyot çarpanlarına dayalı bir THz kaynak Galvo aynaların yardımıyla görüntüleme alanını nokta nokta taramaktadır. İkinci bir sistemde yine Schottky diyot çarpanlarına dayalı başka bir THz kaynak ile Compressive Sensing prensibine uygun olarak görüntüleme yapılmaktadır. Bir başka proje kapsamında ise görüntüleme için GDD alıcı dizgiler geliştirilmektedir. Bu projede THz bariyer arkası görüntüleme sistemlerinin bariyerden gelen gürültüyü yok etmesi için gerekli elektronik bileşenler geliştirilecek ve böylece bahsi geçen üç çalışmanın da kazanç sağlaması sağlanacaktır. Hedeflenen bu amaç için gereken optik dizayn standart üretimi ve satışı yapılan lensler ile sağlanamamaktadır. Bu yüzden gerekli parametrelere sahip lenslerin tasarımı, üretimi ve testi gereklidir. Proje kapsamında uygun bir 3B yazıcı alınıp farklı frekansları akromat şekilde hedefe odaklayan THz merceklerin üretilmesi planlanmıştır

Extraction of dielectric and magnetic material properties for a periodic hole array THz filter

With the increase in optical applications in the terahertz frequency range there is a greater need for devices that work at these frequencies. One such device which has applications which range from spectroscopy to imaging is the band pass filter. The frequency selective nature of this component can be used for filtering noise during signal processing or can be even used for reducing radiation background levels for imaging purposes [1, 2]. The simplest form of this filter structure has been shown to be that of a free-standing metal sheet with a hole array. The frequency selective nature of the filter is realized by changing the structural parameters of the hole array with respect to the propagating input THz wave. Due to the sub-millimeter wave nature of the radiation the patterns are easier to fabricate for low frequency terahertz radiation applications (<1THz). Here, by using a high power Yb:doped pulsed nanosecond fiber laser system with exceptional beam quality, aluminum metal surfaces were machined with high precision leading to a high quality band pass filter that was shown to work in the terahertz frequency range [3]. We modeled the THz transmission through the the structure that was produced using FDTD simulation tools and also characterized them using the existing home-built time-domain terahertz spectrometers in our laboratories. Results of FDTD simulations are found to be consistent with transmission coefficient which is obtained from the experiment. In order to understand the efficiency of the composite structure which can be described as a frequency selective surface, the electrical permittivity (ε) and magnetic permeability (μ) is extracted from the transmission and reflection simulations [4]. The significance of these values is discussed with respect to the width and transmission observed for the fabricated and measured filter

Detection of far-infrared radiation using glow discharge detectors

The plasma medium, created inside the neon indicator lamp, enables the detection of a wide wavelength range of differential or modulated EM signals. The interaction between the plasma and various frequency EM waves are still being investigated and in the mm-wave/terahertz range the interaction mechanism is still not well understood. In this study commercially available neon indicator lamps are studied using both time-domain and continuous wave mm-wave/THz measurement systems. The glow discharge detector response is similar to other commercially available room temperature direct detection mm-wave/THz detectors and performs at a fraction of their cost. Using a W-band Schottky diode multiplied source the detection behavior of the detector was investigated using lock-in detection techniques at modulation frequencies up to 90 kHz. These measurements also show that the orientation of the electrode structure was sensitive to the polarization of the incident field. The polarization sensitive behavior indicates a strong coupling between the incident and native field in the plasma. Furthermore, the interaction of the lamp structure with the mm-wave/THz field was investigated between 260 and 380 GHz using both a home-made time-domain THz spectroscopy system as well as a commercially available mm-wave/THz source. Within the constraints on the maximum modulation frequency of the source (<100 Hz-1 kHz), certain mm-wave/THz frequencies are attenuated suggesting the electrode structure of the detector can play a role in the detection. Overall, the measurements show that certain types of indicator lamps perform better than others and future studies in detection and imaging can benefit from their use

Compressive Sensing Imaging with a Graphene Modulator at THz Frequency in Transmission Mode

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