Browsing by Subject "Defect"

Now showing 1 - 6 of 6

Open Access
Defect states in monolayer hexagonal BN: A comparative DFT and DFT-1/2 study
(Elsevier, 2020) Aksu-Korkmaz, Yağmur; Bulutay, Ceyhun; Sevik, C.
Hexagonal boron nitride (h-BN) acts like a semiconductor vacuum to point defects enabling stable and controllable spin states at room temperature which qualifies them for quantum technological applications. To characterize their properties first-principles techniques constitute indispensable tools. The currently established paradigm for such solid-state electronic structure calculations is the density functional theory (DFT). Recently its variant, so-called DFT-1/2 method was introduced with the promise of accurate band gaps without a computational overhead with respect to ordinary DFT. Here, for the monolayer h-BN we contrast DFT and DFT-1/2 results for carbon substitutional impurities (CB, CN), boron and nitrogen single vacancies (VB, VN), divacancy, and Stone-Wales defects. Comparisons with more sophisticated, yet computationally costly techniques namely, hybrid functional DFT and the GW are also made, where available. From the standpoint of defect states embedded in the band gap region we demonstrate a clear advantage of DFT-1/2 in revealing the localized states otherwise buried within either the valence or conduction band continuum due to well-known gap underestimation syndrome of the standard DFT implementations. Thus, DFT-1/2 can serve for the rapid screening of candidate defect systems before more demanding considerations.
Open Access
Dropping of electromagnetic waves through localized modes in three-dimensional photonic band gap structure
(AIP Publishing, 2002-12-03) Bayındır, Mehmet; Özbay, Ekmel
We experimentally demonstrate trapping and dropping of photons through localized cavity modes in three-dimensional layer-by-layer photonic crystal structures. By creating acceptor- and donor-like cavities which are coupled to a highly confined waveguide (HCW), we drop selected frequencies from the waveguide mode. Tunability of the demultiplexing structures can be achieved by changing the properties of cavities and the coupling between the cavity and the HCW. (C) 2002 American Institute of Physics.
Open Access
Electronic structure and optical properties of monolayer semiconductors: a computational study
(2021-09) Korkmaz, Yağmur Aksu
Interest on monolayer semiconductors is rapidly growing in recent years. One of the prominent members is hexagonal boron nitride (h-BN). At room tem-perature, it harbors an environment similar to semiconductor vacuum for point defects which is crucial for stable and controllable spin states. This qualiﬁcation makes h-BN a suitable medium for quantum technological applications. First-principles calculations are essential in order to characterize such systems. Density functional theory (DFT) is one of the most reliable methods used for these type of calculations. Recently, a variant called as DFT-1/2, has been proposed to calculate the band gaps of the materials more accurately without a signiﬁcant additional computational cost. In the ﬁrst part of thesis, we have compared the results of DFT and DFT-1/2 for carbon impurities (CB, CN ), single vacan-cies (VB, VN ), double vacancy (divacancy) and Stone-Wales defect in monolayer h-BN. Subsequently, results from computationally expensive techniques such as hybrid or GW are presented and compared with the obtained DFT-1/2 results. Especially for the defect states seemingly hidden in valence or conduction band, DFT-1/2 technique is instrumental in revealing these states while widening the band gap. Thus, we recommend the DFT-1/2 method for a quick screening of candidate band gap defect states. Another outstanding group of semiconductors is transition metal dichalco-genides (TMDs). They owe their advantages to optically addressable valley and bringing optics and mechanics together as in valleytronics, thanks to their high ﬂexibility. In the second part of this thesis, ten TMDs including their janus coun-terparts (JTMDs), namely, MoS2, MoSe2, MoTe2, WS2, WSe2, WTe2, MoSSe, MoSeTe, WSSe, and WSeTe have been computationally studied. To begin with, the electronic band structure of the speciﬁed materials have been computed using DFT followed by hybrid calculations over these, with the addition of spin-orbit coupling. Biaxial and uniaxial strain calculations are subsequently performed. JTMDs were previously proclaimed to have a good piezoelectric characteristic. According to our DFT results, JTMDs exhibit band structure and electronic properties in between its constituent TMDs, and in this respect they do not dis-play an outstanding behaviour. Based on the acquired DFT data, spinless and spinful k · p parameters are extracted by ﬁtting around optically active K valley. With the help of k·p parametrization, linear and circular dichroic behaviours are studied for unstrained and strained cases. In consideration of all these materials, WTe2 displays the largest linear dichroic responsitivity for uniaxial strain, since it has the smallest band gap and the greatest uniaxial deformation potential at the K valley. Thus, we propose monolayer WTe2 membranes to be considered for optical polarization based strain measurements, as well as, strain adjustable optical polarizers.
Open Access
Physics and applications of coupled-cavity structures in photonic crystals
(2002) Bayındır, Mehmet
We proposed and demonstrated a new type of propagation mechanism for the electromagnetic waves in photonic band gap materials. Photons propagate through coupled cavities due to interaction between the highly localized neighboring cavity modes. We reported a novel waveguide, which we called coupled-cavity waveguide (CCW), in two- and three-dimensional photonic structures. By using CCWs, we demonstrated lossless and reflectionless waveguide bends, efficient power splitters, and photonic switches. We also experimentally observed the splitting of eigenmodes in coupled-cavities and formation of defect band due to interaction between the cavity modes. We reported the modification of spontaneous emission from hydrogenated amorphous silicon-nitride and silicon-oxide multilayers with coupled Fabry-Perot microcavities. We observed that the spontaneous emission rate is drastically enhanced at the coupledmicrocavity band edges due to very long photon lifetime. We also simulated our photonic structures by using the Transfer-Matrix-Method (TMM) and the Finite-Difference-Time-Domain (FDTD) method. The tight-binding (TB) approach, which was originally developped for the electronic structure calculations, is applied to the photonic structures, and compared to our experimental results. The measured results agree well with the simulations and the prediction of TB approximation. The excellent agreement between the measured, simulated, and the TB results is an indication of potential usage of TB approximation in photonic structures. Our achievements open up a new research area, namely physics and applications of coupled-cavities, in photonic structures. These results are very promising to construct for the future all-optical components on a single chip.
Open Access
Physics and applications of photonic crystals
(2000) Temelkuran, Burak
We first fabricated a dielectric based layer-by-layer photonic crystal, with a three-dimensional photonic band gap at microwave frequencies. We investigated the transmission, reflection and defect characteristics of the crystal. A Fabry-Perot cavity analogy was used to understand the localization of the electromagnetic (EM) fields around defects. We then showed the enhancement of the EM held within the defect volumes, and suggested a possible application: resonant cavity enhanced detectors built around photonic crystals. We demonstrated that a detector inserted inside the defect volume benefits from the frequency selectivity and the highly enhanced field of the cavity. Next, we investigated the radiation of the EM fields from a source inserted in the defect volume, and observed that the radiated field has a very high directivity and efficiency. The experimental results agreed well with the theoretical expectations. We demonstrated waveguiding structures built around photonic crystals. We showed that EM waves could be guided through a planar air gap between two photonic crystals, in which the wave is coupled inside the defect volume, and having no where else to go, propagates through this opening. The dispersion diagrams for these planar waveguide structures also agreed well with the theoretical expectations of our waveguide model. We also showed that, the wave could be guided along a single missing rod, and demonstrated the bending of the EM waves for these waveguide structures with “L” shaped openings. We tested metallic photonic crystals built in different dimensions and diflferent filling ratios. We observed many superiorities of these structures when compared to dielectric-based photonic crystals. A full characterisation of various metallic photonic crystals was performed. We also showed that metallic photonic crystals are suitable for some of the applications we have demonstrated for dielectric structures. We also fabricated a new layer-by-layer photonic crystal using highly doped silicon wafers processed by semiconductor micromachining techniques, with a band gap at millimeter wave frequencies. We showed that the transmission and defect characteristics of these structures are analogous to metallic photonic crystals, as we have predicted. The experimental results agree well with the predictions of the transfer matrix method (TMM) simulations. The method can be extended to fabricate these crystals at THz. frequencies.
Open Access
Sözleşme dışı sorumluluk hukukunda otonom sistemler
(2022-05) Polat, Cemre
Yapay zekâ ve robotik bilimindeki gelişmeler otonom sistemlerin yaygınlaşmasını sağlamış; bu tür uygulamalar artık yalnızca bilimsel ya da endüstriyel aktivitelerin alanında olmaktan çıkıp bireylerin günlük hayatlarında da erişilebilir birer ürüne dönüşmüştür. En bilinen örnekleri otonom karayolu araçları ve hava araçları (drone’lar) olan otonom sistemler sağlıktan eğitime; tarımdan finansa kadar hemen her sektörde kendilerine varlık alanı bulabilmektedir. Zaman ve maliyet tasarrufu; insan iş gücüne olan ihtiyacın azalması ya da insanlar tarafından yapılamayacak olan işlerin yapılabilir hale gelmesi gibi pek çok fayda vaat eden otonom sistemler, aynı zamanda bireylerin haklarını ya da diğer menfaatlerini ihlal etme riski de barındırırlar. Bu sistemler, faaliyet göstermek için doğrudan bir insan kontrolüne ihtiyaç duymadıkları gibi, karar ve eylemleri de öngörülemezlik barındırır. Bu nedenle otonom sistemlerin neden oldukları zarardan sorumluluğun kime isnat edileceği sorusu hukukçuların ve kanun koyucuların gündemindedir. Bu çalışmada da otonom sistemlerin neden oldukları zararların sözleşme dışı sorumluluk hukuku uyarınca nasıl giderilebileceği ele alınacaktır.