# Dept. of Physics - Ph.D. / Sc.D.

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Item Open Access Ballistic transport and tunneling in small systems(Bilkent University, 1990) Tekman, A ErkanShow more Ballistic transport and tunneling of electrons in mesoscopic systems have become one of the most important subjects of condensed matter physics. The quantum point contacts and scanning tunneling microscope form the basic experimental tools in this area and have been used for understanding many features of small systems. In this work ballistic transport and tunneling in small systems are investigated theoretically. Ballistic transport through narrow constrictions is investigated for a variety of configurations. It is found that for a uniform constriction the conductance is quantized in units of the quantum of conductance (2e^/A) for long channels. The interference of waves in the constriction gives rise to the resonance structure superimposed on the quantized steps. The lack of the resonance structure in the experimental results are attributed to temperature effects and/or adiabatic transport due to tapering of the constriction. It is shown that elastic scattering by an impurity distorts the quantization of conductance. Novel resonant tunneling effects due to formation of bound states are predicted for an attractive impurity or a local widening at the center of the constriction. It is shown that the probing in scanning tunneling microscopy have very much in common with narrow constrictions. The transition from tunneling to point contact regime is explained by the vanishing effective potential barrier as a result of tip-sample interaction. For noble and simple metals it is conjectured that lateral position dependent interaction between the tip and sample leads to corrugation of the potential barrier and in turn to atomic corrugation observed by scanning tunneling microscopy. The focused field emission of electrons from point sources is analyzed in a systematical way. The effective barrier due to the lateral confinement and nonadiabatic transport through the horn-like opening are found to be responsible for focusing. The nonequilibrium nature of transport is investigated by use of Keldysh Green’s function technique. The effects of elastic and inelastic scattering are analyzed in a strictly one-dimensional geometry. The features of voltage and current probes are studied and the Landauer formulae are examined for multiprobe measurements.Show more Item Open Access Electronic structure of low dimensional semiconductor systems(Bilkent University, 1992) Gülseren, OğuzShow more Recent progress made in the growth techniques has led to the fabrication of the artificial semiconductor systems of lower dimension. Electrons and holes in these materials have quantization different from those of the three dimensional systems presenting unusual electronic properties and novel device applications. In this work, the important features of the free carriers in semiconductor superlattices are examined, and the electronic structure of some novel 2D semiconductor systems are investigated theoretically. This thesis studies various systems of lower dimensionality such as: the strained Si/Ge superlattices, i-doping. Si (100) surface and the tip-sample interaction in scanning tunneling microscopy (STM) study of this surface, and Wannier-Stark localization in finite length superlattices. The electronic energy structure of pseudomorphic Ge„i/Si„ superlattices is investigated by using the empirical tight binding method. Effects of the band offset, sublattice periodicity and the lateral lattice constant on the transition energies have been investigated. It is found that Ge„i/Si„ superlattices grown on Ge (001) can have a direct band gap, if m + n = 10 and m = 6. However, optical matrix elements for in-plane and perpendicular polarized light are negligible for the transition from the highest valence band to the lowest conduction band state at the center of the superlattice Brillouin zone. The electronic structure of the Si i-layer in germanium is explored by using the Green’s function formalism with layer orbitals. We found two dimensional parabolic subbands near the band edges. This approach is extended to treat the electronic structure of a single quantum well without invoking the periodically repeating models. Quantum well formation in Ge,„Si„ superlattices is also studied by using different number of ^-layers. Subband structure is observed by changing the height of the Si quantum well. The confinement of acoustical modes within 2DEG due to only the electronphonon interaction is proposed. The confined modes split out from the bulk phonons, if the 2DEG is created by means of modulation doping. This occurs even if the lattice has uniform parameters. The effect is more pronounced when the wave vector q of the modes increases and is maximum a,t q = 2kp {kp is the Fermi wave vector). In the case of several electron sheets the additional features of the confinement effect appear. Green’s function method is also applied to treat the modifications of electronic state density in STM. The tip-sample interaction in STM study of Si (100) surface is explored by calculating the Gieen’s function within the empirical tight binding method. Both of the proposed reconstruction models, buckled and symmetrical dimer model, is investigated. A dip occurs in the change of density of states of surface atoms at the energy of surface states for small tip-sample distances, and it decreases with increasing tip-sample separation. Although, in-plane tip position (above the up- or down-surface atom) affects the surface atoms differently in buckled dimer model, it influences the surface atoms symmetrically in symmetric dimer model. Recent experimental studies revealed the significant information on the Wannier-Stark localization. Following these experimental results, the WannierStark ladder is investigated by carrying out numerical calculations on a multiple quantum well structure under an applied electric field. The variation of the Wannier-Stark ladder energies and localization of the corresponding wave II function are examined for a wide range of applied electric field. Our results show that Wannier-Stark ladder do exist for finite but periodic system which consists of a large number of quantum well having multi-miniband structure. It is found that the miniband states are localized in the well regions with the applied electric field, while the continuum states preserve their extended character. Energies of the well states show a linear shift with the electric field except the small field values in which a nonlinear shift is resulted. Multiband calculations show that there is a mixing between the different band states although they are localized in different well regions.Show more Item Open Access Superconducting systems of low dimensionality(Bilkent University, 1992) Gedik, M. ZaferShow more It is possible to call the last five years as the golden age of superconductivity. The two most important developments in the field are the discovery of copper oxide and fullerene superconductors. In this work, some possible pairing mechanisms for these materials ai’e examined by giving emphasis on the reduced dimensionality. First, an older problem, spatially separated electron-hole system, is investigated to identify the possible phases in coupled double quantum well structures in electric field. Secondly, the superconducting transition temperature and response to external magnetic fields of layered systems with varying number of layers are studied by means of a microscopic model and its GinzburgLandau version. It is also shown that an interlayer pairing mechanism, phonon assisted tunneling, can induce superconductivity. Finally, effects of the spherical structure of fullerenes are examined by solving a two fermion problem on an isolated molecule where the particles interact via a short range attractive potential. As a possible mechanism of superconductivity in alkali metal doped fullerenes, coupling between electrons and the radial vibrations of the molecule is investigated.Show more Item Open Access Investigation of Si1-xGex alloy formation by using STM(Bilkent University, 1994) Oral, AhmetShow more Item Open Access Hot electron interactions in nanostructures(Bilkent University, 1997) Kaya, Ismet InonuShow more Modern semiconductor growth and processing techiques have provided the capability of fabricating a huge variety of devices which have atomically precise layered structures and lateral patterns with nanometer sizes. This not only provided novel device possibilités but also opened a new field in condensed matter physics, so called mesoscopics. It does not seem likely that the mesoscopic electronic devices will be available in the near future. Two main obstacles for mesoscopic electronics are the low temperature requirements and the breakdown of the phase coherence of the carriers as their energies exceed the Fermi level. This strongly suggests the investigation of the excited carriers with energies well in excess of their thermal equilibrium energy as the dimensions shrink. In this thesis, the interactions of hot electrons in semiconductor and metal structures with deep submicron characteristic dimensions have been studied. Tunneling Hot Electron Transfer Amplifier (THETA) constructed by abrupt semiconductur heterojunctions is a perfect system to analyze the interaction of hot electrons with cold electrons and the other possible excitations in solids. Recently, it has been discovered that an electron multiplication effect took place in such devices under certain conditions and resulted in a transfer ratio of greater than unity. In this work a novel fabrication technique has been developed. It would make it possible to utilize this effect for fabrication of a high frequency oscillator in the THz regime, in a future work. In addition, a kind of lateral THETA device has been constructed using a Two Dimensional Electron Gas structure. Electron multiplication effect for the first time has been observed in 2DEG structures. Moreover, the dependence of the effect on parameters such as injection energy, emitter and collector barrier heights and electron transit length has been investigated. The other direction of the work has been the investigation of metal wires under extremely high current densities. A strong nonlinearity in conductivity is introduced when a free standing submicrometer wire is biased to heat upto very high temperatures. The geometry of two crossing wires has been investigated under this condition.Show more Item Open Access Fabrication and characterization of high-speed, high quantum efficiency, resonant cavity enhanced Schottky photodiodes(Bilkent University, 1998) Ata, Erhan PolatkanShow more Rapidly developing “photonics” technology promises higher bcindwidths of communiccition than any other techniciue did ever. The increasing rate of communication not only alters science and technology, but brings a global cultural exchange, which seems to be one of the most important revolutions in the history. Photodetectors, as vital corniDonents of optoelectronics, cire still being developed to achieve satisfying performances for the increasing communication demcinds. We have designed and fabricated high-speed, high efficiency resonant Ccivity enhanced (RCE) Schottky photodiodes, suitable for 800-850 mil operation wavelengths. We have used two different GaAs/AlGaAs based epitaxial structures to achieve high performance. From one of these structures, we fabricated photodiodes with 50% quantum efficiency and 80 GHz 3-dB bandwidth. The other structure had a design suitable for préfabrication wavelength tuning and adjustable active layer thickness. On this structure, we achieved 20% quantum efficiency along with, world record for RGB photodiodes, over 110 (Hlz 3-dB estimated bandwidth. We investigated effects of active layer, top Au layer, and silicon nitride coating layer thicknesses on the RCE devices. Discrepancy between theory and experiments were also explained briefly. Methods for improving performances of photodiodes has been proposed ¿is possible future work. Possible appliccitions, which may make use of current knowhow on the subject, have also been mentioned.Show more Item Open Access Theory of atomic scale friction(Bilkent University, 1998) Buldum, AlperShow more Friction is an old and important but at the same time very complex physical event. This thesis aims to develop an atomic scale theory of friction. VVe investigate various atomic processes and stick-slip motion by using simple models and by using simulation of realistic systems based on the stateof-the art molecular dynamics and ab-initio electronic structure and force calculations. Theoretical studies of dry sliding friction, which has a close l)earing· on the experiments done by using the atomic and friction force microscope were performed. First, a simple model is used to investigate the basic mechanisms of friction and stick-slip motion, whereby the effect of material parameters and local elastic deformation of the substrate were also examined. Then, atomic scale study of contact, indentation, subsequent |)ulling and dry sliding of a sharp and blunt metal tips on a metal surface were studied. In order to understand the atomic-scale aspects of boundary lubrication such as interesting covera.ge and load dependent behavior and structural transformations, molecular dynamics simulations were performed on a model system that has two .\'i(110) surfaces and a. xenon layer confined between these two surfaces. Finally, in view of the atomic processes revealed from computer simulations an energy dissipation mechanism and quantum heat conduction were studied.Show more Item Open Access Quantum statistics of light interacting with matter(Bilkent University, 1999) Müstecaplıoğlu, ÖEShow more Studies on some systems in which light interacts with matter are performed from quantum statistical point of view. As a result of these studies a novel effect which can be utilized for detecting squeezed phonons is predicted; detection of non-classical states of Bose type excitations in solids and their classification by Raman correlation spectroscopy are discussed; a new approach to the polarization of light is developed.Show more Item Open Access Many-body interaction effects in quasi-one-dimensional photo-excited electron-hole systems(Bilkent University, 1999) Güven, KaanShow more The work in this thesis concerns rnany-body interaction effects in a quasi-onedimensional electron-hole plasma, which may be generated under intense photoexcitation in a semiconductor quantum-well wire. In particular, we investigate how these interactions affect the optical properties of the semiconductor quantum wire. We address this question in two parts: First, the band-gap renormalization (BGR) induced by self-energy corrections of electrons and holes is studied. A two subband model arising from the confinement of the quantum wire is developed to include the multisubband effects. The many-body theoretical formalism of electron (hole) self-energy is given within the GW approximation. We use the dielectric function both in full dynamical random-phase approximation, and in cjuasi-static approximation, in order to emphasize the dynamical properties of screening. The dependence of BGR on the e — h plasma density, temperature and wire width is studied. In the second part, the exciton renormalization induced by e — h plasma screening, and Goulomb correlation effects on the optical spectra of a quantum wire are considered. The optical properties are directly associated with the e — h two particle propogator, which obeys the Bethe-Salpeter equation. Based on recent studies, we review the solution of this equation with screened Coulomb interaction. In particular it is shown* that the dynamical treatment of screening produces an optical absorption/luminescence spectra which is consistent with experimental results. We present a discussion on the interplay of excitons and unbound carriers and on the reflection of this interplay to the optical spectra.Show more Item Open Access Strongly correlated models of high-temperature superconductivity(Bilkent University, 1999) Boyacı, HüseyinShow more Son zamanlarda nanometrc büyüklüğündeki örneklerle ya|)dan tek ek'ktron ta.^ınma deneyleri, üstün iletkenlerin büyüklüklerinin alt sının ile ilgili soruyu yeniden gündeme getirmiştir, iler ne kadar, belirsiz tanecik sayısına dayalı standart BCS teorisi büyük örnekler için iyi sonuçlar vermekteyse de, nanometrc^ büyüklüğündeki örnekler için bu teorinin bazı temel noktalan tc^krar gözdi'iı geçirilmelidir. Bunun iıcin, sabit sayıda parçacık için bir eşleşme IIamiItoni;uı'ı, l'ernıi seviyesi etrafındaki dejenerasyon da göz önüne alınarak incelenmiştir. Dejenera.syona bağlı olarak, eşleşme etkisinin değişimi tartışılmıştır. İkinci bölümde, atomların elektron yörüngelerindeki daralmanın, eıı yakın komşular arasındaki atlama genliğine olan etkisini göz önüne alan bir ınodc'l llamiltonian üzerinde çalışılmıştır. Bu çalışma, zayıf etkileşim limitinde' analitik olarak, orta ve güçlü etkileşim limitlerinde ise sonlu bir atom geoiiK'trisinde sayısal hesaplama ile yapılmıştır. Atom sitelerindeki deşik yerleşiminin yörünge'sel daralmaya etkileri V ve W (to|)lam ve çarpım daralma terimleri) ('tkih'şim parametreleri ile verilmektedir. Çift parçacık Creen fonksiyonundaki belirsizlik noktası, kritik sıcaklık Tc’yj ve Tc üzerinde düzen parametresinin rahatlama hızı r (7 ’) ’yi belirlemektedir. Standart BCS üstımiletkenleriiıdeiı Farklı olarak, r sıfırdan farklı imajiner bir kısma sahiptir. Bu, nstiiailetkenin 7'^. lizeriıuh'ki direııciııin dalgalanmaları üzerine etki ediyor olabilir, 'lemel dnrmn ('iK'ijisi, parçacık sayısı ve manyetik akıya göre hesaplanmı.'jtır. Bir eı^leşnıe |)aramctresi olan A ,,’nin, Cooper kararsızlığı ile aynı bölgede ortaya çıktığı gösteriImiı^tiı·. llııbbard modeli {U > 0) ile yapılan hesaplar herhangi bir dolnInk değerinde hiçbir üstün iletkanlik özelliği göstermemiştir.Show more Item Open Access Structural and electronic properties of carbon-based materials(Bilkent University, 2000) Kılıç, ÇetinShow more In this thesis, some carbon-based materials in nano scale have been investigated by using first-principles methods as well as transferable tight-binding and empirical potential models. The focus of interest has been in the cubane molecule among cage-like structures and in the carbon nanotubes among graphite-related materials. The first-principles calculations predict that cubane-like structures can exist for other group IV elements such as Si and Ge. The energetics and dynamics of such molecules has been studied. By performing quantum molecular dynamics simulations at high temperature a deformation path from cubane to cyclooctatetraene has been established. For solid cubane the structural and electronic properties and doping by alkali metal atoms have been studied. In the study of carbon nanotubes under pressure some new carbon forms due to covalent bonding between the neighboring tubes has been identified. It has been shown that the electronic structure of single wall carbon nanotubes is affected by radial deformations. For example, some zigzag nanotubes have been found to experience semiconductor-to-metal transition as a result of compression. Exploiting this property, it has been shown that variable and reversible quantum structures can be realized on a single carbon nanotube. Finally, other quantum structures which can lead to novel nano-scale molecular devices have been proposed.Show more Item Open Access Physics and applications of photonic crystals(Bilkent University, 2000) Temelkuran, BurakShow more 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.Show more Item Open Access Phonon-mediated electron-electron interaction in confined media: low-dimensional bipolarons(Bilkent University, 2000-09) Senger, R. TuğrulShow more We study the criterion for the formation of confined large bipolarons and their stability. In order to deal with this specific subject of polaron theory, it is required to adopt some particular approximation methods, because the polaronic systems do not admit exact analytic solutions in general. Those approximation techniques, which are applied to the low-dimensional one-polaron problems, are presented to some extent to form a working basis for our main theme, bipolarons. As the model of confined bipolaron, the electrons are treated as bounded within an external potential while being coupled to one another via the Fröhlich interaction Hamiltonian. Within the framework of the bulk-phonon approximation, the model that we use consists of a pair of electrons immersed in a reservoir of bulk LO phonons and confined within an anisotropic parabolic potential box, the barrier slopes of which can be tuned arbitrarily from zero to infinity. Thus, encompassing the bulk and all low-dimensional geometric configurations of general interest, we obtain an explicit tracking of the critical values of material parameters for the bipolarons to exist in confined media. First, in the limit of strong electron-phonon coupling, we present a unified insight into the stability criterion by applying the Landau-Pekar strong coupling approximation. This crude approximation provides us the condition on the ratio of dielectric constants (η = epsilon substcript infinity/epsilon substrcript 0) for large values of electron-phonon coupling constant α. For more reliable results, we consider the path-integral formulation of the problem adopting the Feynman-polaron model to derive variational results over a wide range of the Coulomb interaction and phonon coupling strengths. It is shown that the critical values of α and η exhibit some non-trivial features as the effective dimensionality is varied, and the path integral results conform to those of strong coupling approximation in the limit of large α.Show more Item Open Access Physics and applications of coupled-cavity structures in photonic crystals(Bilkent University, 2002) Bayındır, MehmetShow more 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.Show more Item Open Access Deformation and finite size effects in cooperative molecular motors(Bilkent University, 2002-07) Taneri, SencerShow more Motor protein systems have been of considerable interest lately. In these studies muscle contraction is modeled as the sliding of two filaments made of protein particles over one another, that is the sliding of the backbone filament on the track filament. In order to make the analytical analysis easy these filaments are assumed to be of infinite length or mass. This enables the understanding of the sliding of motility assays with constant velocity and generation of constant force. However, finite size in length and mass brings fluctuationsuctuations in velocity around certain values, and changes in direction through intermittent transitions. It is possible to associate time constants to this kind of behavior. It turns out that the magnitude of the time constant being created during the process is proportional to both the length of the filament and the mass of the protein particles. Deformation phenomenon stems from internally generated forces which so far has been examined as axonemal deformations. The elastic coupling of the protein particles to the backbone has been studied separately, which in fact is also related to the generation of internal forces. Instead of focusing on the axonemal deformations, we implemented an Ising-like potential contribution to our computation to study the elastic coupling which makes the computation easier. We found out that for certain range of parameters that measures the deformation strength, one attains a better motor because of more intense force generation at the expanse of getting a lower sliding velocity.Show more Item Open Access Entanglement in atom-photon systems(Bilkent University, 2004) Can, Muhammet AliShow more In this work we propose a new principle from the point of view of quantum fluctuations of observables. This new principle can be considered as an operational definition of ME states. Moreover, we show the existence of perfect entangled states of a single “spin-1” particle. We give physical examples related to the photons, and particle physics. We show that a system of 2n identical two-level atoms interacting with n cavity photons manifests entanglement and that the set of entangled states coincides with the so-called SU(2) phase states. In particular, violation of classical realism in terms of Greenberger-Horne-Zeilinger (GHZ) and Clauser-Horne-Shimoni-Holt (GHSH) conditions is proved. We also show that generation of entangled states in the atom-photon systems under consideration strongly depends on the choice of initial conditions In order to obtain maximally robust entangled states we have combined maximum principle with minimum of energy requirement for stabilization, called Mini-max principle. We discuss the generation and monitoring of durable atomic entangled state via Raman-type process, which can be used in the quantum information processing. It is shown that the system of two three-level atoms in Λ configuration in a cavity can evolve to a long-lived maximum entangled state if the Stokes photons vanish from the cavity by means of either leakage or damping. We presented some results based on the application of spherical wave representation to description of quantum properties of multipole radiation generated by atomic transitions. In particular, the angular momentum of photons including the angular momentum entanglement, the quantum phase of photons, and the spatial properties of polarization are discussed.Show more Item Open Access High speed and high efficiency infrared photodetectors(Bilkent University, 2004) Kimukin, İbrahimShow more The increasing demand for telecommunication systems resulted in production of high performance components. Photodetectors are essential components of optoelectronic integrated circuits and fiber optic communication systems. We successfully used resonant cavity enhancement technique to improve InGaAs based p-i-n photodetectors. The detectors had 66% peak quantum efficiency at 1572 nm which showed 3 fold increases with respect to similar photodetector without resonant cavity. The detectors had 28 GHz 3-dB bandwidth at the same time. The bandwidth efficiency product for these detectors was 18.5 GHz, which is one of the best results for InGaAs based vertical photodetector. The interest in high speed photodetectors is not limited to fiber optic networks. In the recent years, data communication through the air has become popular due to ease of installation and flexibility of these systems. Although the current systems still operate at 840 nm or 1550 nm wavelengths, the advantage of mid-infrared wavelengths will result in the production of high speed lasers and photodetectors. InSb based p-i-n type photodetectors were fabricated and tested for the operation in the mid-infrared (3 to 5 µm) wavelength range. The epitaxial layers were grown on semi-insulating GaAs substrate by molecular beam epitaxy method. The detectors had low dark noise and high differential resistance around zero bias. Also the responsivity measurements showed 49% quantum efficiency. The detectivity was measured as 7.98×109 cm Hz1/2/W for 60 µm diameter detectors. Finally the high speed measurements showed 8.5 and 6.0 GHz bandwidth for 30 µm and 60 µm diameter detectors, respectively.Show more Item Open Access Robust entanglement in atomic systems(Bilkent University, 2005) Çakır, ÖzgürShow more Various models for generation of robust atomic entangled states and their implementation with current accessible technologies are proposed and worked out. Deterministic creation of long living Bell states with respect to metastable states in three-level Λ type systems is studied. Strong atom-field coupling drives atoms into a transient entangled state followed by an irreversible evolution towards a long-living maximally entangled state featuring robustness against dipole-allowed transitions. First, generation of pairwise atomic entanglement in cavities in ideal case is discussed, extension to multi-party entangled states is made. Observation of photons emitted from the system signals the generation of a Bell state. The interaction of multi-level atoms with body-assisted electro-magnetic field in the presence of dispersing and absorbing media is studied and these results are applied to the description of a pair of Λ type atoms passing by a microsphere. Microspheres give rise to resonances of well defined height and width with easy access to strong and weak coupling regimes for atom-field interaction, thus enabling realization of the proposed scheme of ”robust entanglement of three-level atoms”. Even in realistic settings it is possible to obtain quite high amount of entanglement at spatially well separated distances. Then we focus on steady state entanglement between atomic dipoles. It is shown that two dipoles in free space driven by a classical driving field become entangled in the steady state. The crucial point is that, this entanglement is irrespective of the initial state and may be preserved as long as the engineered system is kept intact. Absorption effects in real cavities are studied, and an input-output relation is formulated in the presence of a source in the cavity. Extraction of non-classical photon states from a cavity is investigated.Show more Item Open Access Monolithic and hybrid silicon-on-insulator integrated optical devices(Bilkent University, 2005) Kiyat, İsaShow more Silicon, the basic material of electronics industry is rediscovered nowadays for its potential use in photonics and integrated optics. The research activity in silicon integrated optics have been speeding up during the last decade and even attracting interest of leading industrial companies. As a contribution to this world wide effort, we have designed, fabricated and characterized a class of monolithic and hybrid silicon integrated optical devices. These devices were realized on high-quality silicon-on-insulator (SOI) wafers. Beam propagation method (BPM) based simulations and analytical calculations were employed for the design. We have demonstrated for the first time an SOI device that splits light into its TE and TM components. An SOI rib waveguide becomes birefringent as its size reduced. This idea is used to design and fabricate a directional coupler polarization splitter based on geometrical birefringence. The device uses 1 µm sized SOI waveguides. This compact device (only 110 µm in length) shows extinction ratios larger than 20 dB. SOI waveguides with the same geometry was used to realize a batch of single and double bus racetrack resonators having radii in the range of 20 to 500 µm. Design of these racetrack resonators are presented in detail. The bending loss and coupling factor calculations were performed using BPM. During the design and analysis of waveguide resonators, we proposed a novel displacement sensor that can be used for scanning probe microscopies. The sensor operates by means of monitoring the changes in transmission spectrum of a high finesse micro-ring resonator due to stress induced by displacement. Operation principles and sensitivity calculations are discussed in detail. SOI resonators with quality factors (Q) as high as 119000 have been achieved. This is the highest Q value for resonators based on SOI rib waveguides to date. Finesse values as large as 43 and modulation depths of 15 dB were observed. Free spectral ranges increased from 0.2 nm to 3.0 nm when radius was decreased from 500 to 20 µm. The thermo-optical tunability of these resonators were also studied. A high-Q racetrack resonator is used to develop a wavelength selective optical switch. The resonator was thermo-optically scanned over its full free spectral range applying only 57 mW of electrical power. A low power of 17 mW was enough to tune from resonance to off-resonance state. The device functioned as a wavelength selective optical switch with a 3 dB cutoff frequency of 210 kHz. We have also demonstrated wavelength add/drop filters using the same racetrack resonators with double bus. Asymmetric lateral coupling was used in order to get better filter characteristics. Filters with crosstalks as low as -10.0 dB and Q-factors of as high as 51000 were achieved. Finally, we introduce the use of a layer transfer method for SOI wafers. Such a layer transfer results in the possibility of using the back side of the silicon layer in SOI structure for further processing. With this method, previously fabricated SOI waveguides were transferred to form hybrid silicon-polymer waveguides. Benzocyclobutene (BCB) polymer was used as the bonding agent. The method is also applied to SOI M-Z interferometers to explore the possibilities of the technology. We additionally studied asymmetric vertical couplers (AVC) based on polymer and silicon waveguides and fabricated them using a hybrid technology.Show more Item Open Access Functionalization of carbon nanotubes(Bilkent University, 2005) Dağ, SefaShow more